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kaolin(kā`əlĭn): see china claychina clay,
one of the purest of the clays, composed chiefly of the mineral kaolinite usually formed when granite is changed by hydrothermal metamorphism. Usage of the terms china clay and kaolin
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(from Kaoling, the name of the area in China, in Kiangsi Province, where it was first found), a rock consisting basically of the mineral kaolinite, commonly with admixtures of quartz granules, feldspar, micas, and, in small quantities, other mineral impurities.
Kaolins are formed through the weathering of various magmatic, feldspathic rocks (primarily low-ferriferous granites; more rarely, argillaceous sediments or arkoses called primary kaolins). They are also formed through the rewashing of these rocks, the kaolins then being redeposited in sedimentary (chiefly sandy) strata (secondary kaolins, or kaolin clays). In primary kaolins the structure of the mother rock is usually clearly visible.
Kaolin deposits are encountered rather frequently. The rock is mined in England (Cornwall), the German Democratic Republic (near Dresden), Czechoslovakia (around Karlovy Vary), and the USA (Georgia). In the USSR, kaolin is mined in the Ukrainian SSR, the Urals, and the Kazakh SSR.
Primary and secondary kaolins are used in an unprocessed form for manufacturing refractory materials. However, in the majority of cases, it is enriched at plants usually located not far from the deposits. According to industrial requirements, the enriched kaolin should contain no more than 0. 3–1. 0 percent iron oxide and titanium oxide (depending upon the grade) and should be free of sand and other impurities, particularly of those that are soluble in water and weak acids. A number of types of production require powdered kaolin of a high degree of whiteness. The most important user of kaolin is the paper industry, which consumes around 40–50 percent of the total output; the kaolin is used to chalk the paper surface and as a filler. In many grades of paper, kaolin makes up as much as 30–40 percent of the total bulk of the paper and, to a significant degree, determines the paper’s quality. In ceramics, kaolin is used in porcelain and earthenware (5–10 percent of the total kaolin output), of which it is a basic component. Around 20 percent of the kaolin mined is used in the rubber industry. In addition, kaolin is used in perfumery, medicine (called “china clay”), and the chemical industry (for making aluminum sulfate).
REFERENCEKurs mestorozhdcnii nemetallicheskikh poleznykh iskopaemykh. Edited by P. M. Tatarinov. Moscow, 1969.
V. P. PETROV