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The AAPG/Datapages Combined Publications Database

Journal of Sedimentary Research (SEPM)


Journal of Sedimentary Petrology
Vol. 38 (1968)No. 1. (March), Pages 13-16

The Effect of Chemical Reduction on the Stability of Pyrophyllite and Kaolinite in Pelitic Rocks

B. Velde


The infrequent occurrence of kaolinite and especially of pyrophyllite in more evolved sedimentary rocks and low grade metamorphic facies of pelitic rocks is considered to be due to chemical controls in the bulk composition of these rocks.

Hydrogen-rich organic gases are released from the original sedimentary organic materials during diagenesis and early metamorphism. These gases in turn reduce the iron present from the ferric to ferrous state. The effect of the valence change of the iron is to shift its position in the silicate minerals present. As a result, aluminum is obliged to readjust to the new conditions moving from an alumino-silicate phase to the mica and perhaps the chlorite phases. The new assemblage does not contain a hydrous alumino-silicate though both kaolinite and pyrophyllite could be stable in the physical environment of the rocks.

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