Genesis and compositional characteristics of natural γ-CrOOH

Natural γ-CrOOH was first found and studied in rischorrite of the Khibina Massif, in which this mineral is associated with gonardite, natrolite, phillipsite, and saponite and usually forms microcrystallites in a matrix of amorphous CrOOH hydrogel or, more rarely, occurs as fine-crystalline, thin acicular, and lumpy aggregates of a green and emerald-green color. X-ray powder diffraction data indicate that crystalline blocks consist of a mineral with a rhombohedral unit cell, Cmcm, a = 3.86 Å, b = 12.78 Å, and c = 3.04 Å. Chromium oxyhydroxide from the Khibina Massif is isostructural with γ-CrOOH from the Iksinskoe deposit, but significantly differs from it in bearing low Al concentrations. The empirical formula of the Khibina γ-CrOOH is (Cr0.94Mg0.03Al0.02Ti0.01Fe0.01)1.01O(OH) · nH2O. The Cr oxyhydroxide and associated zeolites crystallized from low-temperature hydrothermal solutions rich in Na. The most probable source of Cr for the γ-CrOOH in the Khibina Massif was titanomagnetite in xenoliths of ultrabasic rocks in the rischorrites affected by postmagmatic alterations.