Rare and valuable minerals

What properties are most important when diagnosing minerals?

EXTERNAL SIGNS AND PHYSICAL PROPERTIES OF MINERALS When determining minerals using a blowpipe, the main attention is paid to the qualitative chemical composition of the minerals and their chemical properties. These signs are put forward in the first place in this manual. The external signs of minerals, however, cannot be ignored. Knowledge and ability to use them are essential in all those cases where it is necessary to determine a mineral in macroscopically distinguishable samples, which usually occurs when working with a blowpipe. External signs include: appearance of minerals, cleavage, fracture, hardness, etc. Under natural conditions, minerals are found either in the form of crystals with well-defined edges, or in the form of irregular grains that do not have crystallographic outlines, or in the form of amorphous bodies. In addition, liquid and gaseous minerals are known, such as water, mercury, and carbonic acid. Crystals, their shape and properties are discussed in special manuals on crystallography. Here it is only necessary to note that the external shape of a crystal is often an important diagnostic feature of many minerals. Grains or crystals of minerals grow together into peculiar frigates, the shape and structure of which in many cases are also very important in the diagnosis of minerals. The following types of units are distinguished. 1. Granular aggregates are accumulations of improperly fused grains of one or more minerals (for example, marble, holocrystalline rocks). 2. Dense or continuous masses are the same granular aggregates”, consisting of very small particles (for example, kaolin, bauxite, etc.). 3. Earthy formations resembling pieces of loose soil (chalk, brown iron ore, etc.). 4. Amorphous bodies that resemble glass and do not show signs of a crystalline structure (opal, volcanic glasses, etc.). 5. Columnar aggregates, characterized by elongated prismatic crystals (calcite, beryl, scapolite, etc.). 6. Fibrous and jellied minerals that form very thin crystals, which can sometimes be separated into individual fibers (asbestos, gypsum, etc.). 7. Leafy aggregates that split into thin mica-like leaves* (micas, chlorites, etc.). 8. Sinter formations, which are crusts like stalactites and usually have a veined or columnar structure. Cleavage is the property of a crystalline substance to split along planes in a certain direction. The cleavage planes are quite definitely located in relation; to crystal limiting elements. Cleavage is one of the important properties of crystals and is a good diagnostic sign for many minerals. A distinction is made between very perfect cleavage, when the mineral is very easily split by the fingers into individual leaflets that are smooth and perfectly reflect light (micas, some chlorites, gypsum, graphite). Perfect cleavage is considered if the mineral breaks into pieces bounded by cleavage planes, in whatever direction you hit it with a hammer (calcite, lead luster, sphalerite, rock salt, etc.) – Minerals will have an average cleavage, the crushing of which produces approximately the same number of pieces bounded by both irregular and cleavage planes (feldspars, pyroxenes, amphiboles). Imperfect cleavage is difficult to detect, and among the fragments of crushed minerals, pieces with cleavage surfaces are rarely found (for example, garnet). The direction of cleavage is called by the corresponding simple shape of the crystal, parallel to the faces of which it will be located. For example, in the cubic system, cleavage is distinguished by the cube (lead luster, rock salt), by the octahedron (fluorite), and by the rhombic dodecahedron (sphalerite). In the hexagonal system, cleavage is distinguished by pinacoid (based), prism or rhombohedron (calcite). In other syngonies it is distinguished by pinacoids, prism, etc. Minerals with imperfect cleavage produce uneven surfaces when broken or crushed. Depending on the nature of the surface, they are distinguished: conchoidal fracture, reminiscent of the surface of shells, for example, quartz, glass; splintered, when the mineral produces sharp thin needles and splinters, for example in asbestos, flint; uneven if irregular rough surfaces are obtained; granular, when the mineral forms rough surfaces, more likely to be characteristic of aggregates, for example, marble, etc. The hardness of minerals is fairly constant and is an important diagnostic feature. This concept refers to the degree of resistance of a mineral to scratching, grinding, pressure, etc. To assess hardness, they use the Mohs scale, which uses ten minerals as standards, arranged in order of increasing their hardness.

