Mineral Review

What are the main physical properties of minerals?

Main properties of minerals: Shine and transparency (metallic, semi-metallic and non-metallic – diamond, glass, greasy, waxy, silky, pearlescent, etc.) is determined by the amount of light reflected from the surface of the mineral and depends on its refractive index. Based on transparency, minerals are divided into transparent, translucent, translucent in thin fragments, and opaque. Quantitative determination of light refraction and light reflection is possible only under a microscope. Some opaque minerals reflect light strongly and have a metallic luster. This is common in ore minerals such as galena (lead minerals), chalcopyrite and bornite (copper minerals), argentite and acanthite (silver minerals). Most minerals absorb or transmit a significant portion of the light falling on them and have a non-metallic luster. Some minerals have a luster that transitions from metallic to non-metallic, which is called semi-metallic. Minerals with a non-metallic luster are usually light-colored, some of them are transparent. Quartz, gypsum and light mica are often transparent. Other minerals (for example, milky white quartz) that transmit light, but through which objects cannot be clearly distinguished, are called translucent. Minerals containing metals differ from others in light transmission. If light passes through a mineral, at least in the thinnest edges of the grains, then it is, as a rule, non-metallic; if the light does not pass through, then it is ore (but not always). There are, however, exceptions: for example, light-colored sphalerite (zinc mineral) or cinnabar (mercury mineral) are often transparent or translucent. Minerals differ in the qualitative characteristics of their non-metallic luster. The clay has a dull, earthy sheen. Quartz on the edges of crystals or on fracture surfaces is glassy, ​​talc, divided into thin leaves along the cleavage planes, is pearlescent. Bright, sparkling, like a diamond, shine is called diamond. When light falls on a mineral with a non-metallic luster, it is partially reflected from the surface of the mineral and partially refracted at this boundary. Each substance is characterized by a certain refractive index. Because it can be measured with high precision, it is a very useful diagnostic feature of minerals. The nature of the luster depends on the refractive index, and both of them depend on the chemical composition and crystal structure of the mineral. In general, transparent minerals containing heavy metal atoms are characterized by high luster and a high refractive index. This group includes such common minerals as anglesite (lead sulfate), cassiterite (tin oxide) and titanite or sphene (calcium titanium silicate). Minerals composed of relatively light elements can also have high luster and a high refractive index if their atoms are tightly packed and held together by strong chemical bonds. A prime example is diamond, which is composed of only one light element, carbon. 2O3), transparent colored varieties of which: ruby ​​and sapphires, are precious stones. Although corundum is composed of light atoms of aluminum and oxygen, they are so tightly bound together that the mineral has a fairly strong luster and a relatively high refractive index. Some lusters (greasy, waxy, matte, silky, etc.) depend on the state of the surface of the mineral or on the structure of the mineral aggregate; resinous luster is characteristic of many amorphous substances (including minerals containing the radioactive elements uranium or thorium). Color The color of minerals is one of the most important physical properties of minerals, reflecting the nature of the interaction of electromagnetic radiation in the visible range with the electrons of atoms, molecules and ions that make up the crystals, as well as with the electronic system of the crystal as a whole. In mineralogy, color is one of the main diagnostic characteristics of natural compounds, which is of great importance in geological prospecting practice and for the identification of minerals. The color of precious and ornamental stones is one of their main quality (jewelry) characteristics. The color of the mineral is distinguished in crystals and specimens, in transparent sections (under a microscope), in polished polished sections (in reflected light). Studying the color of gemstones is one of the most important aspects of gemology in particular, since the effect of light on a gemstone determines its beauty. Of all the optical properties, color is perhaps the most important, especially for opaque stones, and the attractiveness of transparent stones depends on their “play”, color and brilliance. Color is an important diagnostic feature that allows you to distinguish between precious