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Chegemite Ca₇(SiO₄)₃(OH)₂ – a new humite-group calcium mineral from the Northern Caucasus, Kabardino-Balkaria, Russia

Evgeny V. Galuskin^1,*, Viktor M. Gazeev², Biljana Lazic³, Thomas Armbruster³, Irina O. Galuskina¹, Aleksander E. Zadov⁴, Nikolai N. Pertsev², Roman Wrzalik⁵, Piotr Dzieranowski⁶, Anatoly G. Gurbanov² and Grayna Bzowska¹

¹ Department of Geochemistry, Mineralogy and Petrography, Faculty of Earth Sciences, University of Silesia, Bdziska 60, 41-200 Sosnowiec, Poland
² Institute of Geology of Ore Deposits, Geochemistry, Mineralogy and Petrography (IGEM) RAS, Staromonetny 35, Moscow, Russia
³ Mineralogical Crystallography, Institute of Geological Sciences, University of Bern, Freiestr. 3, CH-3012 Bern, Switzerland
⁴ "NPP Teplochim", Dmitrovskoye Highway 71, 127238 Moscow, Russia
⁵ Institute of Physics, University of Silesia, Uniwersytecka 4, 40-007 Katowice, Poland
⁶ Institute of Geochemistry, Mineralogy and Petrology, Warsaw University, al. wirki i Wigury 93, 02-089 Warszawa, Poland

^* Corresponding author, e-mail: galuskin{at}us.edu.pl

The new mineral chegemite Ca₇(SiO₄)₃(OH)₂ (Pbnm, Z = 4)1, a = 5.0696(1), b = 11.3955(1), c = 23.5571(3) Å; V = 1360.91(4) ų – the calcium and hydroxyl analogue of humite – was discovered as a rock-forming mineral in high-temperature skarns in calcareous xenoliths in ignimbrites of the Upper Chegem volcanic structure, Northern Caucasus, Kabardino-Balkaria, Russia. The chegemite forms granular aggregates with grain sizes up to 5 mm and is associated with various high-temperature minerals: larnite, spurrite, rondorfite, reinhardbraunsite, wadalite, lakargiite, and srebrodolskite, corresponding to the sanidinite metamorphic facies. The empirical formula of the holotype chegemite (mean of 68 analyses) is Ca₇(Si_0.997Ti_0.003O₄)₃(OH)_1.48F_0.52. Chegemite is characterized by the following optical properties: 2V_Z = –80(8)°, = 1.621(2), β = 1.626(3), = 1.630(2); = 0.009; density D_calc = 2.892 g/cm³. The crystal structure, including hydrogen positions, has been refined from single-crystal MoK X-ray diffraction data to R = 2.2 %. Octahedral Ca–O distances are similar to those of -Ca₂SiO₄ (calcio-olivine). As is characteristic of OH-dominant humite-group minerals, two disordered H positions could be resolved. The main bands in the FTIR-spectra of chegemite are at 3550, 3542, 3475, 927, 906, 865, 820, 800, 756, 705, 653, 561, 519 and 437 cm^–1. Those in non-polarized Raman spectra are at 389, 403, 526, 818, 923.5, 3478, 3551 and 3563 cm^–1. The X-ray diffraction powder-pattern (FeK-radiation) shows the strongest lines {d [Å](I_obs)} at: 1.907(10), 2.993(8), 2.700(8), 3.015(7), 2.720(7), 2.834(6), 3.639(5), and 3.040(5).

Key-words: chegemite, new mineral, humite-group, crystal structure, CSH, skarn, xenolith, Caucasus, Russia.