The clays of the carboniferous basin of Coahuila

Abstract

Clays associated with the Carboniferous Cueca of Coahuila, in the Olmos formation, were sampled near Monclova, Coahuila, from top to bottom and between the coal layers. Their mineralogical study through X-ray diffraction, microscopy, and chemical and electron diffraction indicates they contain abundant quartz and, as the clay mineral, disordered dioctahedral kaolinite in both axes, along with interstratified phyllosilicates in three layers resembling illite: montmorillonite, with interstratification varying from 0.7:0.3 to 0.5:0.5, 10:15.4Å. The kaolinite has a pseudohexagonal morphology, with a tendency to coil into tubular shapes, while the interstratified minerals are of indefinite shape and show lower absorption in the electron beam. It is established that their paragenesis involves alteration of volcanic rocks, sedimentation, and metamorphism in a marine environment, under transgressive seas, during the Upper Cretaceous, aligning with what other authors have proposed for the Carboniferous formation of Coahuila.