A INTERAÇÃO DE CROMO-ESPINÉLIO COM FASES SILICÁTICAS NOS PROCESSOS METAMÓRFICOS

Abstract

MgAl 2 O 4 . is one of the most significative components in chrome spinel from ultramafic rocks. It reacts with silicate phases during metamorphic reequilibration depending on PT conditions and mineral phases present. The presence of Cr in some silicate phases (serpentine, chlorite, amphibole) suggests that it may be extrated from chrome spinel as well. In serpentinites, the Mg-Al component is incorporated into the silicate phases, leaving behind the chrome-iron component, which may exist as an only phase, or as two associated phases, depending on the Cr/Fe 3+ ratio and temperature. In the chlorite stability field, the spinel may exhibit compositions varying between chromite and magnetite members, with low Al contents. Under these conditions, Cr and Al contents and Al/(Al+Cr) increase in spinel with increasing metamorphic grade. With chlorite breakdown MgAl spinel is formed regardless of the previously existing spinel phases. The Mg-Al spinel thus formed may constitute individualized grains, or else grow over the pre-existing spinel grains. The occurence of chlorite + spinel associations is due to the multivariant character that this reaction acquires in the presence of other cations, such as Cr, Fe 2+ , Fe 3+ . In the Mg-Al spinel stability field associations with one or more spinels, with broad compositional variations, may occur. Low-P peridotites contain plagioclase and develop aluminous spinel through the olivine + plagioclase reaction, leading to the orthopyroxene + pargasitic amphibole + aluminous spinel association, often with coronitic textures. The formation of ferrichromite is intimately associated with pervasive and straithforward serpentinization processes, without previous reequilibration of the peridotites under progressively lower conditions.