Chromium in Corundum: Ultra-high Contents Under Reducing Conditions

An exploration project run by Shefa Yamim (A. T. M.) Ltd has recovered a variety of gemstone minerals from Cretaceous pyroclastic vents and associated alluvial deposits at Mt Carmel, Israel [1]. Among these are several types of corundum (Al2O3), including rubies with <2 wt% Cr2O3 and sapphires in a variety of colours from yellows through to greens, blues and purples, with a range of chemical impurities e.g. Ti, Fe, V, Ga. The most scientifically interesting type of corundum is the inclusion-rich ‘Carmel SapphireTM’, which contains a variety of mineral phases; some of these have only been seen in meteorites previously, e.g. tistarite (Ti2O3) [2], and others have not previously been described, e.g. carmeltazite (ZrAl2Ti4O11) [3]. These minerals indicate very low oxygen fugacities, at least 7 log units below the Iron-Wustite buffer (DIW-7), and are interpreted as reflecting the presence of CH4+H2-rich fluids [1,4]. These discoveries have led to a new understanding of fluid transfer and redox conditions in the crust and mantle. Here we describe another variety of Cr-rich corundum (Fig. 1) with Cr concentrations up to 32 wt.% Cr2O3, representing a composition in the solid solution series between corundum and eskolaite (Cr2O3), and considerably more Cr-rich than previously known examples. These crystals are a deep purple (Fig. 1), but while purple in corundum usually is due to a combination of Ti and Cr, in this case the crystals are Ti-free and contain much higher concentrations of Cr. The cores of the crystals have relatively low Cr concentrations (1-2 wt.% Cr2O3) and the Cr concentration increases towards the rim. In the highest-Cr areas, the material consists of subgrains with small but distinct variations in Cr content (Fig. 1a, 2). On the surface of the illustrated crystal there are abundant balls (<10µm to 100’s of µm) of native Cr; Transmission Electron Microscopy (TEM) studies show that these are associated with chromium nitride CrN (carlsbergite; Fig. 2), otherwise known only from iron meteorites. Electron Energy Loss Spectroscopy (EELS) analyses show that the valence of the Cr changes from Cr3+ in the corundum (both low-Cr and high-Cr types) to Cr2+ in the carlsbergite and finally Cr0 in the chromium metal. The coexistence of all three valence states suggests that the oxygen fugacity was constrained by the CrO/Cr buffer, and that Cr was undergoing a crystallographically-controlled disproportionation, Cr2+ à Cr3+ + Cr0 The oxygen fugacity implied by this reaction lies at ca DIW-5, less reducing than the conditions inferred from the Ti3+-bearing, but Cr-free, assemblages in the Carmel Sapphire. These unusual high-Cr rubies thus appear to represent an earlier stage in the crystallization of the Mt Carmel magmas.