Basic properties of minerals. Basic physical properties of minerals. Attic insulation scheme

Minerals are natural chemical compounds or native elements found in the earth's crust. Rocks (soils) and soils that are directly under our feet are composed of minerals. The distribution of minerals is extremely uneven. There are about 3000 known minerals, only about 50 of them are widespread. These minerals are called genus-forming. If we consider individual geological provinces, for example, the central part of the Russian Plain, then there are even fewer rock-forming minerals on the earth's surface - about 20.

In general, there are much more chemical compounds than minerals, but most of them are substances obtained artificially. Recently, two more classes of substances have been called minerals:

• what used to be called minerals - inorganic compounds present in food, medicine, cosmetics;
• components formed in the process of manufacturing building materials - bricks, concrete, ceramics, etc.

Minerals are mainly solid, much less often liquid (groundwater) and gaseous (radon, methane). Crystalline, amorphous and colloidal minerals predominate among solid minerals (they are less common). In appearance, the minerals are very diverse and have a large number of features. The same combination of chemical elements can crystallize into different structures and form different minerals - this phenomenon is called polymorphism. For example, modifications of carbon (C) give graphite and diamond; iron sulfide (FS 2) forms two minerals - pyrite and marcasite, calcium carbonate CaCO 3 - minerals calcite and aragonite.

Minerals are isotropic and anisotropic: isotropic are the same in properties in all directions, and anisotropic are different in non-parallel directions.

According to their origin, minerals are usually subdivided into endogenous (deep) and exogenous (formed on the surface; these also include minerals formed at the bottom of the sea). Many minerals can be of both endogenous and exogenous origin. The factor of the presence of the mineral in the rock should not be combined with the factor of origin - many endogenous minerals further compose sedimentary (exogenous) rocks or are present in them (for example, quartz, which is of magmatic or metamorphic origin, forms sands or sandy and dusty). leftist fractions and is an essential component of sedimentary clay rocks).

Diagnostics of minerals

Minerals have various properties, some of which can be determined visually, others - with the help of special equipment. Properties determined visually or using the simplest devices (hydrochloric acid, magnifying glass, knife, hardness scale) are called external, and the corresponding diagnostics are called macroscopic. Usually it is quite enough to determine the names of the rock-forming minerals and the rocks they compose and, in a preliminary, estimated form, to judge the properties of the geological environment.

The external properties of minerals, determined macroscopically, include: form of precipitation, color, powder color (line), luster, fracture, cleavage, hardness, specific gravity, and some special properties.

Allocation form

The most common forms are crystalline, earthy and amorphous masses. Crystals are called isometric if they are approximately equally developed in all three directions. Crystals elongated in one direction are called columnar, prismatic, acicular, and elongated in two directions - tabular, lamellar, leafy. Other forms are brushes (geodes), nodules and secretions, pseudomorphs (fossils), oolites, etc.

One mineral can have different forms of release, while keeping other properties unchanged.

Coloration

Coloring - the color of the mineral. In nature, there are minerals that have either one color or different colors. Graphite is always dark gray, and feldspar can have a color from white to black - pink, red, gray, green, brownish.

Powder color (trait)

As a rule, the color of the mineral is darker than the color of the mineral in powder. Many colored minerals are white powder. The powder is obtained by drawing with a sample on a porcelain plate - hence the name of the property - trait. When drawing on porcelain, a perfect powder is obtained, lying in a thin layer on a white background. For minerals with a hardness greater than that of porcelain (> 6.5), they say that they do not give traits. Some minerals are well diagnosed by trait (for example, black hornblende has a dark green trait, black labrador (feldspar) - white or light gray, dark gray hematite - cherry).

Forms of minerals allocation (diagrams)

a - elongated crystals; b - flat; c - isometric; g - crystalline mass (rock); d - fossil (pseudomorphosis); e - dendrite; g - kidney-shaped drip form; h - stalactites; and - stalagmites; k - nodule; l - secretion; m, n - oolites; o - brush (druse, geode); p - rose (rosette)

Shine

Luster is the property of minerals, like all objects, to reflect, refract, absorb light rays, as well as our perception of reflected light. The gloss of a mineral should be determined by those places where it shines brightest - along the surfaces of a fresh chip (if necessary, a chip must be obtained). One mineral can have a different luster (for example, in plate gypsum - glass and mother-of-pearl; in quartz - bold on chips and glassy on grown edges). Let's name the types of glitter, placing them in the list in descending order of intensity of the reflected light.

• metal. Minerals are like metal objects;
• semi-metallic, diamond resin. These are bright types of glitter; minerals possessing them are quite rare in nature, many are valuable minerals, but are unlikely to be found in works in the field of environmental engineering;
• fatty. The surface of the mineral gives the impression of being covered with a thin layer of oil. It is more often observed in minerals with an uneven surface, for example, in quartz and opal;
• pearl. It is observed on even smooth surfaces, gives a light colored sheen (examples: talcum, to a lesser extent gypsum, mica);
• glass. It is observed on the even edges of many minerals. The entire surface shines at the same time (examples: calcite, anhydrite, feldspars);
• silky. It is observed in minerals with a needle-like break, when the cleavage surface resembles long threads of shiny nylon fabric (examples: asbestos, hornblende, fibrous gypsum);
• ... matte (weak, dull). The surface, even on a fresh cleavage, shines weakly (examples: flint, chalcedony, phosphorite in concretions);
• minerals without gloss (examples: phosphorite in earthy masses, montmorillonite, kaolinite).

Break

Fracture - the shape of the surface of the mineral, resulting from the breaking of the sample. A break in the same sample can be characterized by several words that will complement each other without contradiction. For example, the fracture of limonite is earthy and uneven at the same time, the fracture of sugar-like gypsum is granular and uneven in the entire sample and is stepped if you look closely at the crystals. Some types of fracture, amenable to a schematic representation, are presented below.

Some types of fracture (diagrams)

a - stepped in the crystal; b - stepped in a crystalline mass; c - acicular in a crystalline mass; g - coarse-grained; d - concha

Fracture types:

• stepped. It is easily determined in single crystals with fracture planes, for example, in calcite and micas. It is more difficult to see a stepwise fracture in crystals within crystalline masses. In such cases, you should find crystals and pay attention to their small planes, while the whole sample will give the impression of uneven or grainy, such as, for example, Labrador or dolomite;
• needle-like (splinter, fibrous). Looks like a fracture in wood or some fibrous material; observed in hornblende, asbestos;
• granular (sugar-like). It is observed in minerals with a fine-crystalline form of precipitation; crystals are still visible, but their fracture is already poorly visible (examples: anhydrite, fine-crystalline apatite);
• earthy. It is observed in minerals with a non-smooth surface, in which crystals are not visible due to their small size. The samples are like dry earth, have no shine, and often get dirty hands (examples: limonite, phosphorite, clay minerals);
• conchoidal. It is more often observed in amorphous minerals. Fracture surfaces are shiny, convex or concave, smooth,
  with sharp edges, which was used by ancient people in the manufacture of tools and weapons (examples: flint, chalcedony, obsidian, quartz);
• uneven. When cleaving, the mineral forms irregular, irregular surfaces (examples: fine-crystalline quartz, phosphorite).

Cleavage

Cleavage is the ability of crystalline minerals to split along special directions of the crystal lattice. This property is not observed in objects that surround us in everyday life. Cleavage can form planes, needles or fibers when minerals are split. Cleavage is possessed by most crystalline minerals and cannot be found in amorphous minerals. Cleavage surfaces should not be confused with the faces formed during crystal growth. Cleavage is clearly visible in large crystals (example: mica or feldspar). In broken samples of coarse-crystalline masses, cleavage is determined already because the crystals themselves are visible - each gave its own plane, different from the neighboring one.

Cleavage scheme

a - a large crystal will split only along cracks parallel to the faces; b - in the crystalline mass, chips are clearly visible passing along the cleavage planes

Cleavage is different. It can appear very well, like with mica, and absent, like with quartz crystals. According to the degree of perfection, there are five types of cleavage: very perfect, perfect, average, imperfect, very imperfect (there is practically no cleavage). If there is no cleavage, it is often impossible to understand where one crystal ended and the next began. Cleavage is not at all visible in minerals represented by earthy masses. In this case, it is determined under a microscope, and the data are published. Due to the anisotropy of crystals, even within one mineral, cleavage can manifest itself in different ways, for example, feldspar has perfect cleavage in two directions and an average cleavage in a third. Micas have a very perfect cleavage in one direction and do not have it in the other two.

Mica crystal

Cleavage in one direction, no cleavage in the other two directions, mica breaks like a sheet of paper. Grown faces are not counted.

As can be understood from what has been said, cleavage is rather closely related to a kink. It is present in minerals with a stepped, acicular and coarse-grained fracture and is absent in minerals with a crusty fracture. The cleavage of minerals with a fine-grained, earthy, uneven fracture should be read in reference books.

