What are the physical properties of minerals?

The purpose of this section is to theoretically substantiate the most significant and valuable, in our opinion, energy approach to explaining and predicting the physical properties of crystalline substances, including minerals and various artificial inorganic compounds. Until recently, the energy of atomization of crystals (Е_α, kJ/mol) was considered the main universal energy characteristic, which naturally replaced the concept of crystal lattice energy (Urusov, 1975). This is explained by the fact that the concept of atomization energy is applicable to compounds with any type of chemical bonds, including metallic, covalent, etc. In addition, many important physicochemical properties of crystals are better correlated with atomization energies, which has been convincingly demonstrated in numerous studies ( Lebedev, 1957; Ormont, 1973; Mamyrov, 1975, 1989; The procedure for calculating the atomization energy of a crystal consists of summing the standard enthalpy of its formation and the heats of formation of the constituent atoms (Urusov, 1991; Mamyrov, 1995; Zuev, 1975)[1989]. As an example, let’s find the atomization energy of quartz SiO₂, the enthalpy of formation of which is 911 kJ/mol, and the heats of formation of Si and O atoms are equal to 452 kJ/mol and 249,2 kJ/mol, respectively (Properties of Inorganic Compounds, 1983). Then Е_α(SiO₂) = 911 + 452 + 249,2×2 = 1861,4 kJ/mol. The heats of formation of atoms or, in other words, the enthalpies of atomization of simple substances for the standard states of all elements are known as experimental reference values ​​(Properties of Inorganic Compounds, 1983). Data on the enthalpies of formation of most simple and many complex minerals and other crystalline compounds are also available in reference literature (Bulakh, 1978; Properties of inorganic compounds, 1983;). The enthalpies of complex compounds that are not listed in reference books can be calculated by the method developed by the author (Zuyev, 1987; Zuyev, 1988;), which is used in practice along with other alternative approaches to the theoretical assessment of the enthalpies of complex minerals (Chermak, Rimstidt, 1989; Vieillard, 1994; Reznitsky , 1998). Typically, the atomization energy is referred to a unit amount of a substance (mole) and is expressed in kcal/mol or kJ/mol. However, to correctly compare the atomization energy of various simple and complex minerals (crystals), it is necessary to use specific atomization energies per unit mass (1 g) or per unit volume (1 cm 3 ) of the substance. According to (Zuev, 1995), the transition from Е_α to the specific mass energy of atomization E_m and specific volumetric energy of atomization E_v carried out using the following formulas: In these formulas М – formula (molar) mass of the compound, g/mol; V – its molar volume, cm3/mol, determined by the formula V = M/ρ, where ρ is the density of the substance, g/cm3. From here follow the indicated dimensions E_m и E_v and an important relationship between them: For quartz SiO₂ E_α = 1861,4 kJ/mol, М = 60,1 g/mol and r = 2,65 g/cm3. According to formulas (2.81) and (2.82) E_m = 31,0 kJ/g and E_v = 82,2 kJ/cm3. Similar calculations for such a complex compound as the mineral beryl Be₃Al₂Si₆O₁₈ give E_α = 17837,2 kJ/mol, E_m = 33,2 kJ/g and E_v = 87,6 kJ/cm3. Let us pay attention to the fact that with a colossal difference (by an order of magnitude!) in the parameters E_α beryl and quartz, they turn out to be quite close, quite comparable in parameters E_m и E_v. Options E_α were first widely used by V.S. Urusov to establish correlations with various properties of minerals – thermal, strength, electrical and others. Due to the incomparability of the values E_α For simple and complex minerals, it is in principle impossible to obtain the same required dependencies for them within the framework of the use of atomization energies. Options E_α, as is obvious, are suitable for constructing corresponding correlations for stoichiometrically similar substances. Options E_m were first used by E.M. Mamyrov to identify correlations with various physicochemical properties of minerals and rocks, as well as to identify patterns of differentiation of lithospheric matter (Mamyrov, 1989; 1991). It should be emphasized that interesting dependencies were found and, in particular, a fundamental pattern of growth of parameters was established E_m substances during the evolution (change) of minerals and their parageneses during the transition from the internal zones of the lithosphere to the surface. Paying tribute to the work of E.M. Mamyrov, attention is drawn to a certain limitation of his approach, according to which the measure of the specific energy of atomization of substances is the parameters E_m. In fact, the use of the latter assumes that the energy reserve of chemical bonds is determined by a unit mass of a substance regardless of the volume it occupies, which is a one-sided approach. Obviously, a more correct approach is to link the specific energy of atomization of a substance both with mass and with its distribution in space, i.e., with the volume factor. Both of these factors – mass and volume – can be simultaneously taken into account in the specific energy of atomization of a substance when used in calculations of density (ρ, g/cm 3 ). In other words, when operating with the specific energy of atomization of substances, it is preferable to use the parameters E_v, calculated using formulas (2.82) or (2.83). Parameter E_v according to the method of its calculation and dimension (energy/volume), it represents the specific volumetric concentration of the energy of the chemical bonds of a substance and can be characterized as energy density or abbreviated energy density. The first experiments in applying the concept of energy density are reflected in monographs (Zuev, 1995; Zuev, Denisov, Mochalov et al., 2000). As calculations have shown, minerals and artificial compounds are very clearly differentiated by energy density, varying within very wide limits – from 224 kJ/cm 3 for the supposed substance of the central part of the Earth’s core and 209 kJ/cm 3 for diamond – to very low (≈ 1 kJ/cm cm 3 for crystalline cesium) and zero (noble gases He, Ne, Ar, etc.) values ​​(Table 2.4). Table 2.4 Classification of minerals and inorganic crystals by energy density

