A light-coloured, hydrothermally altered, igneous rock composed of quartz, strongly kaolinized orthoclase (see alkali feldspar), fresh euhedral albite (see plagioclase feldspar), and minor muscovite, topaz, and fluorite. The rock is essentially a granite which is in an arrested state of alteration by the process of kaolinization. The orthoclase, which shows strong kaolinization, can be veined by secondary quartz and fluorite, both products of the kaolinization process. Superb examples are found in the St Austell granite of Cornwall, England, where the product of granite kaolinization, china clay (kaolin), is extracted for use in the ceramic, paper, pharmaceutical, and other industries. See also kaolinite.
Subjects: Earth Sciences and Geography.