Rare and valuable minerals

How do minerals differ?

The most important diagnostic features of minerals include morphological features that characterize the shape of mineral deposits; optical properties: transparency, mineral color, streak color, luster; mechanical properties: cleavage, fracture, hardness, brittleness, elasticity, ductility, flexibility; other physical properties: specific gravity (density), taste, smell, magnetism, etc.

1. Morphological features

Most often, minerals are found in nature in the form of irregularly shaped grains. Well-formed crystals are rarer; their shape is usually a characteristic diagnostic feature. Variety of existing crystal shapes can be divided into three types. Isometric – having similar sizes in all directions: cubes (galena, pyrite), tetrahedra (sphalerite), octahedra (magnetite, pyrochlore), bipyramids (zircon, cassiterite), rhombic dodecahedrons (garnet), rhombohedrons (calcite), etc., as well as various combinations of these simple forms. Extended in one direction – prismatic, columnar, columnar, needle-shaped, fibrous crystals (tourmaline, beryl, pyroxene, amphibole, rutile, etc.). Elongated in two directions (flattened) – tabular, lamellar, leafy, scaly crystals (mica, chlorites, molybdenite, graphite, etc.). As a result of the process of metasomatic replacement or dissolution with subsequent filling of voids, crystalline forms belonging to one mineral turn out to be represented by another mineral; such formations are called pseudomorphoses. Hatching. In addition to the shape of the crystal, a characteristic property of the mineral that helps in its diagnosis is the shading on the faces: transverse parallel (quartz), longitudinal parallel (tourmaline, epidote) or intersecting (magnetite). In nature, it is not single crystals of a mineral that are more widespread, but their various accretion, or aggregates. Many minerals are characterized by oriented regular twin intergrowths of two or more crystals in a certain way. The most widespread specific forms of mineral aggregates, intergrowths and secretions, which have received special names, are given below. Granular aggregates. Depending on the shape of the constituent grains, granular aggregates proper (consisting of isometric grains), as well as lamellar, leafy, scaly, fibrous, needle-shaped, columnar and other aggregates are distinguished. According to the size of the grains, there are coarse-grained aggregates – more than 5 mm in diameter; medium-grained – from 1 to 5 mm and fine-grained – with grains less than 1 mm. In particular, most igneous and metamorphic rocks, as well as many sedimentary rocks, some types of sulfide ores, etc., are composed of granular aggregates. Druze – intergrowths of regular, well-formed crystals of minerals on the walls of voids of various shapes (cracks, caverns, “cellars”, “gnarly holes”, “caves”, etc.). In morphological terms, they are very diverse: “brushes” of crystals, “crystalline crusts” (small closely intergrown crystals, completely covering the walls of narrow cracks), “comb” intergrowths, etc. Crystal druses are typical of pegmatites, some types of hydrothermal veins and alpine-type veins . Secretions – execution of voids of an isometric, often round shape, distinguished by a concentric-zonal structure. The outer zones of secretions are often made of amorphous or cryptocrystalline minerals, and in their inner part there is a cavity, on the walls of which druses of crystals or sinter aggregates of minerals grow. Small secretions found in erupted rocks and tuffs are called tonsils, large, especially characteristic of pegmatites and alpine veins, – geodes. Concretions – spherical or irregularly shaped nodules and nodules formed in loose sedimentary rocks (silts, clays, sands, etc.). Unlike secretions, nodules grow from some center (clastic grain, organic residue, etc.), around which a clot of colloidal substance is formed, subsequently crystallized. Concretions are characteristic of phosphorites, siderites, marcasites and other types of ores of sedimentary origin. Oolites like nodules, they have a spherical shape, but their size is much smaller: from tenths of a millimeter to several millimeters. They are formed by the layering of colloidal material on grains of sand and organic debris that are suspended in mobile aqueous media. Oolites are very characteristic of some limestones, sedimentary iron and manganese ores, and bauxites. Sinter forms mineral deposits form on the walls of various voids and cavities during the slow drainage of solutions. These include calcareous and ice stalactites and stalagmites of caves, similar in shape to ordinary ice icicles, kidney-shaped, cluster-shaped mineral deposits in zones of oxidation and weathering of ore deposits, etc. The sizes and shapes of sinter formations can be very diverse: from fractions of a millimeter to huge pillars (in large caves). Sintered forms of sediments are characteristic of many supergene and low-temperature hydrothermal minerals: calcite, aragonite, malachite, hematite, hydroxides of iron, manganese, opal, gypsum, some sulfides, smithsonite, etc. Earthy masses – loose, soft, mealy aggregates of an amorphous or cryptocrystalline structure, sooty (black) or ocher (yellow, brown and other bright colors). Most often they are formed during chemical weathering of rocks and in the oxidation zone of ores (for example, manganese ores). Plaques and lubricants – thin films of various secondary minerals covering the surface of crystals or rocks. Such are films of limonite on rock crystals, smears of copper green on cracks in rocks containing sulfide deposits with copper minerals, etc. Fading – periodically appearing (in dry weather) and disappearing (in rainy periods) loose crusts, films, deposits, often fluffy or mossy, on the surface of dry soils, ores and rocks and along cracks in them. These formations are most often composed of easily soluble aqueous chlorides, sulfates of various metals, or other water-soluble salts.

