Kaolinite, Al(2)Si(2)O(5)(OH)(4), is a widespread clay mineral in clastic sedimentary rocks, where it occurs as a detrital constituent in near shore deposits as well as an authigenic phase in sandstones. Pyrophyllite, Al(2)Si(4)O(10)(OH)(2) has long been regarded as essentially hydrothermal mineral but its occurrence during incipient regional metamorphism is now well established.

X-ray analyses Regional distribution Coal rank data Fluid inclusions data Water activity SEM data

The reaction-isograd kaolinite + quartz = pyrophyllite + H(2)O has been mapped in the Helvetic Alps over more than 200 km. Metamorphic conditions at the reaction-isograd were estimated from vitrinite reflectance and fluid inclusions data to be 1.3 - 2.1 kbar and 240 - 270oC at a water activity of 0.6 - 0.8. Textural evidence obtained with a scanning electrone microscope suggests that pyrophyllite grew through a dissolution-precipitation process