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

To determine hardness, select an acute angle on a piece of mineral and draw it on the smooth surface of the mineral from the scale. If the mineral under study leaves a noticeable mark, then its hardness is higher, and then they move on to the next number on the scale. This is done until they reach the number on the scale at which the mineral under study will not produce features, but, on the contrary, will be drawn by it itself. For example, a mineral that will be marked by fluorite and will itself be marked by apatite has a hardness of 4-5. For those minerals that mutually draw each other, the hardness can be considered the same. For a less accurate determination of hardness, you can use the following, simpler, practical scale.

2 -2,5 Thumbnail
3 Silver coin
3,5 Bronze coin
5,5-6 Blade of a pocket knife
5,5-6 Window glass
6,5-7 File
  • Brittleness, when a mineral is broken into pieces when struck with a hammer.
  • Malleability, if a light blow with a hammer causes the mineral to be rounded and flattened into a plate (native copper and silver, argentite).
  • Flexibility, if the mineral in the plate bends, but does not take its previous shape when the acting force is removed (chlorites, talc).
  • Elasticity, when a mineral, bent at a significant angle, after removing the acting force, takes on its previous shape (mica)

Color is an important diagnostic feature when identifying minerals, but it must be used with caution, since in addition to its own color inherent in a given substance, color can change due to impurities and a number of other reasons.

The mineral dash color indicates the color of the mineral in the powder. To do this, use an unglazed porcelain plate (called a biscuit), on which a line is drawn with the sharp corner of the mineral. A trace of powder remains on the biscuit, which is used to determine the color of the line.

The brilliance of minerals depends on different strengths of reflection, absorption or refraction of light. There is a metallic luster when a mineral is similar in luster and appearance to metal; metallic luster is inherent in metals, compounds of heavy elements with sulfur, and iron oxides. These minerals are completely opaque, even in fragments, and have a black or darkly colored streak on the porcelain plate. Dark-colored minerals have a metallic or semi-metallic luster, but give, however, a weaker dark or densely colored feature (for example, chromite, limonite). Transparent, colorless or light-colored minerals have a non-metallic luster. They produce a white or light-colored streak. The following types of non-metallic luster are distinguished: glassy (quartz, calcite, gypsum), diamond (diamond, sphalerite, cinnabar), greasy (eleolith), pearlescent (mica, talc) and silky (malachite, asbestos).

Some minerals, such as tourmaline, calamine, etc., become electrified when heated or cooled. This phenomenon can be observed by pollinating a cooling mineral with a mixture of sulfur and red lead powders. In this case, sulfur covers positively charged areas of the mineral surface, and minium covers areas with a negative charge.

Magnetism is the property of some minerals to act on a magnetic needle or be attracted by a magnet. To determine magnetism, use either a magnetic needle placed on a sharp tripod, or a magnetic horseshoe, or a block. It is also very convenient to use a magnetic needle or knife.

When testing for magnetism, three cases are possible: a) when the mineral in its natural form (“by itself”) acts on the magnetic needle, b) when the mineral becomes magnetic only after calcination in the reducing flame of the blowpipe c) when the mineral neither before nor after does not exhibit magnetism when calcined in a reducing flame. For calcination with a reducing flame, you need to take small pieces 2-3 mm in size.

To verify the result of the reaction, you should touch the calcined piece with the tip of a magnetic needle, needle, etc., removed from the tripod.

When an iron-containing mineral is calcined in a reducing flame, the following reaction occurs:

* even partially Fe can be obtained. – The presence of FeO and Fe causes magnetism.

When exposed to an oxidizing flame, the following reactions occur:

In the latter case, the magnetism of the intermediate product in the form of (n-1) Fe2O3 does not take place, since the reaction proceeds very quickly until complete transformation into non-magnetic iron oxide.

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).

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 check for radioactivity, the background value is first measured and recorded, then the mineral is brought as close as possible 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.

Electrical Conductivity

A number of minerals have significant electrical conductivity, which allows them to be clearly distinguished from similar minerals. Can be checked with a regular household tester.

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.

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.

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