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Kumtyubeite, Ca 5 (SiO 4 ) 2 F 2 -the fluorine analog of reinhardbraunsite with a chondrodite-type structure-is a rock-forming mineral found in skarn carbonate-xenoliths in ignimbrites of the Upper Chegem volcanic structure, Kabardino-Balkaria, Northern Caucasus, Russia. The new mineral occurs in spurrite-rondorfite-ellestadite zones of skarn. The empirical formula of kumtyubeite from the holotype sample is Ca 5 (Si 1.99 Ti 0.01 ) Σ2 O 8 (F 1.39 OH 0.61 ) Σ2 . Single-crystal X-ray data were collected for a grain of Ca 5 (SiO 4 )2(F 1.3 OH 0.7 ) composition, and the structure refinement, including a partially occupied H position, converged to R = 1.56%: monoclinic, space group P2 1 /a, Z = 2, a = 11.44637(18), b = 5.05135(8), c = 8.85234(13) Å, β = 108.8625(7)°, V = 484.352(13) Å 3 . For direct comparison, the structure of reinhardbraunsite Ca 5 (SiO 4 ) 2 (OH 1.3 F 0.7 ) from the same locality has also been refined to R = 1.9%, and both symmetry independent, partially occupied H sites were determined: space group P2 1 /a, Z = 2, a = 11.4542(2), b = 5.06180(10), c = 8.89170(10) Å, β = 108.7698(9)°, V = 488.114(14) Å 3 . The following main absorption bands were observed in kumtyubeite FTIR spectra (cm -1 ): 427, 507, 530, 561, 638, 779, 865, 934, 1113, and 3551. Raman spectra are characterized by the following strong bands (cm -1 ) at: 281, 323, 397 (ν 2 ), 547 (ν 4 ), 822 (ν 1 ), 849 (ν 1 ), 901 (ν 3 ), 925 (ν 3 ), 3553 (V OH ). Kumtyubeite with compositions between Ca 5 (SiO 4 ) 2 F 2  and Ca 5 (SiO 4 ) 2 (OH 1.0 F 1.0 ) has only the hydrogen bond O5-H1···F5′, whereas reinhardbraunsite with compositions between Ca 5 (SiO 4 ) 2 (OH 1.0 F 1.0 ) and Ca 5 (SiO 4 ) 2 (OH) 2  has the following hydrogen bonds: O5-H1···F5′, O5-H1···O5′, and O5-H2···O2.

Kumtyubeite Ca 5 (SiO 4 ) 2 F 2 —A new calcium mineral of the humite group from Northern Caucasus, Kabardino-Balkaria, Russia