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What mineral accumulations are called granular?

1.2. Diagnostic properties of minerals Task 1. Study and describe the main forms of occurrence of minerals based on the educational collection. Task 2. Write a description of the diagnostic properties of several minerals from the educational collection. Theoretical part Forms of occurrence of minerals in nature are varied and depend mainly on the conditions of formation. These are either individual crystals or their regular intergrowths (twins), or clearly isolated mineral accumulations, or, more often, accumulations of mineral grains. Individual isolated crystals and crystal twins, i.e. natural intergrowths of crystals arise in conditions favorable for growth. The shape of the crystals is varied and reflects both the composition and internal structure of the mineral, as well as the conditions of formation. Among the isolated mineral accumulations, the most common are druses, representing clusters of crystals attached at one end to a common base. Often druses form on the walls of voids in rocks. Small crystals sitting tightly on any base form brush. Secretions – the result of the gradual filling of voids in rocks with mineral matter deposited on their walls. The accumulation of the substance in this case goes from the periphery to the center, and therefore the secretions usually have a concentric structure, reflecting the stages of formation. Small secretions are called tonsils, large (more than 10 mm) – geodes. Concretions – more or less rounded formations that arose by the deposition of mineral matter around some center of crystallization. In this case, the deposition of mineral matter occurs from the center to the periphery, which is associated with the concentric or radial structure of the nodules. Small round formations, usually of a concentric structure (from a millet grain to a pea in size) are called oolites. They can be cemented into a dense mass or in a loose state. Oolites are formed during the precipitation of minerals from solutions, when grains of sand, skeletal remains of small animals, etc. gradually become enveloped in the released mineral. Oolitic limestones and dolomites are formed in the coastal zone of the seas. Limestone and dolomite oolites are more common, gypsum, anhydrite, limonite, and chalcedony are less common. Sinter formations that complicate the surfaces of voids arise when mineral matter crystallizes from seeping groundwater. Sagging hanging from the vaults of voids are called stalactites, growing upward from the bottom of caves – stalagmites. Flat mineral films with different structures can develop on the surface of cracks. The most widely developed mineral aggregates of a crystalline, amorphous or cryptocrystalline structure make up the rock strata. They are formed by the more or less simultaneous precipitation of many mineral particles from solutions or melts. In crystalline aggregates, minerals are in a crystalline state, but their grains have an irregular shape. The size of the grains depends on the crystallization conditions and varies from large to earthy. According to the shape of the grains, mineral aggregates are divided into: a) grainy (quartz, calcite, galena), having an isometric grain shape; b) fibrous (asbestos), columnar (selenite), pole-shaped (hornblende), having an elongated shape; V) scaly (graphite, talc), lamellar (gypsum, mica), having a flat shape. Mineral aggregates differ in grain size: a) coarse-grained – grain sizes more than 5 mm, b) medium grain – grain sizes 2 – 5 mm, c) fine-grained – grain sizes 0,5 – 2 mm and g) cryptocrystalline, forming dense or earthy-loose masses. There are mineral formations whose composition does not correspond to the form they form – these are the so-called pseudomorphoses (Greek “pseudo” – false). They arise from chemical changes in pre-existing minerals or from filling voids created by the leaching of any mineral or organic inclusions. The first include, for example, the frequently occurring pseudomorphs of limonite on pyrite, when cubic crystals of pyrite transform into cryptocrystalline limonite, the second include pseudomorphs of chalcedony on wood, etc. Physical properties of minerals. The constancy of the chemical composition and internal structure of minerals determines the constancy of their properties. Various methods of mineralogical research and determination of minerals are based on this. Most of them require special equipment. However, every researcher who deals with minerals and rocks must have a method for their field determination, based on the study of external (macroscopic) properties visible to the naked eye. Crystal morphology can be an important diagnostic sign, although it should be noted that in nature the same mineral under different conditions forms crystals of different shapes, and different minerals can produce the same crystals. The entire variety of crystal shapes is conditionally grouped according to the degree of complexity into seven large groups or systems, called syngonies (see above). Optical properties of minerals. Color – an important feature of minerals, which, however, can only be used in conjunction with other properties. For some minerals, color is a permanent feature; for example, pyrite