2. Physical properties

Optical properties. Transparency – the property of a substance to transmit light. Depending on the degree of transparency, all minerals are divided into the following groups: transparent – rock crystal, Iceland spar, topaz, etc.; translucent – sphalerite, cinnabar, etc.; opaque – pyrite, magnetite, graphite, etc. Many minerals that appear opaque in large crystals are translucent in thin fragments or grain edges. Mineral color – the most important diagnostic sign. In many cases, it is due to the internal properties of the mineral (idiochromatic colors) and is associated with the inclusion of chromophoric elements (Fe, Cr, Mn, Ni, Co, etc.) in its composition. For example, the presence of chromium determines the green color of uvarovite and emerald, the presence of manganese determines the pink or lilac color of lepidolite, tourmaline or sparrowite. The nature of the coloring of other minerals (smoky quartz, amethyst, morion, etc.) lies in the violation of the homogeneity of the structure of their crystal lattices, in the occurrence of various defects in them. In some cases, the color of a mineral can be caused by the presence of the finest scattered mechanical impurities (allochromatic colors) – jasper, agate, aventurine, etc. To indicate color in mineralogy, a common method is comparison with the color of well-known objects or substances, which is reflected in the names of colors: bloody- red, azure blue, lemon yellow, apple green, chocolate brown, etc. The names of the colors of the following minerals can be considered standards: violet – amethyst, blue – azurite, green – malachite, yellow – orpiment, red – cinnabar, brown – limonite, lead-gray – molybdenite, iron-black – magnetite, tin-white – arsenopyrite, brass-yellow – chalcopyrite, metallic-gold – gold. Line color – the color of a fine mineral powder. A mineral trait can be obtained by passing the test mineral across the matte unglazed surface of a porcelain plate (biscuit) or a fragment of the same surface of a porcelain chemical vessel. This sign is more permanent compared to coloring. In some cases, the color of the line coincides with the color of the mineral itself, but sometimes a sharp difference is observed: for example, steel-gray hematite leaves a cherry-red line, brass-yellow pyrite leaves a black line, etc. Brilliance depends on the refractive index of the mineral, i.e. a quantity that characterizes the difference in the speed of light when it passes from air to a crystalline medium. It has been practically established that minerals with a refractive index of 1,3–1,9 have glass luster (quartz, fluorite, calcite, corundum, garnet, etc.), with an index of 1,9–2,6 – diamond shine (zircon, cassiterite, sphalerite, diamond, rutile, etc.). Semi-metallic luster corresponds to minerals with a refractive index of 2,6–3,0 (cuprite, cinnabar, hematite) and metal – above 3,0 (molybdenite, stibnite, pyrite, galena, arsenopyrite, etc.). The brilliance of a mineral also depends on the nature of the surface. Thus, in minerals with a parallel-fibrous structure, silky luster (asbestos), translucent “layered” and lamellar minerals often have pearl luster (calcite, albite), opaque or translucent minerals, amorphous or characterized by a disturbed crystal lattice structure (metamictic minerals) differ resinous shine (pyrochlore). Mechanical properties. Cleavage – the property of crystals to split in certain crystallographic directions, due to the structure of their crystal lattices. Thus, calcite crystals, regardless of their external shape, always split along their cleavage into rhombohedrons, and cubic fluorite crystals into octahedra. The degree of perfection of cleavage varies according to the following accepted scale: Cleavage very perfect – the crystal easily splits into thin sheets (mica, chlorite, molybdenite, etc.). Cleavage perfect – when struck with a hammer, cleavage marks are obtained; It is difficult to obtain a fracture in other directions (calcite, galena, fluorite). Cleavage average – a fracture can be obtained in all directions, but on mineral fragments, along with an uneven fracture, smooth shiny cleavage planes (pyroxenes, scapolite) are clearly observed. Cleavage imperfect or no. The grains of such minerals are confined to irregular surfaces, except at the edges of their crystals. Often differently oriented cleavage planes in the same mineral differ in degree of perfection. Thus, gypsum has three directions of cleavage: in one direction the cleavage is very perfect, in the other – average and in the third – imperfect. Cracks separately, unlike cleavage, are rougher and not completely flat; most often oriented transversely to the mineral elongation. Kink. In minerals with imperfect cleavage, fracture