stones. However, sometimes two completely different minerals have the same color. Before the advent of scientific gemology, gemstones were recognized only by color – all red stones were considered carbuncles or rubies, and green ones were usually classified as emeralds, regardless of their origin and composition. The nature of the color of minerals has not been fully elucidated. It is known that in some cases the color is due to the chemical composition of the gem or the admixtures of certain chemical elements-chromophores (Cr, Fe, Mn, V, Ti, etc.). In the latter case, the mechanism for the appearance of a particular color is not always clear, since the same chemical element colors different gemstones in different colors. For example, an admixture of chromium makes a ruby ​​red and an emerald green. The color is also affected by disruption (defects) of the atomic structure of the mineral under the influence of radioactive radiation. White color is formed by mixing all the colors of the rainbow that make up the spectrum. When light hits a clear gemstone, it is partly reflected off the surface, partly absorbed, and partly transmitted through. The color perceived by the eye depends on the extent to which and which parts of the electromagnetic optical spectrum are absorbed or transmitted. The stone will appear black if the light is completely absorbed; a colorless stone transmits all parts of the spectrum. A colored stone absorbs some part of the visible spectrum, thereby acquiring a color complementary to that absorbed (for example, an emerald absorbs red rays and itself becomes green). Line color color of mineral in powder on a white background. To determine the color of the feature, an unglazed porcelain surface (bisque) is used. Compared to the color of minerals, the color of the trait is more constant, as a result of which it has important diagnostic value. Minerals with a metallic luster, as a rule, have a black streak with different shades, minerals with a glassy luster are white, less often slightly colored. The color of a mineral often does not match the color of its trait. Density The mass of atoms of chemical elements varies from hydrogen (the lightest) to uranium (the heaviest). All other things being equal, the mass of a substance consisting of heavy atoms is greater than that of a substance consisting of light atoms. For example, two carbonates – aragonite and cerussite – have a similar internal structure, but aragonite contains light calcium atoms, and cerussite contains heavy lead atoms. As a result, the mass of cerussite exceeds the mass of aragonite of the same volume. The mass per unit volume of a mineral also depends on the atomic packing density. Calcite, like aragonite, is calcium carbonate, but in calcite the atoms are less densely packed, so it has less mass per unit volume than aragonite. The relative mass, or density, depends on the chemical composition and internal structure. Density is the ratio of the mass of a substance to the mass of the same volume of water at 4° C. So, if the mass of a mineral is 4 g, and the mass of the same volume of water is 1 g, then the density of the mineral is 4. In mineralogy, it is customary to express density in g/ cm 3. Density is an important diagnostic feature of minerals and is not difficult to measure. First, the sample is weighed in air and then in water. Since a sample immersed in water is subject to an upward buoyant force, its weight there is less than in air. The weight loss is equal to the weight of water displaced. Thus, density is determined by the ratio of the mass of a sample in air to its weight loss in water. Hardness determined by the mineral’s resistance to scratching. The harder the gemstone, the higher the quality of its polishing and the more beautiful and durable it is. Hard rocks have stronger electronic bonds between atoms. Hardness is a very consistent and reliable indicator, widely used for diagnosing minerals (but it is not always applicable to gemstones, since their edges can be damaged by scratching). Typically, the hardness of gemstones and other minerals is measured using the Mohs scale. The hardness of diamond, the hardest of all substances, is rated at 10. Each mineral on this scale scratches the previous mineral and is in turn scratched by the next one. Quartz, with a hardness of 7 on the Mohs scale, serves as the boundary between hard and soft gemstones. Strength Unlike hardness, a mineral’s toughness is determined by its resistance to splitting. The combination of hardness and toughness determines its strength, which depends on the adhesive forces, i.e. mutual electrical attraction of ions in the crystal structure of a gemstone. Some relatively hard stones (such as zircon) are difficult to scratch, but are very brittle and