Text 16. Halite

Halite is a mineral form of common salt, with the chemical composition sodium chloride, NaCl. Halite, also called rock salt, is a common mineral, formed by the drying of enclosed bodies of salt water; subsequently the beds so formed have often been buried by the rock strata formed from other sedimentary deposits. Beds of halite range in thickness from a few meters to 30 m and have been found at great depths beneath the surface of the earth. This mineral is often found associated with gypsum, sylvite, anhydrite, calcite, clay, and sand. Halite is widely disseminated over the world.

Halite crystallizes in the isometric system, usually in the form of cubes, and shows perfect cubic cleavage. It is colorless and transparent when pure but is often tinted yellow, red, blue, or purple by impurities. It has a hardness of 2.5 and a specific gravity of 2.16.

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Text 17. Gypsum

Gypsum is a common mineral consisting of hydrated calcium sulfate (CaSO4·2H2O). It is a widely distributed form of sedimentary rock, formed by the precipitation of calcium sulfate from seawater, and is frequently associated with other saline deposits, such as halite and anhydrite, as well as with limestone and shale. Gypsum is produced in volcanic regions by the action of sulfuric acid on calcium-containing minerals; it is also found in most clays as a product of the action of sulfuric acid on limestone. It occurs in all parts of the world; some of the best workable deposits are in France, Switzerland, and Mexico. Alabaster, selenite, and satin spar are varieties of gypsum.

Artificial gypsum is obtained as a by-product in an old method for the manufacture of phosphoric acid. Phosphate rock, the essential constituent of which is tricalcium phosphate, is treated with sulfuric acid, producing phosphoric acid and gypsum. The gypsum is compacted into blocks and used for the construction of nonsupporting walls in buildings. By properly controlling the concentration and temperature of sulfuric acid added to phosphate rock, a mixture of monocalcium phosphate, dicalcium phosphate, and gypsum may be obtained. This mixture is a valuable fertilizer, superphosphate.

Gypsum crystallizes in the monoclinic system in white or colorless crystals, massive or foliated in formation. Many specimens are colored green, yellow, or black by impurities. With a hardness ranging from 1.5 to 2, it is soft enough to scratch with a fingernail and has a specific gravity of 2.3.

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NOTES:

• saline – солончак, соленый источник;

• spar - шпат.

Text 18. Selenite

Selenite (Greek selзnз, "moon") is a variety of gypsum that occurs in transparent crystals or crystalline masses. It is so called because, according to a legend of Central Europe, the crystals form when the moon is on the increase. Extremely large crystals of selenite are found in southern Utah in geodes, or hollow rocks. Broad, transparent sheets of selenite can be split into extremely thin, transparent plates, which were used by the ancients in place of glass. Selenite today is quarried and used as ordinary gypsum.

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