plays a significant role in diagnosis – conchoidal (quartz, pyrochlore), splintery (for native metals), small-shelled (pyrite, chalcopyrite, bornite), earthy (kaolinite), uneven and more Hardness, or the degree of resistance of a mineral to external mechanical influence. The simplest way to determine it is by scratching one mineral with another. To assess the relative hardness, it is taken Mohs scale, represented by 10 minerals, of which each subsequent one scratches all the previous ones. The following minerals are accepted as hardness standards: talc – 1, gypsum – 2, calcite – 3, fluorite – 4, apatite – 5, orthoclase – 6, quartz – 7, topaz – 8, corundum – 9, diamond – 10. When diagnosing, very It is also convenient to use for scratching such objects as a copper (hardness 3,0–3,5) and steel (5,5–6,0) needle, knife (5,5–6,0), glass (5,0) . Soft minerals can be scratched with a fingernail (2,5). Fragility, malleability, elasticity. Under fragility in mineralogical practice, the property of a mineral to crumble when drawing a line with a knife or needle is implied. The opposite property – a smooth shiny mark from a needle (knife) – indicates the ability of the mineral to deform plastically. Malleable minerals are flattened by a hammer into a thin plate, elastic are able to restore their shape after removing the load (mica, asbestos). Other properties. Specific weight (density) can be accurately measured in laboratory conditions by various methods; An approximate judgment of the specific gravity of a mineral can be obtained by comparing it with common minerals, the specific gravity of which is taken as a standard. All minerals can be divided by specific gravity into three groups: lungs – with a specific gravity less than or equal to 2,9 (gypsum, muscovite, sulfur, chalcedony, amber, etc.); average – with a specific gravity of about 2,9–5,0 (apatite, biotite, sphalerite, topaz, fluorite, etc.); heavy – with a specific gravity greater than 5,0 (arsenopyrite, galena, cassiterite, cinnabar, etc.). Magneticity. Some minerals are characterized by pronounced ferromagnetic properties, i.e. attract small iron objects – sawdust, pins (magnetite, nickel iron). Less magnetic minerals (paramagnetic) are attracted by a magnet (pyrrhotite) or an electromagnet; Finally, there are minerals that are repelled by a magnet – diamagnetic (native bismuth). The magnetic test is carried out using a freely rotating magnetic needle, to the ends of which the test sample is brought. Since the number of minerals with distinct magnetic properties is small, this feature has important diagnostic value for some minerals (for example, magnetite). Radioactivity. All minerals containing radioactive elements – uranium or thorium – are characterized by the ability to spontaneous α-, β-, γ-radiation. In the rock, radioactive minerals are often surrounded by red or brown rims, and radial cracks radiate from the grains of such minerals included in quartz, feldspar, etc. Radioactive radiation affects photographic paper. Other properties. For diagnostics in field conditions are important solubility minerals in water (chlorides) or acids and alkalis, private chemical reactions into individual elements flame coloring (for example, minerals containing strontium color the flame red, sodium – yellow). Some minerals emit noise when struck or broken. smell (for example, arsenopyrite and native arsenic emit a characteristic garlic odor), etc. Individual minerals are determined to the touch (for example, talc feels greasy to the touch). Table salt and other salt minerals are easily recognized to taste. Luster is loosely defined as the quantity and quality of light reflected from the surface of a mineral. It represents the sum of reflections. Although this definition is not very precise, gloss is a very specific and useful property for identifying minerals. For consultation and delivery, please contact us in any convenient way:
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Brilliance

  • sparkling – reflection of light like a diamond, gives a sharp image of the light source; shining – gives only vague outlines of the light source;
  • brilliant – general reflection of light without a visible image.

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metallic luster

Metallic luster is characteristic of opaque substances with a high absorption coefficient, which are good reflectors. It indicates the presence of a metallic or, to a greater extent, covalent bond between atoms. Such luster is observed in native metals, sulfides and sulfosalts. Almost all of these substances have a reflectivity of at least 20%, and usually even more than 30%, and have high refractive indices, although the latter can rarely be directly determined.

