SAND. If rocks and minerals are broken down by either natural or artificial agencies, the products may be classified as gravels, sands, silts and clays. The term sand is usually applied to the material of diameter ranging from about to 2 mm. Although most of the rock-making minerals occurring on the earth's crust are found in sands, only a limited number are met with at all frequently. For several reasons quartz is by far the commonest ingredient ; it is abundant in rocks, is comparatively hard and has practically no cleavage so that it is not readily worn down to a fine state. Moreover it is nearly insoluble in water and does not decompose. In certain localities felspar, calcareous material, iron ores and volcanic glass, among other substances, have been found to be dominant constituents of sand. Most quartzose sands contain a small quantity of felspar. Small plates of white mica, which, though soft and very fissile, decompose very slowly, are often present. In addition all sands contain small quantities of "heavy" rock-forming minerals among which may be garnet, tourmaline, zircon, rutile, topaz, pyroxenes, amphiboles, iron ores, etc.
In certain shore and river sands these heavier constituents become concentrated as a result of current action and the removal of the lighter constituents. Economically valuable deposits may
then be yielded. Such are the sands worked for diamonds and other gem-stones, those bearing gold, platinum, tin, monazite and other ores. The greensands, widely distributed over the floor of the ocean and found in ancient strata on the continents, owe their colour to the presence of glauconite, a potash-bearing mineral. These sands are used for water-softening and for land-dressing, and attempts have been made to extract potash from them.
In the pottery, glass-making and silicate (water glass) indus tries very pure quartzose sands are used in large quantities as a source of silica. Similar sands are required for lining the hearths of acid-steel furnaces and for foundry mixtures. Moulding-sands, that is, the sands utilized in foundries for making the moulds in which metal is cast, usually have a clayey bond uniting the grains of quartz. On account of the hardness and poor cleavage of quartz, sands are used extensively as abrasives. Garnet-sands, although of more restricted occurrence, are similarly used. Ordi nary sands find a multitude of other uses, among which may be mentioned the preparation of mortar, cement and concrete. They are frequently sources of water-supply. (P. G. H. B.)