Text 13. Calcite

Calcite is a mineral consisting largely of calcium carbonate (CaCO3). Next to quartz, it is the most abundant of the earth’s minerals. Crystallizing in the hexagonal system, calcite is noted for its wide variety of crystalline forms. It also occurs in massive or cryptocrystalline formations. For example, crystalline variety of calcite - Iceland spar - is the only pure form of calcite found in nature. Limestone, chalk, travertine, Oriental alabaster, and marble are among the most common of the massive forms of the mineral. Calcite is also found as stalactites and calcareous tufas, forms deposited by mineral waters.

Colorless, with a hardness of 3 and a specific gravity of 2.72, pure calcite is readily identified by the ease with which it is cut or cleaved and by the rapidity with which it reacts with dilute acids. Such contaminants as magnesium, ferrous iron, manganese, and zinc will alter the properties of the mineral in varying degrees, depending on the amounts present.

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NOTES:

• calcareous tufa – известковый туф;

• manganese – марганец.

Text 14. Stalactite and stalagmite

Stalactite and Stalagmite are mineral formations frequently found in caves. A stalactite is an icicle-shaped mass of accumulated calcium carbonate hanging from the roof or sides of a limestone cavern; it is formed by mineral precipitation from groundwater that has seeped, very slowly, through the roof of the cavern. Water circulating through the ground above a cavern picks up calcium bicarbonate as it percolates through the limestone. When the water trickles through the ground to the roof of a cavern, it tends to cling to the roof in droplets; as the droplets lose some of their water and carbon dioxide, calcium carbonate is precipitated and forms a deposit. As more water seeps through the roof, the precipitation of calcium carbonate continues and the deposits grow in length and width, forming stalactites; these are often of enormous size and beautiful shape.

Pure calcium carbonate is white, but stalactites are often variously colored by impurities in the mineral. Some of the water seepage also falls to the floor and there accumulates into calcium carbonate masses resembling inverted stalactites. These deposits, growing upward from the floor, are called stalagmites. Stalactites and stalagmites often occur in pairs and meet, forming columns that may grow as large as 1.5 m (5 ft) in diameter.

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Text 15. Salt

Salt, also sodium chloride, is a chemical compound that has the formula NaCl. The term salt is also applied to substances produced by the reaction of an acid with a base, known as a neutralization reaction. Salts are characterized by ionic bonds, relatively high melting points, electrical conductivity when melted or when in solution, and a crystalline structure when in the solid state.

Salt is a white solid, soluble in hot or cold water, slightly soluble in alcohol, but insoluble in concentrated hydrochloric acid. In the crystalline form the compound is transparent and colorless, shining with an icelike luster. The compound usually includes traces of magnesium chloride (MgCl2), magnesium sulfate (MgSO 4), calcium sulfate (CaSO4), potassium chloride (KCl), and magnesium bromide (MgBr2).

Salt is widely distributed in nature. It is found in solution in ocean water in concentrations of about 30 g/liter of water, meaning that salt makes up about 3 percent of ocean water by weight. Salt can occur as a surface crust or layer in swamps and dry lake bottoms, especially in extremely arid regions. The mineral halite, more commonly known as rock salt or massive salt, occurs in beds deposited by the dehydration of ancient bodies of salt water. The compound is constantly being formed by the action of rivers and streams on rocks containing chlorides and compounds of sodium. Salt melts at 804° C (1479° F) and begins to vaporize at temperatures just slightly above this; it has a specific gravity of 2.17.

Uses. The most familiar use of salt is as a seasoning. Salt is an essential constituent in the diet of human beings and other warm-blooded animals. Certain peoples restrict the consumption of salt, but they obtain necessary quantities of it by eating salt-containing raw or cooked meat and fish. Wild animals often congregate at salt streams or surface encrustations of salt, called salt licks, where they lick the salt deposits.

Industrially, salt is the source of chlorine and its principal compounds and the source of sodium and its compounds. Chlorine compounds of commercial importance include hydrochloric acid, chloroform, carbon tetrachloride, and bleaching powder. Important sodium compounds include sodium carbonate, sodium sulfate, baking soda, sodium phosphate, and sodium hydroxide. Salt is widely used as a preservative for meats and is employed in some refrigeration processes, in dyeing, and in the manufacture of soap and glass. Because they are transparent to infrared radiation, salt crystals are used for making the prisms and lenses of instruments used in the study of infrared radiation.

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