easily crack or crumble. Others, such as jade, which is no harder than quartz, are quite strong and very difficult to split or cut due to their high viscosity. The durability of a stone is determined by its strength and resistance to chemical attack. Cleavage The ability of a mineral to split or split along one or more directions corresponding to the weakest interatomic bonds in the structure is called cleavage. Cleavage planes are usually parallel to possible crystal faces and are often recognized by stepped cleavages on the surface or by parallel cracks within the crystal. Identifying this property facilitates diagnosis and must be taken into account when cutting a gemstone. There are several degrees of cleavage perfection, depending on the nature of its manifestation in the crystal. For example, diamond and fluorite have perfect octahedral cleavage. This means that the splitting occurs parallel to the faces of the octahedron with the formation of octahedral-shaped cleavages bounded by smooth, even planes. Perfect cleavage facilitates the cutting of diamonds and some other precious stones, which are easily split along the cleavage planes. In other cases (topaz, kunzite) it makes processing very difficult. Separateness (or false cleavage) is the ability of some crystals to split in certain directions, often coinciding with the planes of fusion of twins. The individual surfaces in a crystal are less perfect, and the intervals between them are usually greater. The surface of a split that does not occur along cleavage or separately (i.e., not in accordance with the crystal structure of the stone) is called a fracture. The term is used to describe the cleavage surface of all amorphous gemstones, although crystalline minerals may also be characterized by a specific fracture in addition to indicating the presence of cleavage. Depending on the appearance of the fracture surface, several types are distinguished: conchoidal, stepped, uneven, splintered, hooked, etc. Electrification and polarity Some gemstones exhibit electrical polarity. It is detected by their ability to attract or repel light objects (such as pieces of paper) after these stones are heated by friction or sunlight. Already in 600 BC. It has been observed that amber, if rubbed vigorously, begins to attract fine fibers of wool. Topaz and some other gemstones also exhibit this property during the polishing process. Tourmaline, when compressed or heated, acquires positive or negative charges, which appear simultaneously at opposite ends of its crystals. This phenomenon is called the direct piezoelectric effect. The inverse piezoelectric effect is the change in the volume of a crystal under the influence of an electric field. Crystals of some minerals, such as tourmaline and quartz, are so sensitive to changes in electrical voltage that they begin to vibrate at a high and constant frequency in an electric field. This is the basis for their use in radio electronics and quartz watches. Radioactivity Many minerals containing elements such as niobium, tantalum, zirconium, rare earths, uranium, and thorium often have quite significant radioactivity, easily detectable even by household radiometers, which can serve as an important diagnostic sign. To test for radioactivity, the background value is first measured and recorded, then the mineral is brought, possibly closer to the detector of the device. An increase in readings of more than 10-15% can serve as an indicator of the radioactivity of the mineral. glow Many minerals that do not glow on their own begin to glow under certain special conditions (when heated, exposed to X-rays, ultraviolet and cathode rays, when broken, scratched, etc.).
There are phosphorescence, luminescence, thermoluminescence and triboluminescence of minerals.
Phosphorescence is the ability of a mineral to glow after exposure to one or another ray (willite).
Luminescence is the ability to glow at the moment of irradiation (scheelite when irradiated with ultraviolet and cathode rays, calcite, etc.).
Thermoluminescence – glow when heated (fluorite, apatite).
