Chegemite Ca (sub 7) (SiO (sub 4) ) (sub 3) (OH) (sub 2) ; a new humite-group calcium mineral from the Northern Caucasus, Kabardino-Balkaria, Russia

The new mineral chegemite Ca (sub 7) (SiO (sub 4) ) (sub 3) (OH) (sub 2) (Pbnm, Z=4)1, a=5.0696(1), b=11.3955(1), c=23.5571(3) Aa; V=1360.91(4) Aa (super 3) -- 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 Ca (sub 7) (Si (sub 0.997) Ti (sub 0.003) O (sub 4) ) (sub 3) (OH) (sub 1.48) F (sub 0.52) . Chegemite is characterized by the following optical properties: 2V (sub Z) =-80(8) degrees , alpha =1.621(2), beta =1.626(3), gamma =1.630(2); Delta =0.009; density D (sub calc) =2.892 g/cm (super 3) . The crystal structure, including hydrogen positions, has been refined from single-crystal MoKalpha X-ray diffraction data to R=2.2%. Octahedral Ca-O distances are similar to those of gamma -Ca (sub 2) SiO (sub 4) (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 (super -1) . Those in non-polarized Raman spectra are at 389, 403, 526, 818, 923.5, 3478, 3551 and 3563 cm (super -1) . The X-ray diffraction powder-pattern (FeKalpha -radiation) shows the strongest lines {d[Aa](I (sub 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).