Density (specific gravity)

It is determined by eye. Most of the minerals have a density of 2.5-3.5 g / cm 3. Density helps to recognize light rocks - tripoli, flask, diatomite, dried clay, since they have a density of less than 2.0 g / cm 3, heavy minerals have a density of more than 4 g / cm 3.

Hardness

Hardness - resistance of the material surface to scratching, cutting, indentation, abrasion. This is a very convenient property for the simplest diagnostics of minerals. Minerals have a constant hardness. You can always try to scratch the sample with your fingernail,

with a knife, a piece of glass. The sharp corner of the specimen can also scratch other materials.

In geological practice, with the simplest diagnostics, it is customary to compare the sample under consideration with reference minerals by scratching them against each other. The scale of the German geologist Friedrich Moos is used as a standard. The scale in conventional units has a range from 1 to 10.

Hardness of minerals

Mohs scale			hardness	Mohs scale substitutes
Hardness		Mineral		Materials (edit)	Hardness replace- body
Relate- bodily	kg / cm 2
		Talc	Soft	Soft pencil
		Gypsum		Nail	2,0-2,5
		Calcite		Bronze wash	2,5-4,0
		Fluorite		Iron nail	4,0-4,5
		Apatite		Glass
		Feldspar (microcline, orthoclase, albite, anorthite)	Solid	Plain steel, razor blade	5,0-6,0
	1120	Quartz		Tool steel	7,0-7,5
	1427	Topaz	Highly solid
	2060	Corundum
	10 060	Diamond

With the help of the Mohs scale, it is possible to measure the hardness of minerals with an accuracy of 0.5 or 1. The result obtained is declared, for example, as follows: dolomite has a hardness of 3.5.

Special properties. This includes unusual properties found only in certain minerals.

1. Reaction with acids. Calcite, dolomite and other carbonates enter it: CaCO 3 (calcite) + 2HC1 (hydrochloric acid) -> CaCl 2 + H 2 0 + CO 2.
2. Smell on rubbing. It can have phosphorite.
3. Halite (NaCl) has a salty taste, sylvin (KC1) has a bitter taste.
4. Perception by touch. Talc and kaolinite can be greasy, slippery.
5. Irisation - the appearance of a beautiful blue sheen on the cleavage cleavage of a Labrador retriever.
6. Magnetic. It is checked by the reaction of the compass needle. It is possessed by some minerals containing iron, cobalt, nickel.
7. Double refraction. Some transparent minerals double the image. It is clearly visible if you put such a sample on the text and look through it.

During the repair or construction of the premises, one has to deal with many controversial issues. One of the main ones is the choice of building materials. You need to evaluate the pros and cons of your preference, compare with analogues and make a worthy decision. Mineral wool has gained immense popularity among builders as insulation and soundproofing material.

Wall insulation is economical heating, the absence of fungi, salvation from mold and dampness. In the summer months, good insulation prevents the walls from overheating and maintains a comfortable indoor temperature.

What is rock wool?

Mineral wool is an economical insulation made of natural non-combustible materials. Its production takes place by exposing basalt fiber and metallurgical slags to high temperatures. It has good fire-fighting properties, which is especially important in the construction of houses with stove heating and in hazardous production.

Scope of application

insulation of facades and attic;

internal wall insulation;

insulation of hot structures in production;

in the heating system, in the construction of pipelines, in the construction of flat roofs.

Such widespread use is possible due to the different technical characteristics of mineral wool. It has several varieties, it differs in the structure of the fibers. Each species stands out for its thermal conductivity and moisture resistance.

Types of mineral wool

Glass wool

It is made from broken glass and small crystalline materials. Fiberglass has a good thermal conductivity coefficient - 0.030-0.052 W / mK. The length of its fibers is from 15 to 55 mm, the thickness is 5-15 microns. Working with glass wool requires the utmost care. By its properties, it is prickly, broken threads can penetrate the eyes, damage the skin. Therefore, to work with the material, gloves, glasses, and a respirator are required. It is optimal to heat glass wool up to 450 degrees, do not cool - below 60 degrees. The positive properties of glass wool are good strength and elasticity, easy installation, and the ability to trim.

Slag

The fibers of this blast furnace slag product have a length of about 16 mm. The high hygroscopicity of this raw material does not allow the use of slag wool in the insulation of facades, heating mains. Most often it is used for insulating non-residential buildings. Heating temperature 250-300 degrees. For these and other properties, it is inferior to other types of mineral wool. Its main advantage is low price, easy installation, reliable sound insulation.

Stone wool

It is she who is the highest quality type of mineral wool. In size, its sheets are not inferior to slag fiber. But it is not prickly, very easy to use. It has a fairly high coefficient of thermal conductivity; this fiber can be heated up to 1000-1500 degrees. When heated above permissible degrees, it will not burn, but only melt. When we talk about a modern material for insulating houses, we mean just this type of wool - it is also called basalt.

Internal wall insulation

Production and properties of basalt wool

A bit of history:

For the first time, thin strands of volcanic rock were discovered in Hawaii. After the volcanic eruption, scientists drew attention to interesting finds. The hot lava flew up, and the wind pulled the rocks into thin threads, which solidified and turned into lumps, similar to modern mineral wool.

Basalt insulation production

Due to heat treatment at rather high temperatures, rock materials are converted into fibrous material. After that, binding components are added to them and put under the press. Then the fiber enters the polymerization chamber, where it turns into a solid product.

Basalt insulation can have a high density, which gives the product additional rigidity and good resistance to stress. The porous structure helps to absorb impact noise. During the production process, cotton wool of various structures can be obtained. A more flexible one is used in pipelines, a semi-rigid one is insulated at home, and a rigid structure is indispensable in production.

Properties of basalt mineral wool:

soundproofing;

high thermal insulation;

security;

moisture resistance;

durability;

absolute incombustibility.

Basalt fiber is produced in rolls and slabs. It is very lightweight and easy to cut.

Note!

Recently, the foil type of product has been very popular with builders. Thanks to the foil, an increased level of heat saving is obtained. It is suitable for insulating any surfaces; it is this material that is used for ventilation and refrigeration systems.

Stamps

In the factory, you can get a product of various densities. It is for this property that several brands of mineral wool can be distinguished.

Brand P-75

Has a density of 75 kg per cubic meter. A product of low density is used where there is no need to withstand a serious load. For example, for insulation of some roofs, attics. Also, cotton wool of this brand is used for heating pipes.

Attic insulation scheme

Brand P-125

With its density of 125 kg per cubic meter, it is suitable for floor and interior wall insulation. The material has good noise protection, so it is the ideal mineral wool for sound insulation.

PZh-175 brand

High density material with good rigidity. Indispensable where you need to insulate floors made of reinforced concrete or metal.

PPZh brand - 200

It has the highest rigidity, as indicated by the indicated abbreviation. As well as PZh-175 it is used for thermal insulation of sheet metal walls. But, besides this, this brand should be used where there is an increased likelihood of a fire hazard.

Facade mineral wool

Most often, mineral wool is used to insulate facades. All of the above properties of basalt fiber are significantly superior to the same foam. It is this material that does not easily retain heat, but also helps air penetrate to the walls. Particular attention should be paid to the very choice of the product and the installation of structures.

Insulation of the facade

Important: It is better to purchase products in the form of slabs, which will greatly simplify their installation. The density of the material should not be less than 140 kg / cubic meter. The width of the slab itself is 10 cm.

Mineral wool and health risks

The pessimistic sentiment that the use of mineral wool is seriously harmful to health is based on the technical characteristics of past generations of mineral wool. Indeed, the constant work with glass wool was very dangerous for the lungs. Today these products are used very rarely. Modern basalt fiber is produced using high-quality raw materials, attaching great importance to the technological process. Subject to all sanitary standards, the binding of harmful substances - phenol and formaldehyde - practically lose their negative properties for the environment.

To be sure of the safety of the material, you need to pay attention to the choice of the manufacturer. If stone wool is mined by clandestine organizations, without observing GOSTs and the necessary technical conditions, then there is no guarantee that the action of phenol will not affect the health of others.

The definition of minerals is made according to their physical properties, which are due to the material composition and structure of the crystal lattice of the mineral. These are the color of the mineral and its powder, luster, transparency, fracture and cleavage character, hardness, specific gravity, magnetism, electrical conductivity, malleability, fragility, flammability and odor, taste, roughness, fat content, hygroscopicity. When determining some minerals, their ratio to 5-10% hydrochloric acid (carbonates boil) can be used.

