Megawite, CaSnO_3: a new perovskite-group mineral from skarns of the Upper Chegem caldera, Kabardino-Baikaria, Northern Caucasus, Russia

摘要

Megawite is a perovskite-group mineral with an ideal formula CaSnO_3 that was discovered in altered silicate-carbonate xenoliths in the Upper Chegem caldera, Kabardino-Balkaria, Northern Caucasus, Russia. Megawite occurs in ignimbrite, where it forms by contact metamorphism at a temperature >800°C and low pressure. The name megawite honours the British crystallographer Helen Dick Megaw (1907-2002) who did pioneering research on perovskite-group minerals. Megawite is associated with spurrite, reinhardbraunsite, rondorfite, wadalite, srebrodolskite, lakargiite, perovskite, kerimasite, elbrusite-(Zr), periclase, hydroxylellestadite, hydrogrossular, ettringite-group minerals, afwillite, hydrocalumite and brucite. Megawite forms pale yellow or colourless crystals up to 15 μm on edge with pseudo-cubic and pseudo-cuboctahedral habits. The calculated density and average refractive index are 5.06 g cm~(-3) and 1.89, respectively. Megawite is Zr-rich and usually crystallizes on lakargiite, CaZrO_3. The main bands in the Raman spectrum of megawite are at: 159, 183, 262, 283, 355, 443, 474, 557 and 705 cm~(-1). The unit-cell parameters and space group of megawite, derived from electron back scattered diffraction, are: a = 5.555(3), b = 5.708(2), c = 7.939(5) A, V= 251.8(1) A~3, Pbnm,Z = 4; they are based on an orthorhombic structural model for the synthetic perovskite CaSn_(0.6)Zr_(0.4)O_3.