Semi-metallic luster is observed in some translucent oxides (for example, rutile TiO2 and hematite Fe2O3) with refractive indices of 2 to 3.

Non-metallic luster

This type of shine, inherent in transparent and translucent substances, is divided into the following types:

Rice. 6.2 Electron micrograph showing shimmering platelets in labradorite Compositionally intermediate plagioclase (e.g. labradorite) is a complex intergrowth of slightly different structural units. The electron diffraction pattern (bottom left) shows two types of reflections associated with different structural units

Diamond. High shine due to high transparency. This luster is associated with the presence of a covalent bond in minerals, like diamond, as well as the presence of heavy metal atoms, like cerussite PbO03, or transition group elements, like rutile Ti02. Minerals with a diamond luster have high refractive indices (from 1,9 to 2,6 ,XNUMX) and strong dispersion.

Resinous. This luster is characteristic of ZnS sphalerite and other translucent minerals with a refractive index greater than 2 (sphalerite has a diamond or semi-metallic luster, and a resinous luster is most characteristic of uraninite).

Glass. The shine of broken glass or quartz This is present in many translucent minerals with predominantly ionic bonds of elements having an atomic number less than 26 (i.e., preceding Fe in the periodic table). In particular, many silicates are characterized by a glassy luster. Their refractive index ranges from 1,5 to 2,0.

Pearl. It is found in layered silicates, for example talc Mg3Si4O10(OH)2 and chlorite (Al, Mg)5-6(Si, Al)4O10(OH)8, which have very perfect basal cleavage. Such dim reflected light is also produced by dolomite CaMg(CO3)2 and the adhesive faces of gypsum (in its variety of selenite) CaSO4 • 2H20.

Silky. It is observed in fibrous minerals and is associated more with the texture of mineral aggregates than with the internal structure. An example is fiber gypsum.

Bold. Nepheline (Na, K)AlSiO4 has a greasy luster, to some extent, possibly due to surface changes.

Shimmering shine, play of color and opalescence

These effects are caused by the reflection of light from exsolution plates or other inhomogeneities in the crystal. A case in point is a type of feldspar called moonstone. In the crystals of this mineral, which were homogeneous at high temperatures, exsolution processes are developed, which is manifested in the presence of regularly oriented alternating plates of sodium-enriched and potassium-enriched feldspar. However, such crystals, with appropriate heating, can become homogeneous again, but at the same time their play of color disappears. The strong play of color observed in plagioclase labradorite is due to the presence of shimmering plates located at intervals of 120 HM.

The play of color in opal, an amorphous mineraloid, is due to the diffraction of light. Precious opal is composed of properly packed quartz glass beads. The balls are approximately the same size as the wavelength of visible light. Due to their regular spatial arrangement in the opal, they turn into something like a diffraction grating.

Color

Color in minerals is in most cases associated with the absorption of light radiation of certain wavelengths by the crystal-forming atoms. Those wavelengths of white light that are not absorbed create the visible sensation of color. In many precious minerals, such as corundum and beryl, color is due to the presence of color centers.

Color centers

Color centers, or F-centers (from the German Farbe – color) are associated with defects in the crystal lattice that absorb visible light. Such defects may be caused by the following reasons.

The presence of colloidal particles in the lattice associated with the “coalescence” of excess atoms.

Mechanical deformations of the crystal lattice

Typically, a violation of lattice regularity leads to the formation of vacant anionic and cationic positions. A vacant anionic position (no negative charge) electrostatically acts as a positive charge and can capture an electron. It is believed that the F center is a positively charged vacancy around which an electron moves.

When color occurs as a result of irradiating an initially colorless crystal with ultraviolet or x-rays, it is assumed that some anions have lost an outer electron that has absorbed enough energy to enter the conduction band. If the lattice were ideal, then when the excitation decayed, the electron would have to return to its original place. However, lattice defects create local energy levels between the excited and the original (ground) states, within which electrons can move. At the end of irradiation, energy is redistributed in the crystal and F-centers are formed that absorb light energy, as a result of which the crystal becomes colored.

For each anion, upon loss of an electron, a vacancy is formed at the outer electronic energy level. Such anions are called positive electron-hole centers (a term commonly used to refer to transistors), and they have enough energy to capture electrons. Experiments with crystals of alkali halides have shown that they can be colored by heating in alkali metal vapor, which leads to an excess of its atoms in the lattice. (The resulting color depends only on the crystal and not on the vapor used.) If, at high temperature, an electric field is applied to any part of the crystal colored in this way, the color will move along the crystal towards the anode, indicating that the color centers are moving in this way in the same way as negatively charged particles.