Energy density class	Range of values Ev, kJ/cm 3	Examples of minerals and other compounds
Super energy dense (super energy dense)	150-230	“Nuclear” iron a-Fe(VIII), diamond, C3N4, BN (borazon)
Very highly energy dense	90-150	“Nuclear” Fe oxide2O, bromellite, graphite, stishovite, chrysoberyl, corundum, phenacite, disthene, zircon, spinel, rutile, topaz
High energy density	50-90	Quartz, calcite, the vast majority of complex oxides, silicates, carbonates, phosphates, sulfates and other rock-forming minerals
Medium energy dense	30-50	Some oxides and sulfides, halides and native metals
Low energy density	10-30	Many ore minerals – sulfides, arsenides and their analogues, native metals and non-metals
Very low energy density	1-10	Artificial alkali and alkaline earth metals, as well as some of their halide compounds

The main ore and non-ore (rock-forming) minerals differ quite sharply in energy density. The first of them – sulfides and their analogues, native elements and metals – are low and medium energy dense, while the vast majority of the second – oxides, silicates, carbonates, etc. – are high energy dense. The average energy density parameters of the main ore minerals (≈ 33 kJ/cm 3 ) and the host vein, rock-forming, so-called gangue minerals (≈ 82 kJ/cm 3 ) differ by more than two times (Table 2.5), and the boundary between the two passes approximately through water (E_v = 54 kJ/cm 3 ). This fact should undoubtedly be reflected in further theoretical developments on the separation of ore and non-metallic minerals in enrichment processes based on the contrast of their properties, reflected in the energy density parameters. Table 2.5 Energy density of the most common ore and non-ore minerals (Zuev, 1995) Minerals are determined by physical properties, which are determined by the material composition and structure of the crystal lattice of the mineral. These are the color of the mineral and its powder, luster, transparency, the nature of fracture and cleavage, 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 can be used (carbonates boil).

Mineral color

The question of the nature of the color color of minerals is very complex. The nature of the colors of some minerals has not yet been determined. At best, 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 included in the mineral, finely scattered 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 stripes of agate Some transparent minerals change color due to the reflection of light falling on them from internal surfaces, cracks or inclusions. These are phenomena of rainbow coloring of the minerals chalcopyrite, pyrite and iridescence – blue, blue tints of labradorite. 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 a diagnostic sign. For example, aqueous copper salts are green or blue. 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 determined by the chemical composition of the mineral or the presence of various impurities, which contain chemical elements-chromophores (chrome, manganese, vanadium, titanium, etc.). The mechanism for 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 a ruby ​​red and an 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 line 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, while in brass-yellow pyrite the color is greenish-black. The devil is given by soft and medium-hard minerals, while hard ones only scratch the plate and leave grooves on it. Color of mineral lines on a porcelain plate

Transparency

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

Brilliance

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

Cleavage

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

The number of cleavage planes in different minerals varies, up to six, and the degree of perfection of different planes may not be the same. The following types of cleavage are distinguished:

• very perfect, when a mineral, without much effort, splits into individual leaves or plates with smooth shiny surfaces – cleavage planes (gypsum).
• perfect, detected by a light blow to the mineral, which crumbles into pieces limited only by smooth shiny planes. Uneven surfaces not along the cleavage plane are very rarely obtained (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, fractures are formed both along the cleavage planes and on uneven surfaces (feldspars – orthoclase, microcline, labradorite)
• imperfect. Cleavage planes in minerals are difficult to detect (apatite, olivine).
• very imperfect. There are no cleavage planes in the mineral (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 edges of the crystal from the cleavage planes that appear when the mineral is fractured. The planes can be parallel to the edges and have a more “fresh” appearance and a stronger shine.

