Atoll Garnets in "Achtarandite" Serpentinites: Morphology, Composition and Mode of Origin
Atoll garnets in aposkarn serpentinite from the Wiluy River, Republic of Sakha-Yakutia, Russia, have the classic form comprising a garnet core, an intermediate zone filled with chlorite-group minerals and an outer garnet atoll. The core of an illustrated example is complexly zoned from schorlomite to grossular-andradite. Morphologically, the core is a rhombic dodecahedral crystal. The atoll crystallized as a tetragon-trisoctahedron with minor rhombic dodecahedron faces and is composed of hibschite and "hydroandradite". The atoll garnet formed as the result of selective dissolution and substitution by chlorite of an internal hibschite zone with columnar structure that became unstable under new conditions of crystallization. The pattern of dissolution traces defects in the garnet crystal. The growth of the atoll garnets reflects the main stages in the evolution of the Wiluy deposit itself and is associated with the development of the Siberian traps.
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) Å3 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 (Ca5.73Fe0.03Mg0.02)σ5.78(Zr0.98Hf0.01Ti0.01)σ1Si4(O13.56OH0.44)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).