The role of color in mineral identification

Because of its variability and dependence on impurities, color is thought to be a poor diagnostic feature of minerals. However, such a clear distinctive feature as color is of great importance in identifying minerals. Some minerals are characterized by a permanent color – for example, the green color of malachite, blue azurite, red cinnabar, yellow sulfur. However, in many cases, while remaining a valuable diagnostic feature, color must be used with caution.

Trait

The color of a ground mineral (its trait) is a more constant and therefore more reliable feature than its own color. The trait can be easily obtained by rubbing the mineral on an unglazed porcelain plate, scraping a little powder from the mineral with a knife or file, or crushing a small piece of it.

Appearance (habitus)

Groups of crystals have a general tendency to grow in approximately parallel orientation. This occurs because groups of atoms are deposited on the substrate in some preferred orientation, a process that depends primarily on factors related to surface energy. Added to this is the influence of the direction in which the material flows from the solution. The relationships between crystals do not obey any geometric law, as is observed in twinning, and the parallelism between these processes is rarely exact.

The direction of influx of solutions and the conditions of deposition also determine the formation of crystal aggregates with distinct shapes, which can be characteristic of individual minerals. Such aggregates are given names, often derived from Latin and Greek roots and describing their morphology. Let’s name the most common forms:

Cluster-shaped – resembles clusters of grapes. This form is usually obtained by minerals precipitated as colloidal gels that have been subjected to surface tension. Let us take malachite and romaneshite as an example.

Dense – a continuous mass in which individual crystals are so small that they are barely distinguishable. The structure of such masses is divided into microcrystalline, when the crystals are visible under an optical microscope, and cryptocrystalline, when individual crystals are difficult to distinguish even under a microscope. Examples of the latter are agate and flint, which are cryptocrystalline forms of quartz. Coral-shaped – branched, roughly rounded and intertwined forms, sometimes observed in calcite and aragonite. Dendritic – branched accretion, forming miniature tree-like forms. Observed in native copper and dark-colored manganese oxides (pyrolusite, romaneshite, etc.); the latter are usually identified along cracks in the host rocks. Drusenoid – closely fused crystals growing inside the voids and having a sawtooth-shaped surface of the apical faces. Often observed in quartz veins. Fibrous – thin parallel, closely intergrown crystals. Excellent examples are gypsum crystals grown between bedding surfaces in shales, and chrysotile, an asbestos mineral found as veins in serpentinites. Filiform – thin, long (hair-like) aggregates, like millerite. Granular is a broad term to denote clusters of grains of more or less equal size, covering a significant range of sizes (from large to small) and even capturing dense forms when individual grains are already difficult to distinguish. Lamellar – thin leaves or scales, like mica.

Massive – fused crystals that do not form distinct individuals, but at the same time are not so small that they can be classified as a dense form. An example is the appearance of calcite in marble. Also often observed in sulfide minerals in mixed ores. Mastoid – rounded surfaces that intersect to form open V-shaped grooves, often more reminiscent of irregular stitches. Hematite often has this shape.

Mossy – a miniature form of dendritic aggregates.

Zhelvakovy – forms isolated ellipsoidal secretions. As an example, we can point to siderite nodules in shales. The regularity of the forms varies widely.

Oolitic – small close-packed spheroids or ellipsoids resembling fish eggs. Found in calcite in some limestones and in hematite or other iron minerals in sedimentary ores. Bean-shaped is a coarser variety of oolitic with pea-sized spheroids. Often observed in bauxites, where its development is probably due to deposition from a colloidal environment (cf. grapevine). Kidney-shaped – looks like a kidney. Occurs in hematite and is close to the mastoid.

Radial-radiant – radially arranged needle-shaped or plate-shaped crystals. Examples are gypsum and tourmaline in the case of certain special conditions of their crystallization.

Reticular – in the form of cells that arise as a result of the mutual intersection of crystals. It is observed in cerussite and crocoite. Stalactite – hanging masses converging on a cone, which can interlock with formations (stalagmites) growing upward from the lower level of the cavity, forming columns. Stalactites are formed by the deposition of a substance dissolved in water that seeps through cracks in rocks. It is characteristic primarily of calcite deposits in limestone caves, but other minerals can be formed in the same way.

Wire-like – often seen in native silver and gold.

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