Triboluminescence – glow at the moment of scratching with a needle or splitting (mica, corundum). Light refraction A beam entering a transparent mineral is refracted, since the speed of its propagation becomes less than in air, and the greater the optical density of the stone, the more it decreases. The refractive index of a mineral (the degree to which a light beam deviates from the perpendicular direction) is measured using a refractometer and is expressed mathematically as the ratio of the speed of light propagation in the mineral and in the void. Diamond has a very high refractive index. The light coming out of the stone is also refracted, because it leaves the optically denser medium and enters the air again. If stones that have a high refractive index are cut correctly, the light rays are bent in such a way that they are eventually refracted and re-emerged through the top rather than being lost through the bottom. This enhances the shine of the cut stone. The value of the refractive index is a specific feature of each mineral (including precious stones) and contributes to its reliable diagnosis. The refraction of minerals such as diamond, titanite, zircon, andradite and demantoid garnets cannot be measured with a conventional diffractometer – their luster is too strong and the refractive index value is outside its scale. When white light enters a gemstone, not only does it refract, but also decomposes into different colors of the spectrum, since each of the colored rays that make up white light (red, orange, blue, green, etc.) is refracted differently and at the exit from the crystal, a white beam “splits” into all the colors of the rainbow. This phenomenon is called the “play” of the stone, “fire” or dispersion. In diamond, the dispersion value is approximately the same as in demantoid or titanite, but its “fire” seems much brighter, since the “game” of colorless stones is more noticeable. One turn of the diamond causes a whole sheaf of rainbow sparks. All transparent minerals (with the exception of cubic and amorphous minerals) divide light into two differently deflected beams. This phenomenon is called birefringence, or birefringence. At the same time, if you look through the faceted stone, you can see that the ribs of the rear facets seem to bifurcate. In titanite, birefringence is so pronounced that it can be observed with the naked eye, in olivine – chrysolite and zircon – with a magnifying glass. A microscope is required to observe this effect in other gemstones. Some chemical impurity elements present in the composition of a jewelry stone absorb some of the light rays and thus darken certain parts of the light spectrum. The absorbed portion of the light can be determined by means of a spectroscope, in which the portions of the spectrum corresponding to the absorbed rays are represented by dark vertical stripes or lines. Each chemical element has a characteristic arrangement and combination of bands representing its absorption spectrum. Dichroism The effect of bicolor (dichroism) is observed in a number of jewelry stones, characterized by the presence of birefringence when their orientation changes relative to the line of sight. The color change becomes noticeable if you rotate the crystal or look at it either through the top or through the side faces. This property enhances the charm of the stone and its attractiveness, for example, in tourmaline, dichroism is so strong that it can be observed without the help of a dichroscope (a device that enhances the effect of dichroism; both colors can be seen side by side, within a single field of view). For some gemstones, dichroism testing is one of the most obvious diagnostic methods. For example, a ruby ​​immediately stands out from other red stones by the presence of two clearly defined shades of red. Pleochroism In anisotropic crystals, light oscillating in different crystallographic directions can be absorbed differently. One of the possible consequences of this phenomenon, called pleochroism, is a change in the color of the crystal when the direction of vibration changes. In other crystals, light oscillating in one crystallographic direction can propagate with almost no loss of intensity, and at right angles to it can be almost completely absorbed. The action of polarizing filters such as Polaroid is based on differences in the absorption of light by thin oriented crystals. Asterism The star-like effect found in only a few gemstones is called asterism. It is caused by the reflection (diffraction) of light from inclusions in the stone, oriented along certain crystallographic directions. The best examples are star sapphire and star ruby. In minerals with a fibrous structure, such as cat’s eye, there is a stripe of light that changes its position when the stone is rotated (iridescence). The play of light in the opal or the brilliant peacock colors of labradorite are explained by the interference of light, i.e. mixing of light rays when they are reflected from layers of regularly arranged silica balls (opal) or from the thinnest lamellar crystalline growths (labradorite, moonstone). Strictly speaking, properties are described not only physical, but also chemical. There are more than 3,5 thousand mineral species in the world. In order to distinguish them from each other, descriptions are used according to their properties. It is especially necessary to highlight diagnostic properties: a set for each mineral, allowing it to be uniquely identified.

