Text 19. Alabaster

Alabaster is a varietal name applied to two different minerals. One, Oriental alabaster, was extensively used by the ancient Egyptians. It is a variety of calcite, with a hardness of 3; it is usually white and translucent, but is often banded with dark or colored streaks. The other mineral, true alabaster, is a variety of gypsum, usually snow-white in color with a uniform, fine grain. True alabaster is softer than Oriental alabaster; it has a hardness of 1.5 and is easily carved into intricate shapes. Deposits of fine gypsum alabaster are found in Italy, England, Iran, and Pakistan.

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Text 20. Basalt

Basalt is the most common variety of volcanic rock, composed almost entirely of dark, fine-grained silicate minerals, chiefly plagioclase feldspar and pyroxene, and magnetite. The extrusive equivalent of gabbro, it forms by the outpouring of lava all along the world’s mid-ocean ridges, where sea-floor spreading continually adds new crust to counterbalance that lost by subduction. Usually dark-gray in color, basalt often has a vesicular texture, preserving vestiges of bubbles produced by expanding steam as lava cools and solidifies. Also characteristic are pillow-shaped masses caused by rapid cooling of lava erupted on the sea floor. In addition to lava flows, basalt is also found in the form of dikes and sills. Columnar jointing, as exhibited by Devils Tower in Wyoming and the Giant’s Causeway in Ireland, is a common feature of these shallow intrusive bodies.

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NOTES:

• gabbro – габбро.

Text 21. Gold

Gold, symbol Au (from Latin aurum,"gold"), is a soft, dense, bright yellow metallic element. Gold is one of the transition elements of the periodic table; its atomic number is 79.

Properties. Pure gold is the most malleable and ductile of all the metals. It can easily be beaten or hammered to a thickness of 0.000013 cm, and 29 g could be drawn into a wire 100 km long. It is one of the softest metals (hardness, 2.5 to 3) and is a good conductor of heat and electricity. Gold is bright yellow and has a high luster. Finely divided gold, like other metallic powders, is black; colloidally suspended gold ranges in color from ruby red to purple.

Gold is extremely inactive. It is unaffected by air, heat, moisture, and most solvents. It will, however, dissolve in aqueous mixtures containing various halogens such as chlorides, bromides, or some iodides. It will also dissolve in some oxidizing mixtures, such as cyanide ion with oxygen, and in aqua regia, a mixture of hydrochloric and nitric acids. The chlorides and cyanides are important compounds of gold. Gold melts at about 1064° C (about 1947° F), boils at about 2808° C (about 5086° F), and has a specific gravity of 19.3; its atomic weight is 196.97.

Occurrence. Gold is found in nature in quartz veins and secondary alluvial deposits as a free metal or in a combined state. It is widely distributed although it is rare, being 75th in order of abundance of the elements in the crust of the earth. It is almost always associated with varying amounts of silver; the naturally occurring gold-silver alloy is called electrum. Gold occurs, in chemical combination with tellurium, in the minerals calaverite and sylvanite along with silver, and in the mineral nagyagite along with lead, antimony, and sulfur. It occurs with mercury as gold amalgam. It is generally present to a small extent in iron pyrites; galena, the lead sulfide ore that usually contains silver, sometimes also contains appreciable amounts of gold. Gold also occurs in seawater to the extent of 5 to 250 parts by weight to 100 million parts of water. Although the quantity of gold present in seawater is more than 9 billion metric tons, the cost of recovering the gold would be far greater than the value of the gold that could thus be recovered.

Uses. The metal has been known and highly valued from earliest times, not only because of its beauty and resistance to corrosion, but also because gold is easier to work than all other metals. In addition, gold was easier to obtain in pure form than the other metals. Because of its relative rarity, gold became used as currency and as a basis for international monetary transactions. The unit used in weighing gold is the troy ounce; 1 troy ounce is equivalent to 31.1 grams.

The major portion of the gold produced is used in coinage and jewelry. For these purposes it is alloyed with other metals to give it the necessary hardness. The gold content in alloys is expressed in carats. Coinage gold is composed of 90 parts gold to 10 parts silver. Green gold used in jewelry contains copper and silver; white gold contains zinc and nickel, or platinum metals.

Gold is also used in the form of gold leaf in the arts of gilding and lettering. Purple of Cassius, a precipitate of finely divided gold and stannic hydroxide formed by the interaction of auric chloride and stannous chloride, is used in coloring ruby glass. Chlorauric acid is used in photography for toning silver images. Potassium gold cyanide is used in electrogilding. Gold is also used in dentistry. Radioisotopes of gold are used in biological research and in the treatment of cancer.

South Africa is the world’s leading supplier of gold, producing about 600 metric tons annually; its most important gold mines are in the Witwatersrand region. Some 70 other countries produce gold in commercial quantities, but about 80 percent of the total worldwide production now comes from South Africa, the United States, the former Soviet republics, Australia, Canada, China, and Brazil.

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NOTES:

• malleable - ковкий (о металле);

• alluvial - рассыпной (о золоте);

• antimony - сурьма;

• troy ounce - тройская унция (31,1035 г);

• gilding - позолота, золочение.