Text 7. Chalcedony

Chalcedony is a mineral, a cryptocrystalline variety of quartz of various shades of white, gray, yellow, brown, green, and blue. Like quartz it has the formula SiO2, a hardness of 7, and a specific gravity of 2.65. Although it is usually translucent, some of the milk-white varieties are opaque. The color variations occur as the result of the presence of such impurities as iron, aluminum, and nickel. Chalcedony occurs as a lining or filling of cavities in rocks. The principal varieties are agate, which is banded and striped, having alternating layers of chalcedony and opal; jasper, or other forms of quartz; carnelian, which is clear and of various shades of red; chrysoprase, an apple green variety, in which the color is due to nickel oxide; heliotrope or bloodstone, of a dark green color, with small spots of jasper; onyx, consisting of bands of opal and chalcedony of different colors, usually black and white; plasma, of a deep green color; and sardonyx, a red-and-white-banded variety of chalcedony.

The many colors and the high luster that chalcedony takes by polishing render it valuable for brooches, necklaces, and other ornaments, and some varieties are cut as sealstones. Chalcedony is found in many parts of the world; superior varieties prized for gems are mined in Uruguay and in the Lake Superior area of the United States and Canada.

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NOTES:

• translucent – просвечивающий, полупрозрачный;

• jasper – яшма.

Text 8. Sand

Sand is loose, incoherent mass of mineral materials in a finely granular condition, usually consisting of quartz (silica), with a small proportion of mica, feldspar, magnetite, and other resistant minerals. It is the product of the chemical and mechanical disintegration of rocks under the influences of weathering and abrasion erosion. When freshly formed the particles are usually angular and sharply pointed, becoming smaller and more rounded by attrition by the wind or by water.

Sand is an important constituent of most soils and is extremely abundant as a surface deposit along the courses of rivers, on the shores of lakes and the sea, and in arid regions.

One specific form of sand is the major ingredient in glassmaking. Other types of sand are used in foundries to make casting molds and in ceramics, plasters, and cements. Sand is used as a grinding and polishing abrasive in the form of sandpaper, which is a sheet of paper covered on one side with sand or a similar abrasive substance. Sandblasting is an important technique used for cleaning stone or for smoothing rough metal surfaces by blowing a stream of sand under air or steam pressure.

(970)

Text 9. Sandstone

Sandstone is a coarse-grained, sedimentary rock consisting of consolidated masses of sand deposited by moving water or by wind. The chemical constitution of sandstone is the same as that of sand; the rock is thus composed essentially of quartz. The cementing material that binds together the grains of sand is usually composed of silica, calcium carbonate, or iron oxide. The color of the rock is often determined largely by the cementing material, iron oxides causing a red or reddish-brown sandstone, and the other materials producing white, yellowish, or grayish sandstone. When sandstone breaks, the cement is fractured and the individual grains remain whole, thus giving the surfaces a granular appearance. Sandstones of various geologic ages and of commercial importance are widely distributed in the U.S.A. and Europe. Besides serving as a natural reservoir for deposits of oil and gas, sandstone is used in building flagstone pavings and in the manufacture of whetstones and grindstones.

(850)

NOTES:

• whetstone – точильный камень;

• grindstone – шлифовальный камень.