A high-Cr mineral assemblage is observed in the unusual Matoush uranium deposit (Quebec, Canada), which is associated with the bimodal Matoush dike intruding Otish Basin sandstones. In addition to uraninite, both silicate (chromium-dravite and chromphyllite) and oxide (eskolaite) high-Cr minerals are present in major amounts. While high-Cr dravite and high-Cr muscovite have been described previously, eskolaite, Cr2O3, has not been studied extensively. The highest Cr contents observed in the Matoush silicates, obtained by electron microprobe, are comparable with the highest ever documented in the literature, with Cr2O3 of up to ca. 22 wt.% in chromphyllite and ca. 37 wt.% in chromium-dravite. Complete solid solutions among hematite (Fe2O3), eskolaite, and karelianite (V2O3) are theoretically possible, but until now only complete Cr-V substitution had been documented in natural samples. Matoush eskolaite contains significant amounts of Fe, with up to 37% wt.% Fe2O3 substitution, and minimal V substitution (typically under 0.5 wt.% V2O3).
A hydrated Fe-Cr oxide is also observed closely associated with eskolaite at Matoush, having the same Cr:Fe:V ratio as eskolaite, but with significant assumed H2O. Rietveld analysis of X-ray powder diffraction data is best fit using a model that includes eskolaite and a second hydrated crystalline Fe-Cr oxide with a similar atomic structure to eskolaite, but a significantly larger c-axis dimension.