has a brass-yellow color, malachite has a green color, azurite has a blue color, gold has a golden yellow color, etc. The names of a number of minerals already carry characteristics of their color: rhodonite – pink, chlorite – green ; cinnabar – mercury sulfide of bright red, scarlet color – translated from Arabic means “dragon’s blood”, etc. For most minerals this sign is not constant. Feldspars come in white, yellow, red, green, and dark gray colors. Calcite is found colorless, white, yellow, green, blue, purple, brown, black. The color of minerals is determined primarily by their chemical composition. Each chemical element that makes up minerals and each chemical compound gives them a certain, very characteristic color. Minerals containing copper carbonates are green or blue (malachite, azurite). The mineral beryl in its pure form is colorless and transparent, and in the presence of an admixture of chromium oxide it becomes green (emerald); minerals containing iron oxide are characterized by red, brown, yellow colors (brown iron ore). Tarnishing. Some minerals, especially those containing copper, have a variegated thin film on their surface: pinkish, reddish, yellowish, bluish, etc., due to chemical weathering processes. The color of this film differs from the color of the mineral itself. This phenomenon is called tarnish (for example, chalcopyrite). For opaque and strongly colored weakly transparent minerals, an important diagnostic feature is color of mineral in powder or stroke color. It may be the same as in the piece (for example, magnetite), but it may differ from it (for example, pyrite, hematite). To determine the color of the powder, a mineral is passed along the rough surface of a porcelain plate, on which a line remains that corresponds to the color of the powder. Transparency, characterizing the ability of a mineral to transmit light depends on its crystal structure, as well as on the nature and homogeneity of the mineral accumulation. Based on this feature, minerals are distinguished: transparent, transmitting light like ordinary glass; translucent or translucent, transmitting light like frosted glass; translucent only in a thin plate и opaque, not transmitting light rays. Brilliance depends on the refractive index of the mineral and the nature of the reflecting surface. Release minerals from metal luster, which includes opaque minerals that have a dark-colored feature. A shine that resembles the shine of tarnished metal is called metal-like. A much larger group consists of minerals with non-metallic shine, the varieties of which include: diamond, glass, greasy, mother-of-pearl, silky, waxy and in case of lack of shine, mat. Mechanical properties of minerals. Kink determined by the surface along which the mineral is split. It may resemble the ribbed surface of a shell – conchoidal fracture may have an indefinitely uneven character – uneven kink In fine-grained aggregates, it is not possible to determine the fracture of individual mineral grains; in this case, they describe a fracture of the unit – grainy, splintery, or needle-shaped, earthy. Cleavage – the ability of crystalline minerals to split along smooth surfaces – cleavage planes, corresponding to the directions of least cohesion of particles in the crystalline structure of the mineral. Depending on how easily chips form along the planes and how well they are sustained, different degrees of cleavage are distinguished: very perfect – the mineral easily splits into thin plates; perfect – upon impact, the mineral splits along the cleavage planes; average – upon impact, the mineral splits both along planes and along an uneven fracture; imperfect cleavage – against the background of an uneven fracture, chips along the planes only occasionally form; very imperfect – an uneven or conchoidal fracture always forms. Cleavage can be expressed in one, two, three, less often four and six directions. Hardness – the ability to withstand external mechanical influence is an important property of minerals. Usually in mineralogy, relative hardness is determined by scratching the surface of the mineral under study with a reference mineral: the harder mineral leaves a scratch on the less hard one. The Mohs hardness scale adopted in geology includes ten standard minerals, arranged in order of increasing hardness: talc – hardness 1, gypsum – 2, calcite – 3, fluorite – 4, apatite – 5, orthoclase – 6, quartz – 7, topaz – 8, corundum – 9, diamond – 10. To determine the hardness of minerals, you can use some common objects, the hardness of which is close to the hardness of standard minerals. Thus, soft pencil graphite has a hardness of 1; about 2-2,5 – nail; 4 – iron nail; 5,5-6 – steel knife, needle. Each mineral is characterized by a more or less constant density. Based on this feature, minerals are divided into lungs и heavy. When studying minerals macroscopically, it is important to be able to assign a mineral to a group by simply weighing it in the palm of your hand light – with a density of up to 2,5 g/cm 3 , secondary – up to 0,4, heavy – 4-6, very heavy minerals – with a density of over 6 g/cm 3 . For minerals that contain heavy metals, high density is an essential diagnostic feature. In addition to the properties listed above, some minerals have magnetism, radioactivity, malleability и elasticity. Table salt (NaCl) has salty taste; Iceland spar(CaCO 3 ) is birefringent. Guidelines 1. Study the theoretical part of the work. 2.Describe and sketch in a notebook the main forms of mineral aggregates available in the educational collection (druzes, brushes, nodules, secretions, granular and earthy aggregates, sinter forms, pseudomorphs). 