The question of the nature of the color coloration of minerals is very complex. The nature of the coloration of some minerals has not yet been determined. In the best case, the color of a mineral is determined by the spectral composition of the light radiation reflected by the mineral or is determined by its internal properties, some chemical element that is part of the mineral, finely dispersed inclusions of other minerals, organic matter and other reasons. The coloring pigment is sometimes distributed unevenly, in stripes, giving multi-colored patterns (for example, in agates).

Irregular agate stripes

Some transparent minerals change color when incident light reflects off internal surfaces, cracks, or inclusions. These are the phenomena of iridescent color of chalcopyrite, pyrite and iridescence minerals - blue, blue overflows of Labrador.

Some minerals are multicolored (polychrome) and have different colors along the length of the crystal (tourmaline, amethyst, beryl, gypsum, fluorite, etc.).

The color of a mineral can sometimes be diagnostic. For example, aqueous copper salts are green or blue in color. The nature of the color of minerals is determined visually, usually by comparing the observed color with well-known concepts: milky white, light green, cherry red, etc. This feature is not always characteristic of minerals, since the colors of many of them vary greatly.

Often, the color is due to the chemical composition of the mineral or the presence of various impurities in which chemical chromophore elements are present (chromium, manganese, vanadium, titanium, etc.). The mechanism of the appearance of this or that color on gems is still not always clear, since the same chemical element can color different gems in different colors: the presence of chromium makes the ruby ​​red, and the emerald green.

Line color

A more reliable diagnostic feature than the color of a mineral is the color of its powder, which is left when the test mineral scratches the matte surface of a porcelain plate. In some cases, the color of the trait coincides with the color of the mineral itself, in others it is completely different. So, in cinnabar, the color of the mineral and powder is red, and in brass-yellow pyrite, the line is greenish-black. The trait is given by soft and medium hard minerals, while hard ones only scratch the plate and leave furrows on it.

Color line of minerals on a porcelain plate

Transparency

According to their ability to transmit light, minerals are divided into several groups:

• transparent(rock crystal, rock salt) - transmitting light, objects are clearly visible through them;
• translucent(chalcedony, opal) - objects, through which objects are poorly visible;
• translucent only in very thin plates;
• opaque- light is not transmitted even in thin plates (pyrite, magnetite).

Shine

Luster is the ability of a mineral to reflect light. There is no rigorous scientific definition of shine. There are minerals with a metallic luster like polished minerals (pyrite, galena); with semi-metallic (diamond, glass, matte, greasy, wax, mother-of-pearl, with iridescent tints, silky).

Cleavage

The phenomenon of cleavage in minerals is determined by the cohesion of particles inside crystals and is due to the properties of their crystal lattices. The cleavage of minerals occurs most easily parallel to the densest networks of crystal lattices. These grids most often and in the best development manifest themselves in the external limitation of the crystal.

The number of cleavage planes for different minerals is not the same, reaching six, and the degree of perfection of different planes may be different. The following types of cleavage are distinguished:

• very perfect, when the mineral without much effort splits into separate leaves or plates with smooth shiny surfaces - cleavage planes (gypsum).
• perfect, found when lightly hitting the mineral, which crumbles into pieces, limited only by smooth shiny surfaces. Uneven surfaces not along the cleavage plane are obtained very rarely (calcite splits into regular rhombohedrons of different sizes, rock salt - into cubes, sphalerite - into rhombic dodecahedrons).
• average, which is expressed in the fact that when a mineral is struck, kinks are formed both along cleavage planes and along uneven surfaces (feldspars - orthoclase, microcline, labrador)
• imperfect... Cleavage planes in the mineral are difficult to detect (apatite, olivine).
• very imperfect... Cleavage planes in the mineral are absent (quartz, pyrite, magnetite). At the same time, sometimes quartz (rock crystal) is found in well-cut crystals. Therefore, it is necessary to distinguish the natural crystal faces from the cleavage planes that are revealed during the fracture of the mineral. The planes can be parallel to the edges and have a fresher look and higher gloss.

Break

The nature of the surface formed during the fracture (splitting) of the mineral is different:

1. Smooth break if the mineral is split along cleavage planes, as, for example, in crystals of mica, gypsum, and calcite.
2. Stepped fracture is obtained when there are intersecting cleavage planes in the mineral; it can be observed in feldspars, calcite.
3. Uneven kink characterized by the absence of shiny areas of cleavage, as, for example, in quartz.
4. Grainy fracture observed in minerals with a granular-crystalline structure (magnetite, chromite).
5. Earthy fracture typical for soft and highly porous minerals (limonite, bauxite).
6. Crustaceous- with convex and concave areas like shells (apatite, opal).
7. Splinter(acicular) - an uneven surface with splinters oriented in one direction (selenite, chrysotile asbestos, hornblende).
8. Hooked- hooked irregularities appear on the surface of the split (native copper, gold, silver). This kind of fracture is typical for malleable metals.

Smooth fracture on mica Uneven fracture on rose quartz Stepped fracture on halite. © Rob Lavinsky Grainy fracture of chromite. © Petr Sosonovski
Earthy fracture of limonite Crusty fracture on creme Splinter fracture on actinolite. © Rob Lavinsky Hooky kink on copper

Hardness

Hardness of minerals- this is the degree of resistance of their outer surface to the penetration of another, harder mineral and depends on the type of crystal lattice and the strength of the bonds of atoms (ions). Determine the hardness by scratching the surface of the mineral with a fingernail, knife, glass or minerals with known hardness from the Mohs scale, which includes 10 minerals with gradually increasing hardness (in relative units).

The relative position of minerals in terms of the degree of increase in their hardness is visible when comparing: accurate determinations of the hardness of diamond (hardness on a scale of 10) showed that it is more than 4000 times higher than that of talc (hardness - 1).

Mohs scale

Most of the minerals have a hardness of 2 to 6. The harder minerals are anhydrous oxides and some silicates. When determining a mineral in a rock, make sure that it is the mineral and not the rock that is being tested.

Specific gravity

The specific gravity varies from 0.9 to 23 g / cm 3. For most of the minerals, it is 2 - 3.4 g / cm 3, ore minerals and native metals have the highest specific gravity of 5.5 - 23 g / cm 3. The exact specific gravity is determined in laboratory conditions, and in normal practice - by "weighing" the sample on the hand:

1. Light (with a specific gravity of up to 2.5 g / cm 3) - sulfur, rock salt, gypsum and other minerals.
2. Medium (2.6 - 4 g / cm 3) - calcite, quartz, fluorite, topaz, brown iron ore and other minerals.
3. With a high specific gravity (more than 4). These are barite (heavy spar) - with a specific gravity of 4.3 - 4.7, sulphurous ores of lead and copper - a specific gravity of 4.1 - 7.6 g / cm 3, native elements - gold, platinum, copper, iron, etc. .d. with a specific gravity of 7 to 23 g / cm 3 (osmous iridium - 22.7 g / cm 3, platinum iridium - 23 g / cm 3).

Magnetic

The property of minerals to be attracted by a magnet or to deflect a magnetic compass needle is one of the diagnostic signs. Magnetite and pyrrhotite are highly magnetic minerals.

Malleability and fragility

Malleable are minerals that change their shape when struck with a hammer, but do not disintegrate (copper, gold, platinum, silver). Fragile - crumble on impact into small pieces.

Electrical conductivity

The electrical conductivity of minerals is the ability of minerals to conduct electric current when exposed to an electric field. Otherwise, minerals are classified as dielectrics, i.e. non-conductive.

Flammability and odor

Some minerals catch fire from a match and create characteristic odors (sulfur - sulfur dioxide, amber - an aromatic smell, ozokerite - a suffocating smell of carbon monoxide). The smell of hydrogen sulfide appears when hitting marcasite, pyrite, when rubbing quartz, fluorite, calcite. When the pieces of phosphorite rub against each other, the smell of burnt bone appears. Kaolinite, when wetted, acquires the smell of a stove.

Taste

Only minerals readily soluble in water evoke taste sensations (halite - salty taste, sylvin - bitter salty).

Roughness and greasiness

Talc, kaolinite are greasy, slightly smearing, rough - bauxite, chalk.

Hygroscopicity

This is the property of minerals to moisturize, attracting water molecules from the environment, including from the air (carnallite).

Some minerals react with acids. To identify minerals that are, by chemical composition, salts of carbonic acid, it is convenient to use the reaction of boiling them up with weak (5-10%) hydrochloric acid (calcite, dolomite).

Radioactivity

Radioactivity can be an important diagnostic feature. Some minerals containing radioactive chemical elements (such as uranium, thorium, tantalum, zirconium, thorium) often have significant radioactivity that can be easily detected with household radiometers. To check the radioactivity, the background value of the radioactivity is first measured and recorded, then the mineral is placed on the detector of the device. An increase in readings of more than 15% indicates the radioactivity of the mineral. Radioactive minerals are: abernatiite, bannerite, gadolinite, monazite, ortite, zircon, etc.

