Dovyrenite Ca6Zr[Si2O7]2(OH)4 - A New Mineral from Skarned Carbonate Xenoliths in Basic-Ultrabasic Rocks of the Ioko-Dovyren Massif, Northern Baikal Region, Russia

Dovyrenite, simplified formula Ca₆Zr[Si₂O₇]₂(OH)₄, occurs as an accessory mineral in vein skarns developed in carbonate xenoliths in subvolcanic layered plagiodunite-troctolite series in the Ioko-Dovyren Massif of Proterozoic age, Northern Baikal Region, Buryatia, Russia. Dovyrenite is a late mineral of altered pyroxene and melilite-monticellite skarns. Associated minerals are Zr-bearing phases: fassaitic pyroxene, perovskite and hydrogarnets; and also monticellite, vesuvianite, diopside, foshagite, brucite, calzirtite, tazheranite, baghdadite, apatite, calcite, native bismuth, sphalerite, selenian galena, clausthalite, safflorite, rammelsbergite, pyrrhotite, pentlandite, valleriite, laitakarite, nickeline, nickel-skutterudite. The average structure of dovyrenite is orthorhombic, space group Pnnm, with subcell parameters A = 5.666(16) Å, B = 18.844(5) Å, C = 3.728(11) Å, V = 398.0(2) ų and Z = 1. Dovyrenite shows a new type of modular structure with stacking of the tobermorite-like and the rosenbuschite-like layers parallel to (010). Single-crystal structural data point to an incompletely occupied Ca(2) site from the rosenbuschite module which is confirmed by microprobe analyses: ZrO₂ 16.47, SiO₂ 32.83, TiO₂ 0.14, HfO₂ 0.16, Cr₂O₃ 0.01, CaO 43.87, FeO 0.25, MgO 0.13, MnO 0.02, Nb₂O₃ 0.03; total 99.38 wt% with calculated H₂O. The empirical formula is (Ca_5.73Fe_0.03Mg_0.02)_σ5.78(Zr_0.98Hf_0.01Ti_0.01)_σ1Si₄(O_13.56OH_0.44)_σ14(OH)₄. The presence of two types of OH group in the dovyrenite structure is corroborated by FTIR and Raman spectroscopy. Dovyrenite is an optically positive biaxial mineral: α 1.659(2), β 1.660(2); γ 1.676(2); 2Vz 30(5)° (measured), 28° (calculated). The coexistence of monticellite, foshagite and dovyrenite points to a narrow interval of crystallization 560-630°C under subvolcanic conditions (P < 10⁸ Pa).