I. Optical properties.

1. Color.

Describes the color of the mineral in the piece. The color description must consist of more than one word and contain a description of both the range (green, blue.) and shades (dark, light, bluish.). Examples: “lead gray; bluish green, swamp green; bright orange» In addition, it is used:

Powder color (trash color)

A strip of unglazed porcelain (“biscuit”) is used. You need to follow it with a sample. Describe the color of the resulting feature. If the sample is harder than the biscuit, a scratch will remain, in which case the description states: “leaves no features“/”no line». The color of the dash may be white and not noticeable on white porcelain.

2. Shine.

Shine:

Gloss is a characteristic of the reflection of light by a sample. A descriptive characteristic is used: a comparison of the brilliance of the sample with the brilliance of well-known objects. It is impossible to increase gloss artificially, but to reduce it is easy (scratch, stain, etc.). Therefore, when describing a mineral, it is necessary to indicate its brightest observed brilliance. Shine cannot be absent: a perfectly black body is a physical abstraction.

3. Transparency.

The mineral can be:

Transparency is a characteristic of the passage of light through a sample. A descriptive characteristic is used: a four-stage gradation. It is very difficult to artificially increase transparency, but to decrease it is easy (scratch, stain, etc.). Therefore, when describing a mineral, it is necessary to indicate its best transparency. All other things being equal, finer-grained aggregates appear less transparent.

II. Mechanical properties

4.Hardness

Mohs scale

Hardness Name Formula Analogs and replacements 1TalcMg3[And4O10] (OH)2(soft pencil, grade M, 2M) 2GypsumCaSO4· 2H2O(about 2 – human nail) 5CalciteCaCO3(copper – a piece of wire or a coin) 4FluoriteCaF2 5ApatiteCa5[P.O.4]3(OH,Cl,F)(5 – glass, 5,5 – steel: knife, wire, nail.) 6 Feldspars K[AlSi3O8] 7QuartzSiO2(File [Bondar, 1999]) 8TopazAl2SiO4(OH, F)2 9CorundumAl2O3(emery is one of the varieties of corundum) 10DiamondC Hardness is the resistance offered by a crystal to a scratching, drilling, grinding or pressing object. [GeoWikipedia] The material being tested either scratches the standard and its hardness is higher, or it scratches the standard and its hardness is lower than the standard. The Mohs scale is used to determine the relative hardness of minerals. A relative characteristic is used: a ten-step gradation, from the softest mineral to the hardest (in the known part of the Universe, at least). Most minerals are widely available and there is no need to carry the entire scale with you. For most standards, analogs have been selected (shown in parentheses). If a sample has a hardness between two standards, a fractional hardness is compared to it. (for example, a sample is scratched by quartz, but feldspar itself is scratched: its hardness will be 6,5) Materials partially from GeoWikipedia. More details in GeoWiki

Kink

  • Smooth or stepped Characteristic of minerals with cleavage (see below).
  • Conchoidal Similar in shape to the valve of a shell. characteristic of amorphous and similar aggregates (opal, glass, quartz).
  • Splinter is characteristic of minerals with a needle-like structure [Bondar, 1999]
  • Earthy Characteristic of clay minerals
  • Grainy looks like sandpaper or a lump of sugar.
  • Uneven

5. Cleavage

Cleavage is the ability of mineral crystals to split along certain directions.

A double characteristic is used: a five-stage gradation according to the degree of perfection and an indication of the number of directions.

By number of directions:

  • In one direction (mica, etc.)
  • In two directions (and then the angle between the cleavage planes should be specified)
  • In three, four or six directions (and then you should indicate the figure – a simple shape that is limited by these directions. Example: halite: perfect cleavage, in 3 directions along the faces of the cube)

By degree of perfection

  • Very perfect, perfectly even chips; often you can split the crystal simply with your hands (for example, mica).
  • Perfect, regular, fairly even chips are visible; the sample has to be split with a hammer.
  • The middle one is poorly distinguishable, it requires skill, or better yet, a thin section and a microscope 🙂
  • Imperfect is visible only under a microscope
  • A very imperfect analogue of the expression “no cleavage” After all, if you chop the same crystal for a long time and persistently, the same directions of chipping will appear, as the theory of probability says. 🙂

III. Special properties

Unlike those listed above, they are not inherent in all minerals, and by default, if they are not present, they are not indicated in the description.

If they exist, they are always diagnostic for a given mineral.

Использованная литература:

  • GeoWikipedia. http://wiki.web.ru
  • Bondar V.P. Geology. Laboratory workshop. M.: Forum: Infra-M, 2002, 190 p.
  • Practical guide to general geology. Ed. N.V. Koronovsky. M.: “Academy”, 2004, 160 p.

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