DIAGENETIC/ANCHIMETAMORPHIC CHANGES ON THE PROTEROZOIC GLAUCONITE AND GLAUCONY FROM THE PARANOÁ GROUP, MID-WESTERN BRAZIL

Abstract

Petrographic, X-ray and chemical studies were made on glauconies from sandstones belonging to the same tectonic and stratigraphic level of the Paranoá Group, a Proterozoic sedimentary unit of Mid-West of Brazil. Three types of mineralogically different glauconitic sandstones were recognized, i.e., white feldspatic, green-reddish ferric and green-grayish calcitic sandstones, with different content of glauconies. In each sandstone, glauconite shows a non-expansible structure, as well as high contents of Al and K, but other chemical features and mineralogical assemblages vary from one rock to another. Differences found in chemical composition of the glauconies from different rocks and their mineralogical evolution are attributed to lithologically dependent changes since all the studied sandstones have the same origin, and have undergone the same diagenetic/anchimetamorphic conditions.