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

Abstract

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) A; V = 1360.91(4) A3 – 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(Si0.997Ti0.003O4)3(OH)1.48F0.52. Chegemite is characterized by the following optical properties: 2VZ = −80(8)°, α = 1.621(2), β = 1.626(3), γ = 1.630(2); Δ = 0.009; density D calc = 2.892 g/cm3. The crystal structure, including hydrogen positions, has been refined from single-crystal Mo K α X-ray diffraction data to R = 2.2 %. Octahedral Ca–O distances are similar to those of γ-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 (Fe K α-radiation) shows the strongest lines {d \[A\]( 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).