Sobre análisis microestructural por DRX y condiciones de formación de mullitas naturales de la cuenca de Oliete (Teruel)

Abstract

Several occurrences of natural mullites have been found in Spain corresponding to products of thermal transformation of clays by the action of coal burning. A microstructural X-ray diffraction analysis of mullite from two new localities in the Oliete basin (Lower Cretaceous) is presented. The 220 reflection of mullite has been studied by using the Voigt function method. The observed crystallite size [Dv] values range from 442 to 1014 A and the observed values for microstrains (e) range from 0.0012 to 0.0025. The microstructural characteristics could be used to evaluate the formation conditions of mullite by comparison to experimental bibliographic results obtained at normal pressure and maximal temperatures and times of firing range from 995 to 1050ºC, with times from 50 hours to 1000 hours, for the Castel de Cabras deposit and from 1100º C, 100 hours to 1300º C, 2 hours in the Oliete deposit, with normal pressure conditions. The corundum + mullite asociation found could be originated from thermal transformation of kaolins under fluid pressure and above 575° C according to the Si02-Al2O3-H2O system, and that enables the possibility of using experimental data from experiments at normal pressure in the interpretation of time - thermal conditions of formation in our materials. The release of gases from the burning coal as well as from the thermally affected rocks allows to explain as hydrothermal sorne secondary minerals present in mullitic materials and in related mineral assemblages.