Vorlanite (CaU⁶⁺)O₄—A new mineral from the Upper Chegem caldera, Kabardino-Balkaria, Northern Caucasus, Russia

Abstract

The new mineral vorlanite, (CaU⁶⁺)O₄, D_calc = 7.29 g/cm³, H = 4-5, VHN₁₀ = 360 kg/mm², was found near the top of Mt. Vorlan in a calcareous skarn xenolith in ignimbrite of the Upper Chegem caldera in the Northern Caucasus, Kabardino-Balkaria, Russia. Vorlanite occurs as aggregates of black platy crystals up to 0.3 mm long with external symmetry 3̄m. The strongest powder diffraction lines are [d(Å)/(hkl)]: 3.107/(111), 2.691/(200), 1.903/(220), 1.623/(311), 1.235/(331), 1.203/(420), 1.098/(422), 0.910/(531). Single-crystal X-ray study gives isometric symmetry, space group Fm3̄m, a = 5.3813(2) Å, V = 155.834(10) ų, and Z = 2. X-ray photoelectron spectroscopy indicate that all U in vorlanite is hexavalent. The mineral is isostructural with fluorite and uraninite (U⁴⁺O₂). In contrast to synthetic rhombohedral CaUO₄, and most U⁶⁺ minerals, the U⁶⁺ cations in vorlanite are present as disordered uranyl ions. ^[8]Ca²⁺ and ^[8]U⁶⁺ are disordered over a single site with average M-O = 2.33 Å.

Vorlanite is believed to be a pseudomorphic replacement of originally rhombohedral CaUO₄. We assume that this rhombohedral phase transformed by radiation damage to cubic CaUO₄ (vorlanite). The new mineral is associated with larnite, chegemite, reinhardbraunsite, lakargiite, rondorfite, and wadalite, which are indicative of high-temperature formation (>800 °C) at shallow depth.