Tourmaline is a complex hydrous silicate of aluminum and boron, with varying amounts of magnesium, iron, and alkalies. It is a rather common mineral in silicated zones in limestones near igneous contacts, but gem tourmalines are found principally in pegmatite dikes. They have a wide variety of colors, the red and green gems being the most prized. Maine, California, and Connecticut are the principal American producers.
Turquoise is a hydrated copper-aluminum phosphate. It is found in veinlets near the surface in altered granites and other igneous rocks. It is usually associated with kaolin and frequently with quartz, and is believed to have been formed by surface alterations. In the United States it is produced chiefly in Nevada, Arizona, and Colorado.
In general the principal gem minerals, except pearl and turquoise, occur as original constituents in igneous intrusives, usually of a pegmatite or peridotite nature. Sapphire, ruby, emerald, and tourmaline result also from contact metamorphism of sediments in the vicinity of igneous rocks. Weathering softens the primary rocks, making it possible to separate the gem stones from the matrix. When eroded and transported the gems are concentrated in placers.
SALT
Economic Features
The principal uses of salt are in the preserving and seasoning of foods and in chemical industries. Chemical industries require salt for the manufacture of many sodium compounds, and also as a source of hydrochloric acid and chlorine. A minor use of salt is in the making of glazes and enamel on pottery and hardware.
Because of the wide distribution of salt in continental deposits and because of the availability of ocean and salt-lake brines as other sources, most countries of the world either possess domestic supplies of salt adequate for the bulk of their needs, or are able to obtain supplies from nearby foreign countries. Certain sea salts preferred by fish packers and other users are, however, shipped to distant points. About a fifth of all the salt consumed in the world annually is produced in the United States, and other large producers are Great Britain, Germany, Russia, China, India, and France.
The United States produces almost its entire consumption of salt, which is increasing at a very rapid rate. Salt is produced in fourteen states, but over 85 per cent of the total output comes from Michigan, New York, Ohio, and Kansas. Reserves are practically inexhaustible.
Exports and imports of salt form a very minor part of the United States industry, each being equivalent to less than 5 per cent of the domestic production. A large part of the imported material is coarse solar-evaporated sea salt, which is believed by fish and pork packers to be almost essential to their industry. Imports of this salt come from Spain, Italy, Portugal, and the British and Dutch West Indies; during the war, on account of ship shortage, they were confined chiefly to the West Indies. A considerable tonnage of specially prepared kiln-dried salt, desired by butter-makers, is imported from Liverpool, England. There are also some small imports from Canada, probably because of geographic location. Exports of domestic salt go chiefly to Canada, Cuba, and New Zealand, with smaller amounts to practically all parts of the world.
Salt is recovered from salt beds in two ways. About a fourth of the salt produced in the United States is mined through shafts in the same manner as coal, the lumps of salt being broken and sized just as coal is prepared for the market. The larger part of the United States production, however, is derived by pumping water down to the beds to dissolve the salt, and pumping the resulting brine to the surface where it is then evaporated. A considerable amount of salt, also, is recovered from natural brines—which represent the solution of rock salt by ground-waters—and from the waters of salt lakes and the ocean.