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

Abstract

The new mineral vorlanite, (CaU 6+ )O 4 , D calc = 7.29 g/cm 3 , H = 4–5, VHN 10 = 360 kg/mm 2 , 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 3 m. The strongest powder diffraction lines are [ d (A)/( 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 Fm 3 m , a = 5.3813(2) A, V = 155.834(10) A 3 , and Z = 2. X-ray photoelectron spectroscopy indicate that all U in vorlanite is hexavalent. The mineral is isostructural with fluorite and uraninite (U 4+ O 2 ). In contrast to synthetic rhombohedral CaUO 4 , and most U 6+ minerals, the U 6+ cations in vorlanite are present as disordered uranyl ions. [8] Ca 2+ and [8] U 6+ are disordered over a single site with average M-O = 2.33 A. Vorlanite is believed to be a pseudomorphic replacement of originally rhombohedral CaUO 4 . We assume that this rhombohedral phase transformed by radiation damage to cubic CaUO 4 (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.