Glow

Luminous fluorite

Some minerals, which by themselves do not glow, begin to glow under various special conditions (heating, irradiation with X-rays, ultraviolet and cathode rays; when broken or even scratched). There are the following types of minerals glow:

1. Phosphorescence - the ability of a mineral to glow for minutes and hours after exposure to certain rays (willemite glows after exposure to short ultraviolet rays).
2. Luminescence - the ability to glow at the moment of irradiation with certain rays (scheelite glows blue when irradiated with ultraviolet and rays).
3. Thermoluminescence - glow when heated (fluorite glows purple-pink).
4. Triboluminescence - glow at the moment of scratching with a knife or splitting (corundum).

Asterism

Asterism or star effect

Asterism, or the star effect, found in few minerals. It consists in the reflection (diffraction) of light rays from inclusions in the mineral, oriented along certain crystallographic directions. The best representatives of this property are star sapphire and star ruby.

In minerals with a fibrous structure (cat's eye), a thin strip of light is observed that can change its direction when the stone is turned (iridescence). The playing light on the surface of the opal or the shining peacock colors of the Labrador are explained by the interference of light - the mixing of light rays when they are reflected from the layers of packed silica balls (in opal) or from the thinnest lamellar crystalline growths (Labrador, moonstone).

Minerals have a complex of physical properties, by which they are distinguished and determined. Let's consider the most important of these properties.

Optical properties. Coloration or Colour mineral is an important diagnostic characteristic. Some minerals have a specific color, by which it can be almost accurately identified. The color of other minerals can vary widely within the same mineral individual. The color of minerals depends on their chemical composition, internal structure, mechanical impurities and, mainly, on chemical impurities of chromophore elements: Cr, V, Ti, Mn, Fe, Al, Ni, Co, Cu, U, Mo, etc.

By color, all minerals are generally subdivided into dark-colored and light-colored. When characterizing the color of a mineral for diagnostic purposes, one should strive for its most accurate description. When describing color, complex definitions are used, for example, bluish green, milky white, etc., the main color in such adjectives is in second place.

Mineral color in powder, or line color , is also an important characteristic that sometimes plays a decisive role in determining the mineral. The color of the mineral in the powder may differ significantly from the color of the mineral aggregate in the lump. To determine the color of the mineral in powder, the mineral is carried out over the rough surface of a porcelain plate, cleaned of enamel. Such a plate is called a biscuit (from the French Biscuite - uncoated porcelain). It is on it that the line remains, which makes it possible to evaluate the color of the mineral in the powder. However, if the hardness of the mineral exceeds the hardness of the biscuit, it is impossible to get a trait in this way.

Transparency- the ability of minerals to transmit light without changing the direction of its propagation. Transparency depends on the crystal structure of the mineral, the intensity of its color, the presence of fine inclusions and other features of its structure, composition and formation conditions. According to the degree of transparency, minerals are divided into: transparent, translucent, translucent, opaque.

Transparent- transmit light throughout the entire volume. You can see through such minerals as through window glass.

Translucent- only outlines of objects are visible through them. Light passes through the mineral like frosted glass.

Translucent- transmit light along a thin edge or in thin plates.

Opaque- do not transmit light even in thin plates.

All other things being equal, finer-grained aggregates appear less transparent.

Shine- the ability of the mineral to reflect light. The reflection of light from the surface of the mineral is perceived as a luster of varying intensity. This property also depends on the structure of the mineral, its reflectivity and the nature of the reflective surface. There are the following types of gloss.

Metal- strong luster typical of native metals and many ore minerals.

Metal-like or semi-metallic- reminiscent of the shine of a tarnished metal surface.

Diamond brilliance (brightest) is characteristic of diamond, some varieties of sphalerite and sulfur.

Glass- is quite widespread and resembles the luster of glass.

Fatty- shine, in which the surface of the mineral seems to be covered with a film of fat or oiled. Oily sheen occurs due to irregularities in the surface of a fracture or the face of a mineral, as well as due to hygroscopicity - absorption of water with the formation of a water film on the surface.

Wax- in general, it is similar to fat, only weaker, dull, characteristic of cryptocrystalline mineral aggregates.

Pearl- resembles the iridescent sheen of the surface of a mother-of-pearl shell.

Silky- observed in aggregates with a fibrous or needle-like structure. It resembles the sheen of silk fabric.

Matt shine is characteristic of fine-grained aggregates with an uneven earthy surface. A matte gloss practically means no gloss.

Sometimes the luster on the crystal edges, on its cleavage and on the cleavage surface may differ, for example, in quartz the luster on the edges can be glassy, ​​while on the cleavage it is almost always oily. As a rule, the luster on the cleavage surfaces is brighter and more intense than on the crystal edges.

Mechanical properties. Cleavage - the ability of a mineral to split in certain directions with the formation of relatively smooth surfaces (cleavage surfaces).

Some minerals, when exposed to them, are destroyed along regular parallel planes, the direction and amount of which is due to the peculiarities of the crystal structure of the mineral. Destruction occurs preferably along those directions along which the weakest bonds exist in the crystal lattice. The nature of cleavage is established by studying individual mineral grains. Amorphous minerals do not possess cleavage.

According to the ease of splitting and the nature of the formed surfaces, several types of cleavage are distinguished.

Very perfect cleavage- the mineral is easily cracked or split by hand into thin plates. Cleavage planes are smooth, even, often mirror-even. A very perfect cleavage usually appears in only one direction.

Perfect cleavage- the mineral is easily cracked with a weak blow of a hammer with the formation of smooth shiny surfaces. The number of cleavage directions for different minerals is not the same (Fig. 8).

Average cleavage- the mineral splits upon impact into fragments, which are bounded approximately to the same extent by both relatively flat cleavage planes and irregular fracture planes.

Imperfect cleavage- splitting of the mineral leads to the formation of fragments, most of which are limited by uneven fracture surfaces. Recognition of such cleavage is difficult.

Very imperfect cleavage, or lack of cleavage - the mineral splits in random directions and always gives an uneven fracture surface.

The number of cleavage directions, the angle between them, the degree of its perfection are some of the main diagnostic features in the determination of minerals.

Rice. 8. Perfect cleavage:

a - gouges by cleavage - halite cube, rhombohedrons of calcite; b - visible cracks developed along the cleavage directions; c - different orientation and number of cleavage planes: 1 - cleavage in one direction, mica; 2 - cleavage in two mutually perpendicular directions, orthoclase; 3 - cleavage in two non-perpendicular directions, amphibole; 4 - cleavage in three mutually perpendicular directions, halite; 5 - cleavage in three non-perpendicular directions, calcite; 6 - cleavage in four directions parallel to the faces of the octahedron, diamond; 7 - cleavage in six directions, sphalerite

Break- the type of surface formed by the splitting of the mineral. This characteristic is especially important in the study of minerals with imperfect and very imperfect cleavage. For such minerals, the appearance of the fracture surface can be an important diagnostic feature. There are several characteristic fracture types.

In some minerals, a characteristic concave or convex concentric-ribbed surface, resembling a shell, may appear at the fracture. Such a break is called conchoidal... Most often, the mineral splits along an uneven surface that does not have any characteristic features. Such a break is called uneven, it is possessed by many minerals devoid of cleavage. Native metals, copper, iron and other minerals are found hooked break; native silver has chopped break. Minerals with perfect cleavage in 1-2 directions give smooth break. In addition, the kink can be stepwise, splinter, grainy.

Hardness- the ability of the mineral to resist external mechanical stress - scratching, cutting, indentation. This feature, like most others, depends on the internal structure of the mineral and reflects the strength of bonds between lattice sites in crystals. In the field, the relative hardness of minerals is determined by scratching one mineral with another.

To assess the relative hardness of the mineral, an empirical scale is used, proposed at the beginning of the last century by the Austrian mineralogist F. Moos (1772-1839) and known in mineralogy as the Mohs hardness scale (Table 1). The scale uses ten minerals with known and constant hardness as references. These minerals are arranged in increasing order of hardness. The first mineral - talc - corresponds to the lowest hardness taken as 1, the last mineral - diamond - corresponds to the highest hardness 10. Each previous mineral on the scale is scratched by the next mineral.

Table 1 - Mineral hardness scale

The Mohs scale is a relative scale, the increase in hardness within its limits occurs very unevenly from standard to standard, for example, the measured absolute hardness of diamond is not 10 times more than the hardness of talc, but about 4200 times. The absolute value of hardness increases with decreasing radii and increasing the charge of the ions that make up the crystal. To determine the relative hardness of a mineral on its fresh (non-weathered) surface, press with an acute angle of the reference mineral. If the standard leaves a scratch, it means that the hardness of the studied mineral is less than the hardness of the standard, if it does not leave - the hardness of the mineral is higher. Depending on this, the next standard is selected higher or lower on the scale until the hardness of the mineral to be determined and the hardness of the reference mineral coincide or turn out to be close, i.e. both minerals do not scratch each other or leave a faint mark. If the investigated mineral in terms of hardness is between two standards, its hardness is determined as intermediate, for example 3.5.