Kink

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

1. Smooth break, if the mineral splits along cleavage planes, as, for example, in mica, gypsum, and calcite crystals.
2. Step fracture obtained when there are intersecting cleavage planes in the mineral; it can be observed in feldspars and calcite.
3. Uneven fracture characterized by the absence of shiny areas of cleavage splitting, as, for example, in quartz.
4. grainy fracture observed in minerals with a granular-crystalline structure (magnetite, chromite).
5. Earthy fracture characteristic of soft and highly porous minerals (limonite, bauxite).
6. Crustaceous – with convex and concave areas like shells (apatite, opal).
7. Splinter (needle-shaped) – 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 type of fracture is characteristic of malleable metals.

Smooth fracture on mica. Rough fracture on rose quartz. Stepped fracture on halite. © Rob Lavinsky Granular fracture of chromite. © Piotr Sosonowski
Earthy fracture of limonite Conchoidal fracture on flint Splinter fracture on actinolite. © Rob Lavinsky Hooked fracture on copper

Hardness

Mineral hardness – 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). Hardness is determined by scratching the surface of the mineral with a fingernail, knife, glass, or minerals of known hardness from the Mohs scale, which includes 10 minerals with gradually increasing hardness (in relative units).

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

Mohs scale

Mineral	Hardness
Talc	1
Гипс	2
Calcite	3
Fluorite	4
Apatite	5
Feldspar	6
Quartz	7
Topaz	8
Corundum	9
Diamond	10

The main mass of minerals has a hardness of 2 to 6. Harder minerals are anhydrous oxides and some silicates. When determining a mineral in a rock, you need to make sure that it is the mineral that is being tested, and not the rock.

Specific weight

The specific gravity varies from 0,9 to 23 g/cm 3 . For most minerals it is 2–3,4 g/cm3; ore minerals and native metals have the highest specific gravity of 5,5–23 g/cm3. The exact specific gravity is determined in the laboratory, 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). This is barite (heavy spar) – with a specific gravity of 4,3 – 4,7, sulfur ores of lead and copper – specific gravity of 4,1 – 7,6 g / cm 3, native elements – gold, platinum, copper, iron, etc. .d. with a specific gravity from 7 to 23 g/cm 3 (osmic iridium – 22,7 g/cm 3, platinum iridium – 23 g/cm 3).

Magneticity

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

Malleability and fragility

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

Electrical Conductivity

Electrical conductivity of minerals is the ability of minerals to conduct electric current under the influence of an electric field. Otherwise, minerals are classified as dielectrics, i.e. non-conductive.

Flammability and odor

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

Taste

Taste sensations are caused only by minerals that are highly soluble in water (halite – salty taste, sylvite – bitterly salty).

Roughness and fat content

Fatty, slightly smearing are talc, kaolinite, rough – bauxite, chalk.

absorbability

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

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

Radioactivity

Radioactivity can serve as an important diagnostic sign. Some minerals containing radioactive chemical elements (such as uranium, thorium, tantalum, zirconium, thorium) often have significant radioactivity, which is easy to detect with household radiometers. To test for radioactivity, the background amount of radioactivity is first measured and recorded, then a mineral is placed near the detector of the device. An increase in readings by more than 15% indicates the radioactivity of the mineral. Radioactive minerals are: abernathyite, bannerite, gadolinite, monazite, orthite, zircon, etc.

glow

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

1. Phosphorescence is the ability of a mineral to glow for minutes and hours after exposure to certain rays (willite glows after irradiation with short ultraviolet rays).
2. Luminescence is the ability to glow when irradiated with certain rays (scheelite glows blue when irradiated with ultraviolet and rays).
3. Thermoluminescence – glow when heated (fluorite glows violet-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, is characteristic of 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), there is a thin strip of light that can change its direction when the stone is turned (iridescence). The playful light on the surface of opal or the shining peacock colors of labradorite are explained by the interference of light – the mixing of light rays when they are reflected from layers of packed silica beads (in opal) or from the thinnest lamellar crystal growths (labradorite, moonstone).

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