Hydrothermal association of pyrophyllite, kaolinite, diaspore, dickite, and quartz in the Coromandel Area, New Zealand

Hydrothermal action has formed a clay deposit with a zone containing over 80% pyrophyllite and other zones containing kaolinite‐dickite‐quartz and kaolinite‐diaspore from andesite in the Coromandel area New Zealand. The deposit was formed deep in the earth at pressures over 2,000 p.s.i. and temperatures above 335°C. The pyrophyllite in the deposits has the formula [Al(OH) 2 )]+ 0.1 Al 2 [Si 3.90 Al 0.10 ] O 9.05 (OH) 2.10 , and an X‐ray diffraction pattern and differential thermogram similar to, but not identical with, those given in the literature. Dickite and lepidocrocite were identified in the deposits by X‐ray diffraction techniques; diaspore by X‐ray diffraction and optical techniques.