For an approximate assessment of the relative hardness of minerals in the field, you can use a pencil lead (hardness 1), nail (2-2.5), copper wire or coin (3-3.5), steel needle, pin, nail or knife (5 -5.5), glass (5.5-6), file (7).

Density minerals vary from 0.8-0.9 (for natural crystalline hydrocarbons) to 22.7 g / cm 3 (for osmous iridium). The exact determination is carried out in laboratory conditions. By density, all minerals can be divided into three categories: light - density up to 2.5 g / cm 3 (gypsum, halite), medium - density up to 4 g / cm 3 (calcite, quartz, feldspars, micas) and heavy - density more than 4 g / cm 3 (galena, magnetite). The density of most minerals is from 2 to 5 g / cm 3.

Mechanical properties that can be used as diagnostic signs of minerals also include brittleness and malleability.

Fragility- the property of a substance to crumble under pressure or upon impact.

Ductility- the property of a substance under pressure to flatten into a thin plate, to be plastic.

Special properties. Some minerals are characterized by special, only inherent properties - taste(halite, sylvin) smell(sulfur), combustion(sulfur, amber), magnetism(magnetite), reaction with hydrochloric acid(minerals of the carbonate class), electrical conductivity (graphite) and some others.

Magnetic properties of minerals are determined by their magnetic structure, i.e. firstly, the magnetic properties of the atoms that make up the mineral, and, secondly, the location and interaction of these atoms. Magnetite and pyrrhotite are two of the most important magnetic minerals that act on the magnetic needle.

Electrical conductivity. For the most part, minerals are poor conductors of electricity, with the exception of sulfides, some oxides (magnetite) and graphite, the resistivity of which is less than 10 Ohm m.However, the total electrical conductivity of mineral aggregates depends not only on the properties of the mineral itself, but also on the structure, and, most importantly, on the degree of porosity and water cut of the unit. Most minerals conduct electricity through pores filled with natural mineralized waters - electrolyte solutions.

When we examine minerals in museum showcases or trays with specially selected samples, we are involuntarily amazed at the variety of external signs by which they differ from each other.

Some minerals appear transparent (rock crystal, rock salt), others - cloudy, translucent or completely impervious to light (magnetite, graphite).

A remarkable feature of many natural compounds is their color. For a number of minerals, it is constant and very characteristic. For example: cinnabar (mercury sulphide) always has a carmine red color; for malachite, a bright green color is characteristic; cubic pyrite crystals are easily recognizable by their metallic golden color, etc. Along with this, the color of a large number of minerals is variable. These are, for example, the varieties of quartz: colorless (transparent), milky white, yellowish brown, almost black, purple, pink.

Gloss is also a very characteristic feature of many minerals. In some cases, it is very similar to the luster of metals (galena, pyrite, arsenopyrite), in others - to the luster of glass (quartz), mother-of-pearl (muscovite). There are also many such minerals that, even in a fresh fracture, look dull, that is, do not have a shine.

Often minerals are found in crystals, sometimes very large, sometimes extremely small, established only with a magnifying glass or microscope. For a number of minerals, crystalline forms are very typical, for example: for pyrite - cubic crystals, for garnets - rhombic dodecahedrons, for beryl - hexagonal prisms. However, in most cases, mineral masses are observed in the form of continuous granular aggregates, in which individual grains do not have crystallographic outlines. Many minerals are also distributed in the form of drip masses, sometimes of a bizarre shape that has nothing to do with crystals. Such are, for example, kidney-shaped masses of malachite, stalactite-like formations of limonite (iron hydroxides).

Minerals also differ in other physical properties. Some of them are so hard that they easily leave scratches on the glass (quartz, garnet, pyrite); others are themselves scratched by fragments of glass or by the edge of a knife (calcite, malachite); still others have such a low hardness that they can be easily drawn with a fingernail (plaster of Paris, graphite). Some minerals, when cleaved, are easily split along certain planes, forming fragments of a regular shape, similar to crystals (rock salt, galena, calcite); others give curves at the fracture, "concave" surfaces (quartz). Properties such as specific gravity, fusibility, etc. also vary widely.

The chemical properties of minerals are just as different. Some are readily soluble in water (rock salt), others are soluble only in acids (calcite), and still others are stable even with respect to strong acids (quartz). Most minerals retain well in the air. However, a number of natural compounds are known that readily undergo oxidation or decomposition due to oxygen, carbon dioxide and moisture contained in the air. It was also established long ago that some minerals gradually change their color under the influence of light.

All these properties of minerals are causally dependent on the characteristics of the chemical composition of minerals, on the crystal structure of the substance and on the structure of the atoms or ions that make up the compounds. Much that previously seemed mysterious, now, in the light of modern achievements of the exact sciences, especially physics and chemistry, is becoming clearer and clearer.

In this regard, let us recall some of the most important provisions for us in physics, chemistry, crystal chemistry and colloidal chemistry.

State of aggregation of minerals... As already indicated, according to the existing three aggregate states of matter, solid, liquid and gaseous minerals are distinguished.

Any substance of an inorganic nature, depending on temperature and pressure, can be in any state of aggregation, and when these factors change, it passes from one state to another.

The stability limits of each state of aggregation are in the most different temperature ranges, depending on the nature of the substance. At atmospheric pressure at room temperature, most minerals are in a solid state and melt at high temperatures, while mercury under these conditions exists in a liquid form, and hydrogen sulfide and carbon dioxide are in a gaseous state.

Most solid minerals are represented by crystalline substances, that is, substances with a crystalline structure. Each crystalline substance has a certain melting point, at which a change in the state of aggregation occurs with heat absorption, which clearly affects the behavior of the heating curves (Fig. 5, A). At a certain time interval, the heat inflow imparted to the system is spent on the melting process (the curve flattens out).

Crystallization of a cooled homogeneous liquid substance should occur at the same temperature as the melting of a solid of the same composition, but usually it occurs with some hypothermia liquids, which must always be borne in mind.

Solid chemically pure substances characterized by a disordered structure, i.e., the absence of a regular arrangement of atoms, are called amorphous(glassy) bodies. They belong to the group of isotropic substances, i.e., possessing the same physical properties in all directions. A characteristic feature of amorphous substances, in contrast to crystalline ones, is also gradual transition from one state of aggregation to another along a smooth curve (Fig. 5, B) like sealing wax, which, when heated, gradually becomes flexible, then viscous and, finally, drip-liquid.

Amorphous substances are often obtained by solidification of molten viscous masses, especially when the cooling of the melt occurs very quickly. An example is the formation of the mineral lechatellerite - amorphous quartz glass when lightning strikes quartz crystalline rocks. The transition of amorphous substances into crystalline masses can occur only when they are kept in a softened state for a long time at a temperature close to the melting point.

It should be added that not all substances can be easily obtained in an amorphous state. These are, for example, metals which, even when quenched, do not form glassy substances.

Polymorphism... Polymorphism ("poly" in Greek - a lot) is the ability of a given crystalline substance to undergo two or more modifications of the crystalline structure when external factors (mainly temperature) change, and in this regard, changes in physical properties. The most striking example in this respect is the dimorphism of natural carbon, which crystallizes, depending on the conditions, either in the form of diamond (cubic system) or in the form of graphite (hexagonal system), very different from each other in physical properties, despite the identity of the composition. When heated in the absence of oxygen, the crystal structure of diamond at temperatures above 3000 ° at atmospheric pressure is rearranged into a more stable (stable) structure of graphite under these conditions. The reverse transition of graphite to diamond is not established.

Fig 6. Changing the properties of quartz when heated. I - rotation of the plane of polarization; II - the magnitude of birefringence; III - refractive index Nm (for the D line of the spectrum)

Sometimes polymorphic transformation is accompanied by a very insignificant change in the crystal structure of the substance, and therefore, without sophisticated research, it is not possible to notice any significant changes in the physical properties of the mineral. Such, for example, are the transformations of the so-called α-quartz into β-quartz and vice versa. However, a study of the optical properties (Fig. 6) unambiguously shows an abrupt change at the transition point (about 573 °) in such properties as refractive indices, birefringence, and rotation of the plane of optical polarization.

Differences of a given crystalline substance that are stable under certain specific physicochemical conditions are called modifications, each of which is characterized by a certain crystal structure peculiar to it. A given substance can have two, three, or several such polymorphic modifications (for example, six modifications have been established for sulfur, of which only three are found in nature, for SiO2 - nine modifications, etc.).