3. Describe in your notebook the physical properties of several minerals from the educational collection (as chosen by the teacher). test questions 1. Define the concepts “druze”, “brush”, “secretion”, “nodule”. 2. What is pseudomorphosis? Give examples. 3. List the main diagnostic properties of minerals. 4. What is cleavage? Give an example of a mineral with very perfect cleavage. 5. What is the hardness of quartz on the Mohs scale? Recommended reading 1. Dobrovolsky V.V. Geology. – M.: VLADOS, 2001. 2. Muzafarov V. G. Key to minerals, rocks and fossils. – M.: NEDRA, 1979. Grainy – fused grains of minerals (apatite, pyrite, which are sources of significant energy). Earthy– externally resemble loose soil; (kaolinite).Needle-shaped—prismatic crystals Leafy-scaly – minerals that can be separated into plates or flakes (mica).Sinter forms of minerals — formations of unearthly origin; form in voids and caves. When evaporated, they look like icicles, stalactites or buds (malachite; hematite).Concretions – minerals of spherical, kidney-shaped, palmate, elongated and other shapes, characterized by a radical radiant structure. Crystals of such minerals grow in the form of radial rays running from the center to the periphery Secretions — are formed as a result of filling voids in rocks with mineral substances. druses – large crystals attached at one end to a common base, Oolites – small-sized balls, characterized by a concentric-shell structure. They, as a rule, are cemented into a dense mass, but sometimes they are in a loose state. Dendrites – forms resulting from the rapid crystallization of minerals. They are born in narrow cracks or in a viscous substance, for example, in clay. (according to Betekhtin further) Mineral aggregates.As a result of crystallization and solidification of a solution or melt, a mixture of crystalline grains fused together is formed, which is called a mineral aggregate. Aggregates are monomineral, i.e. i.e., consisting of crystalline grains of one mineral (for example, a piece of marble or magnetite ore), and polymineral, represented by several minerals of different composition and properties (for example, a piece of granite or copper-zinc sulfide ore). Mineral aggregates are very diverse in their structure and morphological characteristics . Many of them are so typical that they have acquired special names. The most characteristic morphological features of mineral aggregates are determined by the degree of crystallinity of the substance. From this point of view, first of all, two large groups differ significantly from each other: 1) clearly crystalline aggregates; 2) cryptocrystalline and collomorphic masses. Let us list the main types of mineral aggregates. 1. Granular aggregates composed of crystalline grains, sometimes in combination with well-formed crystals of some minerals. This type of aggregates is most widespread in the earth’s crust. Examples include fully crystalline igneous rocks, many sulfide and other ores of mineral deposits, etc. Based on the size of the constituent grains, they are distinguished: 1) coarse-grained aggregates – with a grain size of more than 5 mm in diameter; 2) medium-grained – with grains 1–5 mm in diameter, easily visible to the naked eye; 3) fine-grained, the grain sizes of which are less than 1 mm. The structure of cryptocrystalline aggregates can only be established under a microscope in thin sections The shape of the constituent grains also leaves its mark on the morphological features of the aggregates. If the aggregate is composed of grains of a more or less isothermal shape, then it is simply called granular. If the grains have a lamellar appearance, then such aggregates are called leafy or scaly, depending on the size of the composing individuals. Finally, there are aggregates, the individuals of which have a shape elongated in one direction, sometimes with a radial arrangement they are called columnar, needle-shaped, fibrous aggregates. Aggregates composed of minerals of various shapes are also common, for example, mica schists with isometric garnet crystals, granular masses of quartz with columnar tourmaline crystals, etc. Based on the degree of space filling, dense and loose granular aggregates are distinguished. An example of the latter is loose crystalline formations at the bottom of drying salt lakes. 2. Drusen are intergrowths of well-formed crystals that have grown on the walls of any voids. An example is the commonly found druses of quartz crystals. Drusen are interesting not only from a crystallographic point of view, but also because in them it is often possible to study the sequence of release of various minerals that crystallized from the last portions of solutions. The very fact of the presence of well-formed crystals in drusen indicates that they arose in free space, i.e. that is, in any primary voids, hollow cracks, crushed rocks, etc. The sizes of voids vary widely, from small pores to caves, sometimes called “crystal cellars,” in which the walls are studded with large crystals of transparent quartz and other minerals. 