THE ROLE OF ALKALIS AND CALCIUM IN ANDALUSITIZATION OF CLAY SUBSTANCE

摘要

An andalusite concentrate from the Republic of South Africa with the grain size from 1 to 4 mm was subject in laboratory to further andalusitization as it contains thermally affected but not fully transformed remnants of clays. The sample was treated with basic aluminium acetate in an autoclave at 250℃ and 3 MPa for 4 hours at pH close to 3. Under those conditions andalusitization proceeds indirectly, involving crystallization of a whole range of alkali and calc-alkali micas gradually enriched in aluminium. As the changes take place within a group of minerals with the same layered structure, they require less energy than the direct transformation of kaolinite into andalusite (different structures). The laboratory observations point to the role of the alkalis and calcium (contained in micas) in natural andalusitization of clay substance.