kaolinite

kaolinite (dickite, nacrite, China clay, kaolin) A very important group of clay minerals belonging to the 1:1 group of phyllosilicates (sheet silicates), and with the general formula Al₄[Si₄O₁₀](OH)₈, kaolinite represents the final product from the chemical weathering of feldspars to give clays; sp. gr. 2.6–2.7; hardness 2.0–2.5; monoclinic; white, greyish, or stained a variety of colours; dull earthy lustre; on a microscopic scale crystals are hexagonal plates, but it is usually massive; cleavage {001}; occurs as a secondary mineral produced by the alteration of aluminosilicates, especially alkali feldspars; is widely distributed in igneous rocks, gneisses, and pegmatites, and in sedimentary rocks due to the action of acidic solutions on a wide variety of rocks by processes of weathering and low-temperature hydrothermal reactions. It is distinguished by its plastic feel, but normally it has to be identified by optical and physical tests. It is extensively used when pure as a cheap, general-purpose filler and coating material for paper, in ceramics, and also in chemicals and paints. It is distinctive in soils for its physical stability during wetting and drying and for its small cation-exchange capacity. Kaolinite dominates the clay minerals present in certain acid and very old soils, but not in Oxisols, some Latosols, or soils formed in arid climates. The name is derived from the Chinese kau-ling (‘high ridge’), referring to the hill from which the first samples were taken.