Zn-rich h�gbomite formed from gahnite in the metabauxites of the Menderes Massif, SW Turkey

Abstract

Gahnite, ZnAl2O4, present as an accessory mineral in regionally metamorphosed low-grade diasporites, has reacted in adjacent higher-grade, corundum-bearing metabauxite equivalents (emeries) to form Zn-rich hogbomite, (Zn,Fe2+,Mg,Ni)t-2x (Ti,Sn)xAl2O4, of the 4H polytype. Commonly, the initial hogbomite crystals grew epitactically along the octahedral faces of gahnite, which was subsequently dissolved, so that hogbomite now forms spectacularly intergrown sets of eight crystals in perfect crystallographic orientation to each other. This indicates a metamorphic reaction, probably involving a fluid, transporting mainly the elements Zn and Al. Reactant Ti minerals in the diasporites were rutile and titanian hematite (10–15 mol% FeTiO3). In the emeries hogbomite coexists with still more Ti-rich hematites containing between 26 and 37 mol% FeTiO3. The overall reaction relations involving partial reduction may be subdivided into the intial univariant reaction, gahnite+diaspore+Ti-hematite+rutile=hogbomite+H2O+O2. This was followed, in the absence of gahnite, by compositional readjustments of hogbomite and Ti-hematite and the appearance of magnetite. Core to rim zoning profiles indicate that, with continued growth, the hogbomite crystals became poorer in Zn and Ti, but richer in Fe2+, while the Ti-contents of coexisting hematite increased. Hogbomite formation at the expense of gahnite started at temperatures as low as about 400° C for an estimated pressure of 5–6 kbar.