Características morfológicas y cristalográficas del grafito de Osumita, Venezuela, y su uso como geotermómetro

Abstract

A morphologic, petrographic and crystallographic study was performed in some representative samples of disseminated graphite of the Cerro Osumita, Cojedes State, Venezuela with the aim of establishing the degree of mineral crystallinity (graphitization) and the highest temperature reached by the rock. From the original rock (La Aguadita quartz-feldespatic-graphitic Gneiss), fractions enriched in graphite were obtained by means of leaching with HCI-HF and physical procedures (flotation and accumulation in liquid interfaces). The isolated graphite was analyzed by X-Ray Diffraction, whereas morphology was studied by Scanning Electronic Microscopy (SEM). The mineralogical association exhibited by the gneiss (quartz- oligoclase- hornblende (green)- microcline - chlorite- biotite- epidote) allows establishing the metamorphism of the rock in the upper zone of the green schist facies, in the biotite zone. Graphite grains present aspect of flakes, with a concentration oscillating between 7 and 10% in weight. The used enrichment process allowed obtaining fractions with more than 92% in graphi- te. Under Scanning Electronic Microscopy, graphite samples showed morphology dominantly as sheets, very well defined and ordered. The crystallinity index (measured by XRD) is around 58 and 60, with a mean graphitization degree of 60, suggesting a high order in the crystalline net. The higher temperature reached by the rock, according to the graphitization degree, is of 472 f 27 "C.