Various polymorphic modifications are usually indicated by prefixes to the name of the mineral of the Greek letters α, β, γ, etc. (for example: α-quartz, stable below 573 °; β-quartz, stable above 573 °, etc.). There is no uniformity in the order of naming the modifications in the literature: some adhere to the designation of various modifications with the letters α, β ... in the order of increasing or decreasing the transformation temperature, other order of designations is used according to the degree of prevalence or in the order of discovery. The first order of designation should be considered more rational.

The phenomena of polymorphism are very widespread among natural compounds. Unfortunately, they are still far from being sufficiently studied. Polymorphic modifications of various minerals can be stable in the most varied ranges of changes in external factors (temperature, pressure, etc.). Some have a wide field of stability at very significant fluctuations in temperature and pressure (diamond, graphite), while others, on the contrary, undergo polymorphic transformations within narrow limits of changes in external factors (sulfur).

The very fact of the rearrangement of the crystal structure with a change in the external factors of equilibrium, as V.M. ... In the simplest case, at the moment of the critical state, a change in the coordination number occurs, indicating a radical change in the structure of the substance.

It often happens that the high-temperature modification of a mineral, when converted into a lower-temperature modification, retains the external shape of the original crystals. Such cases of false forms are called paramorphosis... An example is the paramorphoses of calcite over aragonite (CaCO 3).

If a given modification of a crystalline substance, let’s say α, has the property of changing into another β-modification when external conditions (for example, temperature) change, and when the previous conditions are restored, it turns back into the α-modification, then such polymorphic transformations are called enantiotropic*. Example: conversion of rhombic α-sulfur to monoclinic β-sulfur and vice versa. If the reverse transition cannot take place, then this type of transformation is called monotropic... An example is the monotropic transformation of rhombic aragonite (CaCO 3) into trigonal calcite (upon heating).

* ("Enantios" in Greek - opposite, "tropos" - change, transformation)

In nature, the simultaneous existence of two modifications under the same physicochemical conditions is often observed even next to each other (for example: pyrite and marcasite, calcite and aragonite, etc.). Obviously, the transition of one of the modifications to stable, i.e., stable, was delayed for some reason, and the substance in this case is in metastable(or, as they call it otherwise, a labile, instable) state, just as there are supercooled liquids.

It should be emphasized that a stable modification, in comparison with an unstable one, has:

1. lower vapor pressure,
2. less solubility and
3. higher melting point.

Phenomena of destruction of crystal lattices... The main features of the spatial lattices of crystalline bodies are the regular arrangement and strictly balanced state of their constituent structural units. However, it is enough to create conditions under which the internal bonds of structural units will be shaken, as from a crystalline substance with an ordered spatial lattice, we get an amorphous mass that does not have a crystalline structure.

An excellent example in this regard is the mineral ferrobrusite - (Mg, Fe) 2, which contains up to 36% (by weight) iron oxide as an isomorphic impurity. In its fresh state, this mineral, being extracted from the deep horizons of the mines, is completely colorless, transparent and has a glassy luster. Over the course of several days, its crystals in air gradually change their color, becoming golden yellow, then brown and, finally, opaque dark brown, retaining their outer crystalline form *. Chemical analysis shows that almost all ferrous iron is converted into ferric iron (i.e., oxidation occurs), and X-ray examination does not establish signs of a crystalline structure. Obviously, the oxidation of iron broke the internal bonds in the crystal lattice, which led to the disorganization of the structure of the substance.

* (Iron-free brucite is quite stable under similar conditions)

What happens to ferrobrusite in an oxidizing environment at room temperature and atmospheric pressure, for other minerals can take place at elevated temperatures and pressures, as has already been established for a number of cases.

Very interesting phenomena have been studied in minerals containing rare earth and radioactive elements (orthite, fergusonite, eshinite, etc.). In them, too, very often, but not always, the transformation of a crystalline substance into an amorphous one is established, which, as it is assumed, is due to the action of α-rays during radioactive decay *. These altered glassy minerals, not belonging to the cubic system, are optically isotropic and do not exhibit X-ray diffraction, that is, they behave like amorphous bodies. In this case, a partial hydration of the substance occurs. Such bodies were named by Brugger metamict.

* (According to V.M. Goldschmidt, in order to achieve an amorphous state in these cases, the radioactivity of the mineral alone is not enough, and the following two conditions are also necessary:

1. the initially emerging crystalline substance must have a weak ionic lattice, allowing rearrangement or hydrolysis; such lattices are formed mainly when weak bases are combined with weak anhydrides;
2. the lattice should contain one or more types of ions that can be easily recharged (for example, rare earth ions) or even turn into neutral atoms (for example, the formation of atomic fluorine in fluorite under the influence of radioactive radiation from the side)

The very process of disintegration VM Goldschmidt presents as a rearrangement of matter. For example, the compound YNbO 4 turns into a finely dispersed mixture (solid pseudo-solution) of oxides: Y 2 O 3 and Nb 2 O 5. With this concept, it is clear why there are no transformations into an amorphous substance of simple compounds like ThO2 (thorianite), or salts of strong acids with weak bases, for example (Ge, La ...) PO 4 (monazite))

A number of other analogous examples illustrating the formation of amorphous or colloidal masses can be cited to confirm the phenomena of the decomposition of crystalline media. However, one cannot think that these new formations are a stable form of the existence of a substance. There are many known examples of the secondary rearrangement of matter with the formation of new crystalline bodies that are stable under changed conditions. So, known "crystals of ilmenite" (Fe .. TiO 3), which, when microscopically examined, are composed of a mixture of two minerals: hematite (Fe 2 O 3) and rutile (TiO 2). Apparently, after the moment of formation of ilmenite at some period of the mineral's life, under the influence of the changed oxygen regime, sharply oxidizing conditions were created, which led to the transition of Fe 2+ to Fe 3+ with the simultaneous decomposition of the crystal structure, and then to the gradual rearrangement of the substance with the formation of a mixture resistant minerals. In the same way, for example, cases of formation of tillite (PbSnS 2), galena (PbS) and cassiterite (SnO 2) in the closest germination with each other were observed, but with the relict (i.e., previous) lamellar-granular structure of the aggregate preserved. characteristic of tillite. Obviously, due to the increased concentration of oxygen in this medium at some point, tin, having a high affinity for oxygen, separated from the initially homogeneous mineral mass in the form of an oxide, and lead passed into the form of an independent sulfur compound.

The concept of colloids*. In addition to clearly crystalline formations, the crystalline nature of which is easily established by eye or under a microscope, colloids are also widespread in the earth's crust.

* ("Kolla" in Greek - glue, "colloid" - glue-like)

Colloids are heterogeneous (dissimilar) dispersed * systems consisting of "dispersed phase" and "dispersion medium".

* (Dispersion - scattering; in this case - the state of matter in the form of the smallest particles. The degree of dispersion is determined by the size of these particles)

The dispersed phase in these systems is represented by finely dispersed particles (micelles) of a substance in any mass (dispersion medium). The sizes of particles of the dispersed phase range from about 100 to 1 mμ (from 10 -4 to 10 -6 mm), that is, much larger than the sizes of ions and molecules, but at the same time are so small that with the help of a conventional microscope are not distinguishable. Each such particle can contain from several to many tens and hundreds of molecules of a given compound; in solid particles, ions or molecules are bound into a crystal lattice, that is, these particles are the smallest crystalline phases.

The aggregate state of the dispersed phase and the dispersion medium can be different (solid, liquid, gaseous), and a wide variety of their combinations can be observed. Denoting the state of aggregation of the dispersion medium in capital letters and the state of the dispersed phase in small letters, we give the following examples:

• G + t: tobacco smoke; soot
• G + W: fog
• W + t: yellow peat waters; healing mud
• W + g: hydrogen sulfide sources; foam
• F + F: typical emulsoids (e.g. milk)
• T + w: crystals of native sulfur with liquid bitumen sprayed into them; opal
• T + t: red calcite with finely dispersed iron oxide
• T + g: milky white minerals containing gases

Among colloidal formations are distinguished sols and gels.

Typical sols, otherwise called colloidal solutions or pseudo-solutions, are those formations in which the dispersion medium strongly predominates over the dispersed phase (for example: tobacco smoke, yellow-brown ferrous waters, milk). To the eye, such solutions appear completely homogeneous and often transparent, indistinguishable from true (ionic or molecular) solutions. In sols in which the dispersion medium ("solvent") is water, the particles of the dispersed phase easily pass through ordinary filters, but do not penetrate through animal membranes. If their size exceeds 5 mμ, then they can be easily detected in an ultramicroscope using the so-called Tyndall light cone, which is created by side illumination of a special glass vessel filled with a colloidal solution. The effect created in this case is completely analogous to what we usually observe in a darkened room in a beam of light emanating from a projection lamp: in a luminous cone, particles of the dispersed phase become visible, performing Brownian motion, which is never observed in true solutions, with the exception of solutions of some organic compounds with very large molecules.

