Kumtyubeite, Ca5(SiO4)2F2—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 Ca5(Si1.99Ti0.01)∑2O8(F1.39OH0.61)∑2. Single-crystal X-ray data were collected for a grain of Ca5(SiO4)2(F1.3OH0.7) composition, and the structure refinement, including a partially occupied H position, converged to R = 1.56%: monoclinic, space group P21/a, Z = 2, a = 11.44637(18), b = 5.05135(8), c = 8.85234(13) Å, β = 108.8625(7)°, V = 484.352(13) Å3. For direct comparison, the structure of reinhardbraunsite Ca5(SiO4)2(OH1.3F0.7) from the same locality has also been refined to R = 1.9%, and both symmetry independent, partially occupied H sites were determined: space group P21/a, Z = 2, a = 11.4542(2), b = 5.06180(10), c = 8.89170(10) Å, β = 108.7698(9)°, V = 488.114(14) Å3. 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 (nu;2), 547 (nu;4), 822 (nu;1), 849 (nu;1), 901 (nu;3), 925 (nu;3), 3553 (nu;OH). Kumtyubeite with compositions between Ca5(SiO4)2F2 and Ca5(SiO4)2(OH1.0F1.0) has only the hydrogen bond O5-H1· · ·F5′, whereas reinhardbraunsite with compositions between Ca5(SiO4)2(OH1.0F1.0) and Ca5(SiO4)2(OH)2 has the following hydrogen bonds: O5-H1···F5′, O5-H1···O5′, and O5-H2···O2.

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