In samples of slate from the Fomeque Formation near Bogota, Colombia, pyrophyllite was found to occur together with mixed-layered illite-smectite, chlorite and illite. Other minerals were quartz, K-feldspar, dolomite and pyrite. X-ray diffraction patterns revealed that the mixed-layer represents an R1 ordered rectorite with 80-90% illite layers. The microfabric is developed as a closely spaced cleavage in the phyllosilicate-rich rocks, and grades into a fracture cleavage with coarsening of the grain size. No cleavage was observed in the interbedded siltstones. It is suggested that the microfrabrics developed in these rocks correspond to high diagenetic to anchizonal conditions. The illite crystallinity from the slate has been measured on glycolated samples and ranges from 0.47 to 0.55 degrees Delta 2theta , with a mean of 0.52 degrees Delta 2theta . Based on the stability of R1 ordered rectorite, the illite crystallinity and the microfabric development, it is proposed that the rocks have been subjected to a temperature of approximately 200 degrees C at low pressure. At this temperature, pyrophyllite can only be stabilized at the expense of kaolinite and quartz if alpha H 2 O<<1.

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