3. Secretions are formed as a result of filling the voids of an irregular, usually rounded forms of crystalline substance. A characteristic feature of many secretions is successive concentric layer-by-layer deposition of mineral matter in the direction from the walls of the void towards the center. In this case, individual layers often differ from each other in color and often in composition. Small voids are usually completely filled with mineral matter. Sometimes the central part is made of radial fibrous aggregates of some mineral, for example zeolites. In large voids, a cavity is often observed in the center, the walls of which are covered with druses of crystals or sinter formations. 4. Concretions are spherical or not quite regularly shaped nodules and nodules that occur in loose sedimentary rocks, mainly in clays, sands and earthy products of rock destruction. The sizes of nodules vary widely limits – from millimeters to tens of centimeters, sometimes up to meters in diameter. Growing and joining together, they form large bodies of complex shape. 5. Oolites in their method of formation are in many ways similar to nodules. These are the same spherical formations, but small in size (from tenths of a millimeter to 5–10 mm), arising in aqueous environments around suspended foreign bodies – grains of sand, fragments of organic residues and even gas bubbles . A characteristic feature of oolitic nodules is their pronounced, fairly regular concentric layering, sometimes shelliness. Formations that are similar in shape but do not have concentric layering are called pseudoolites (beans). 6. The so-called sinter forms of mineral formations, like druses of crystals, are observed in voids, including in natural caves. Previously it was assumed that they appear as a result of crystallization of colloids. Such aggregates almost always exhibit a spherulite or microspherulite structure; they are observed in the form of stalactite or pseudostalactite, kidney-shaped, grape-shaped and other forms. In the lower parts of the voids, due to falling drops, cone-shaped stalagmites appear that rise upward, which, however, are not observed for all mineral formations. The formation of “sinter” forms in the overwhelming majority of cases is associated with mass crystallization from true solutions, accompanied by splitting and geometric selection of individuals. The diversity of the resulting aggregates is associated with differences in the rates of nucleation and in the growth mode of individuals, determined by the nature of the contact of the surface of the substrate and growing crystals with liquid media in different states (film, droplet or stagnant solutions, laminar or turbulent flows of solutions, etc.). Pseudolactites appear, in all likelihood, through the nucleation and crystallization of spherulite crusts on flexible colloidal tubes in an aqueous environment. 7. Earthy masses, as the name itself shows, are soft powdery formations in which it is impossible to distinguish any crystalline formations even with a magnifying glass. We usually see them in the form of crusts or accumulations, most often arising during the chemical weathering of ores and rocks. Depending on the color, such masses are sometimes called sooty (black formations) or ocher (clumps and crusts of yellow and brown color). Such, for example, are earthy mineral formations of various colors of nickel hydrosilicates, sooty formations of manganese hydroxides, ocher formations of iron hydroxides and other residual weathering products. 8. Plaques and greases, sometimes found in the form of thin films on the surface of crystals, can be substances of different compositions. These include, for example, thin films of brown iron hydroxides on rock crystal crystals, smears of copper green and blue in rocks hosting copper deposits, etc. 9. Efflorescences are usually called loose films and crusts that periodically appear on the surface of ores, rocks, dry soils and in cracks, or sporadically scattered mossy and fluffy formations of any salts, most often easily soluble aqueous sulfates. During rainy periods of the year, they usually disappear, and in dry weather they reappear. Quite often observed dendrite-like precipitations of manganese hydroxides on the surface of rocks along thin cracks should also be attributed to this type of formations 10. “Rings and spirals of Liesegang.” This term refers to rhythmically alternating banded formations resulting from the periodic deposition of any compounds during diffusion in microporous media, very similar to those obtained by R. Liesegang in the form of concentric rings or spirals. The essence of his experiment was that around a drop of AgNO3 on gelatin impregnated with K2Cr2O7, during the diffusion of the solution during the reaction process, microscopic crystals of Ag2 Cr2O7 appeared, which initially moved along with the solution, but then, as they grew, were retained in the pores of the gelatin, not being able to move further due to their size, and therefore were periodically deposited in the form of concentric rings.

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