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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 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) Å 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 Å.

Vorlanite (CaU 6+ )O 4 —A new mineral from the Upper Chegem caldera, Kabardino-Balkaria, Northern Caucasus, Russia