Urusovite, Cu[AlAsO5], a new mineral from the Tolbachik volcano, Kamchatka, Russia

Urusovite, ideally Cu[AlAsO5], has been found in a fumarole in the North Br each of the great fissure Tolbachik eruption (1975-76, Kamchatka peninsula, Russia). It occurs as light-green plates up to 0.4 mm in maximal dimension , tabular on {100}, elongated along [001]; well-developed forms are {100}, {010}, {110}, {011}, {111}. Associated minerals are: ponomarevite, piypite, sylvite, dolerophanite, euchlorine, tenorite, hematite and two unknown As- bearing minerals. Urusovite is brittle and has a perfect cleavage on (100), a vitreous luster and a white streak. H = 378 kg/mm(2). Biaxial, optically negative. alpha = 1.672(2), beta = 1.718(2), gamma = 1.722(2), 2V(means) s imilar to 30(2)degrees, 2V(calc) = 32.2 degrees. Optical orientation is b = beta, c similar to alpha. Pleochroism is in the light-green tones: alpha - colourless, beta - light-green, gamma - light-green. The mineral is monocl inic, space group P2(1)/c, a = 7.314(2), b = 10.223(3), c = 5.576(2) Angstr om, beta = 99.79(3)degrees, V = 410.9(2) Angstrom(3). The diagnostic lines of the X-ray powder diffraction pattern are (I-d-hkl): 100-7.20-100; 9-4.84 4-011; 23-4.327-111; 10-3.604-200; 10-3.174-121; 20-3.125-211; 6-2.656-012; 8-2.458-221. Urusovite is isostructural with Fe2+[AlPO5]. Microprobe analy sis gave the following chemical composition (wt. %): CuO 32.23 (30.97-32.82 ), Al2O3 20.89 (20.44-21.44), Fe2O3 0.32 (0.10-0.72), ZnO 0.25 (0.10-0.43), As2O5 46.02 (44.97-47.17), V2O5 0.12 (0.00-0.40), Sigma 99.83 (98.71-100.6 4). The empirical formula of urosovite, (Cu0.99Zn0.01)(Sigma 1.00)Al1.00As0 .98O5.00. calculated on the basis of 5 O atoms, is close to the ideal one, Cu[AlAsO5], confirmed by crystal-structure analysis. The mineral is named u rusovite in honour of Vadim Sergeevich Urusov (1936-), crystal chemist, Cor responding Member of the Russian Academy of Sciences, chair of the Departme nt of Crystallography and Crystal Chemistry of Moscow State University.