V gels the dispersed phase is presented in such a significant amount that the individual dispersed particles are, as it were, stuck together, forming gelatinous, glue-like, glassy masses. The dispersion medium in these cases, as it were, occupies the remaining space between dispersed particles.Examples of gels include: soot, dirt, opal (silica gel), limonite (iron hydroxide gel), etc.

Depending on the nature of the dispersion medium, there are: hydrosols and hydrogels (dispersion medium - water), aerosols and aerogels (dispersion medium - air), pyrosols and pyrogels (dispersion medium - any melt), crystalline sols and crystallogels (dispersion medium - any or crystalline substance), etc.

The most widespread in the earth's crust are hydrosols, crystal sols, and hydrogels. Further we will talk only about them.

Hydrosols most simply can be obtained mechanically, by fine spraying the substance in one way or another to the size of the dispersed phase in water. In nature, coarse and finely dispersed systems are often formed during grinding and abrasion of rocks and minerals under the influence of driving forces (water flows, glaciers, tectonic displacements, etc.).

However, the greatest role in the formation of natural colloidal solutions is played by chemical reactions in aqueous media leading to the condensation of molecules: oxidation, reduction and, especially, exchange decomposition reactions. For the most superficial part of the earth's crust, the vital activity of organisms (biochemical processes) is also of no less importance in the formation of colloids.

It is important to note that dispersed particles in colloidal solutions are electrically charged, which is easy to verify when passing an electric current through the solutions. The sign of the charge is the same for all particles of a given colloid, due to which, repelling from each other, they are suspended in a dispersion medium. The appearance of a charge is explained by the adsorption of dispersed particles of certain ions contained in solutions. It is necessary to dwell on this issue in more detail.

Let us imagine, for example, a solid dispersed particle of AgBr. Despite its ultramicroscopic size, it should have a crystal structure, which is schematically shown in section in Fig. 7. Each of the Ag 1+ cations and Br 1- anions inside this lattice is in a six-fold environment of ions of opposite charge: four in the plane of the drawing, one above the given ion and one below it. Thus, the internal ions of the dispersed particle are completely saturated with valences. The situation is different with boundary ions in the crystal lattice. In the same way, it is easy to calculate that most of the external ions on the face perpendicular to the plane of the figure receive saturation only from five ions of the opposite sign (three in the plane of the figure, one above and one below the plane of the figure). Consequently, the Ag and Br ions located on the flat surface of the dispersed particle have 1/6 of the unsaturated valence each, and on the edges they have 2/6, and the corner ions even have 3/6 of the unsaturated valence. This residual uncompensated charge is responsible for the absorption (adsorption) of a certain amount of additional bromine or silver ions from the solution, which are retained at the surface of dispersed particles in the form of a so-called diffuse layer.

In practice, the AgBr colloid is obtained by mixing solutions of AgNO 3 and KBr, reacting according to the following scheme: AgNO 3 + KBr = AgBr + KNO 3. If these solutions are mixed in equivalent amounts, a crystalline AgBr precipitate (but not a colloid) is formed. If silver nitrate is poured into potassium bromide, then a sol appears, the dispersed particles of which AgBr are charged negatively due to the adsorption of Br 1- ions. In the reverse order of merging, the resulting dispersed particles of AgBr adsorb Ag 1+ cations and are therefore positively charged.

To get a more realistic idea of ​​hydrosols and the structure of dispersed phases, let us turn to their characteristics from the point of view of electrochemistry.

>
Rice. 8. Diagram of the structure of the dispersed phase in an aqueous medium containing electrolytes. 1 - cations in the crystal lattice of the dispersed phase; 2 - anions in it; 3 - anions with unsaturated valences protruding at the corners; 4 - adsorbed ion swarm cations; 5 - H 2 O dipoles (partially deformed)

In fig. 8 schematically depicts a colloidal particle surrounded by a dispersion medium, in this case, water containing ions Na 1+, K 1+, Ca 2+, Mg 2+, Cl 1-, 2- and others, usually found in soil waters, which contain this or that amount of dissolved salts. The dispersed particle itself, as in the previous case, is shown in the form of a crystalline phase, in which incomplete saturation with valences should take place at the corner points. Consequently, these protrusions will accumulate adsorbed ions, in our case, cations Na 1+, K 1+, NH 4 1+, Mg 2+, Ca 2+, which positively charge the dispersed particle and form a diffuse layer.

Anions protruding at the corners of the lattice exert their influence not only on ions in solution, but also on electrically neutral water molecules. As we learn later, the H2O molecule is a dipole and has an original structure. It can be represented in the form of one oxygen ion O 2-, the negative charge of which is neutralized by two H 1+ protons embedded in it. Both protons are located on one side (from the center of the oxygen ion), which is positively charged, and the opposite side is negatively charged. This structure of the H 2 O molecule allows it to orient itself in a certain way (Fig. 8): by the side opposite to the two H 1+ protons, it is attracted to the cations. Since electrically neutral H 2 O molecules do not neutralize the cation charge affecting them, this charge spreads further, to the next nearest H 2 O molecules, which are also oriented.

Thus, a whole swarm of ions and oriented molecules of H 2 O is established around the dispersed particle (Fig. 8). The thickness of the water shell depends on the type of hydrated cations (holding H 2 O molecules). The most strongly hydrated cations of alkali metals. For example, the Na 1+ ion in an aqueous medium is capable of retaining 60-70 oriented H 2 O molecules, while Ca 2+ is only up to 14 H 2 O molecules.

It should also be noted that in some cases, when exposed to acids, the cations of the diffuse layer can be replaced by anions, for example: Cl 1-, 2-, etc. The latter, as well as cations, can be hydrated; however, the orientation of water molecules in this case will be reverse what is the case for cations (see the right side of Fig. 8).

From all that has been said, the following conclusions can be drawn:

1. From an electrochemical point of view, a charged dispersed phase can be considered a large ion ("macroion"), capable of moving in sols towards one or another electrode when an electric current is passed (the phenomenon of electrophoresis).
2. The dispersion medium for the dispersed phase is by no means a solvent in the usual sense of the word, although it can and usually contains certain compounds dissociated into ions.
3. The cations of the diffuse layer can be replaced by others if, for some reason, the composition and concentration of electrolytes in the dispersion medium change. Mutual replacement or displacement of some ions by others in adsorbents (adsorbing colloids) occurs according to the law of mass action.

The described phenomena of unsaturated valences on the surface of dispersed particles and the associated adsorption of cations or anions from a solution, no doubt, should also take place for large crystals or crystalline grains. But if we approach this issue from the point of view of the energetics of phenomena, we will find a colossal difference between real crystals and dispersed phases.

Since the phenomena of adsorption in colloids are confined to the boundaries between dispersed phases and the dispersion medium, the total surface of dispersed particles per unit volume is very important for expressing the total surface of the energy of a substance. This surface, called specific surface, increases sharply as the degree of dispersion of the substance increases. This is not difficult to show.

Let us assume that we have a cubic crystal of some mineral with an edge equal to 1 cm. Its total surface will be equal to 6 cm 2 (specific surface-6). If we divide this cube into eight parts, as shown in Fig. 9, then the total surface of the resulting eight small cubes will already be equal to 12 cm 2, and when dividing into cubes with an edge of 1 mm - 60 cm 2. If we bring further division to cubes with an edge of 1 mμ, i.e., to the size of the colloidal dispersed phase, then the total surface will reach a huge value of 6000 m 2 with a total mass volume of 1 cm 3 (i.e., the specific surface will be equal to 6 10 7). In this case, the number of cubes will reach 10 21.

Thus, between the specific surface NS and grain size at we have an inversely proportional relationship expressed by a simple formula: x = 6 / y... This dependence is easy to represent in the form of a graph (Fig. 10).

It can be seen from the data presented that for coarse-crystalline systems, the specific surface area, and hence the surface energy associated with it, is so negligible that the latter can be practically neglected. On the contrary, in colloidal systems it acquires exceptional importance. It is because of this that a number of physical and chemical properties of colloidal formations, widely used for practical purposes, are very different from the properties of coarse crystalline substances.

The phenomena of diffusion in colloidal solutions are incomparably weaker than in true solutions, which is explained by the much larger size of the particles of the dispersed phase in comparison with ions. This circumstance is reflected in the fact that mineral masses formed from colloidal solutions often have an extremely heterogeneous composition and structure.

Crystal ash, i.e., typical crystalline media containing any substance in the form of a dispersed phase are often formed as a result of crystallization of hydrosols. The process of their formation can be compared with crystallization (transformation into ice) of turbid water, that is, water containing dispersed particles in suspension. The formed ice will also be cloudy, i.e. contaminated with the same dispersed phase that was present in the water. In other words, it will be a crystalline ash.

