Rectorite-Pyrophyllite-Chlorite-lllite Assemblage in Pelitic Rocks from Fómeque Formation, Central Eastern Cordillera, Colombia
Palabras clave:
Clay Mineral Analysis, Fómeque Formation, Eastern Cordillera-Colombia, Illite Crystallinity, High Diagenetic-Anchizonal Metamorphism (en)Clay Mineral Analysis, Fómeque Formation, Illite Crystallinity, High Diagenetic-Anchizonal Metamorphism (es)
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The research described in this paper was performed on samples of slate from the Fómeque formation near Bogotá in Colombia. Pyrophyllite was found to occur together with mixed layered illite/smectite, chlorite, and illite. Other minerals were quartz, K-feldspar, dolomite and pyrite. Evaluation of the x-ray diffraction patterns reveals that the mixed layered sheet-silicate represents an R1 ordered rectorite with 80-90% illite layers. The microfabric i.e. the formation of cleavage is developed as a closely spaced cleavage in the phyllosilicate-rich rocks, which grades into a fracture cleavage as the grain size becomes coarser. In the interbedded siltstone no cleavage is observed. It is suggested that the stage of microfrabric developed in these rocks represents high diagenetic to anchizonal conditions. The illite crystallinity has been measured on glycolated samples from the slate and ranges from 0.47 to 0.55°∆2θ, with a mean of 0.52°∆2θ. 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 high diagenetic condition i.e. a temperature of about 200°C at low pressure. At this temperature pyrophyllite can only be stabilized at the expense of kaolinite and quartz if aH20« 1.