These include, first of all, many minerals colored in one color or another, usually observed in the form of colorless transparent crystals. Such are, for example, reddish carnallite, red barite (due to the content of iron oxide in the form of a dispersed phase), black calcite, the color of which in some cases is due to finely dispersed sulfides in it, in others - organic substances, etc. numbered milky white quartz, calcite, etc., in which the role of the dispersed phase is played by finely dispersed gases or liquids, often visible in thin sections under a microscope. There are crystals, such as quartz, calcite and other minerals, with a crystalline-zonal structure, due to the alternation of transparent and colored or milky-white zones.

There is no doubt that crystal sols are also found among opaque minerals. This is evidenced by the impurities of such elements captured by chemical and spectral analyzes, which cannot be explained from the crystallochemical point of view as a result of isomorphic impurities. Such, for example, are the facts of the content of copper in crystals of pyrite, gold in pyrite, galena, arsenopyrite, etc. Microscopic studies of polished sections prepared from such crystals, at high magnifications, it is often possible to detect the smallest inclusions of chalcopyrite, native gold, etc. that they probably also contain more finely dispersed particles that cannot be captured with ordinary microscopes *.

* (The resolution (limit of distinguishability) of modern conventional microscopes is 0.5-1.0 μ. Smaller particles are not captured at all or at any magnification)

Hydrogels in nature, they are often formed from hydrosols by coagulation or, as they say, their coagulation, expressed in the formation of clots in the aquatic environment. The process of coagulation occurs only when, for one reason or another, dispersed particles lose their charge, becoming electrically neutral. In this case, the forces of repulsion of the particles from each other disappear, the particles are combined into larger bodies, called polyions, with their subsequent settling under the action of gravity.

The neutralization of the charges of dispersed particles, causing coagulation, can be obtained in various ways:

• a) adding electrolytes (ionic solutions) to the colloidal solution, and depending on the charge of the dispersed phase, neutralization will be carried out by anions or cations of the electrolyte; in this way, many silty sediments are formed at the mouths of large rivers carrying colloidal solutions; the latter, when meeting with sea waters containing dissolved salts, which play the role of electrolytes, undergo coagulation and precipitation in the coastal zones of sea basins;
• b) by mutual neutralization of colloidal solutions containing oppositely charged colloidal particles and taken in appropriate quantitative ratios; as a result of this, mixed gels are obtained (for example, brown iron ores rich in colloidal silica);
• c) by spontaneous coagulation of colloidal solutions over time, especially if a dispersion medium (water) is lost in the system due to its evaporation; in this case, naturally, an increase in the concentration of electrolytes contained in colloidal solutions occurs; an example is silt and mud in drying up lakes;
• d) when circulating colloidal solutions through capillaries in rocks; due to the high dielectric constant of water, the wetted walls of the capillaries are negatively charged with [OH] 1- ions, which causes the precipitation of positively charged particles from circulating colloidal solutions in the form of flakes or deposits; an example is the often observed "ferruginization" of limestones and other rocks, which is expressed in the coloring of the rock from the surface or along cracks with flaky iron hydroxides in a brown color;
• e) during the processes of metasomatism (replacement) of certain rocks that easily react with chemically active salt solutions to form colloidal solutions that immediately coagulate (for example, the formation of malachite due to vitriolic waters in limestones), etc.

In the biosphere, gels of organic origin are widespread. In some cases, the formation of gels is associated with the vital activity of bacteria. For example, it has been established that the so-called iron bacteria, while processing muddy lacustrine sediments, gradually deposit colloidal iron hydroxides (limonite).

Colloids in which dispersed particles have the ability to clothe a layer of water molecules from the surface are called hydrophilic, otherwise - hydrophobic... Hydrophilic colloids are much more difficult to coagulate than hydrophobic ones. In the case of coagulation of hydrophilic colloids, mucous, glue-like, gelatinous gel precipitates are usually formed.

From hydrophobic colloidal solutions, gels are most often formed in the form of powdery and flaky masses.

Gels, especially those that arise from hydrophilic colloids, easily lose water (dispersion medium) over time, i.e. are dehydrated. Water-rich hydrogels have an almost liquid consistency at the time of formation. As the dispersion medium evaporates when standing in air, they become more elastic and, finally, hard and brittle. However, water can be completely removed only by calcining.

When a dispersion medium is added, some gels are capable of not only swelling (like gelatin), but also transforming into sols again. This process of converting gels to sols is called peptization... Such gels are called reversible and are widely represented in the organic world. On the other hand, almost all inorganic colloidal formations belong to the group of irreversible, i.e., not transforming into sol, gels.

The phenomenon of adsorption in gels, of course, retains its significance. Moreover, in many cases, selective, i.e., selective, adsorption... For example, clay substances have the ability to adsorb mainly potassium and radioactive cations, and manganese dioxide gel - Ba, Li, K cations (but does not adsorb anions), etc.

Thus, as we have seen, colloids differ significantly in their properties from both true solutions and coarsely dispersed systems (with particles larger than 100 mμ). In colloids, the first place is not the vectorial properties of crystal lattices, not the forces of chemical affinity, but the enormous surface energy and the electrical forces associated with it. Nevertheless, there are gradual transitions between colloidal and true solutions, just as there are gradual transitions to coarse dispersions.

W. Ostwald gave the following scheme of dispersed systems:

Scheme of disperse systems by W. Ostwald

This scheme should be equally attributed to both liquid and solid systems.

At present, it has been precisely established that "the colloidal state is the general state of matter" (Weimarn), that is, any substance can be obtained in the form of a colloid. It is important to emphasize that colloids can form at a wide variety of temperatures and pressures and under a wide variety of conditions.

From a strictly theoretical point of view, colloids cannot be considered as independent special minerals, since they are essentially mechanical mixtures of various substances (dispersed phase and dispersion medium). However, by purely external signs, that is, macroscopically, they are completely indistinguishable from typical minerals. It is not possible to establish differences between them and minerals in the strict sense of the word also by means of microscopic research methods available to us. Therefore, in descriptive mineralogy courses, colloidal formations are conventionally considered along with typical minerals.

Earlier, solid colloids (gels) belonged to the number of amorphous minerals, since they are not observed in the form of clearly crystalline formations (if we do not take into account crystalline sols). However, X-ray studies of these substances often show that they are cryptocrystalline substances and therefore cannot belong to typical amorphous homogeneous bodies, despite the fact that they are very similar in appearance.

About recrystallization of gels... It has been established that hydrogels formed as a result of coagulation undergo aging over time, i.e., a gradual change in their composition and structure. This change is primarily expressed in the fact that the substance gradually loses water, that is, it undergoes dehydration (dehydration).

Such, for example, are widespread in nature silica hydrogels - opals. Water-rich silica hydrogels have the consistency of semi-liquid mass-jellies. With a gradual loss of water, they become more and more hard, glassy or semi-matte in a fracture. This is what naturally occurring opals look like, in most cases poor in water. These formations are characterized by the finest porosity, imperceptible to the eye and under a microscope, which can only be established by staining them with any organic substances. The water remaining in them can only be removed by heating.

In the case of a strong manifestation of dehydration in water-rich gels, porosity is noticeable to the eye, and sometimes wrinkling of the mass or the appearance of characteristic drying cracks in the form of nets is observed, as is often the case when mud dries in puddles.

The study of typical solid and semi-solid gels using X-rays by the Debye-Scherrer method shows that many of them do not give interference fringes, while aged colloidal formations show a clearly crystalline structure of the substance. In some cases, this can be seen when studying such gels under a microscope. Such are, for example, many stalactite formations of calcium carbonate. In place of opals (solid silica hydrogels), as a result of recrystallization, cryptocrystalline aggregates of anhydrous chalcedony or quartz are formed. Examples include flints and agates. Gels that have passed into crystalline-granular aggregates are called metacolloids(former colloids).

The essence of the recrystallization of gels is expressed in the combination of randomly oriented dispersed phases into larger units with a single crystal lattice. This phenomenon is known as collective crystallization... It expresses the natural tendency of substances to assume a state with the lowest specific surface area and, consequently, with the lowest surface energy.

In this case, often, especially in kidney-shaped gel masses, fine-fibrous aggregates appear with a radial arrangement of individuals, which is well observed at a fracture. On the peripheral parts of the crusts, spherical and kidney-shaped formations, for some minerals in these cases, crystalline faces are characteristic, which end in radially growing individuals.

The factors influencing the recrystallization of gels are varied. The most important are temperature and pressure, the increase of which accelerates the recrystallization process. Climatic conditions also play an undoubted role: in areas with a dry and hot climate, dehydration and recrystallization of hydrogels formed on the surface are much more pronounced in comparison with areas characterized by a temperate and humid climate. Of course, the time during which, under the most varied geological conditions, the gels are gradually transformed into explicit crystalline aggregates is of indisputable importance.