Rippite, K2(Nb,Ti)2(Si4O12)O(O,F), a New K-Nb-Cyclosilicate from Chuktukon Carbonatite Massif, Chadobets Upland, Krasnoyarsk Territory, Russia

Rippite K-2(Nb,Ti)(2)(Si4O12)(O,F)(2), a new K-Nb-cyclosilicate, has been discovered in calciocarbonatites from the Chuktukon massif (Chadobets upland, SW Siberian Platform, Krasnoyarsk Territory, Russia). It was found in a primary mineral assemblage, which also includes calcite, fluorcalciopyrochlore, tainiolite, fluorapatite, fluorite, Nb-rich rutile, olekminskite, K-feldspar, Fe-Mn-dolomite and quartz. Goethite, francolite (Sr-rich carbonate-fluorapatite) and psilomelane (romanechite +/- hollandite) aggregates as well as barite, monazite-(Ce), parisite-(Ce), synchysite-(Ce) and Sr-Ba-Pb-rich keno-/hydropyrochlore are related to a stage of metasomatic (hydrothermal) alteration of carbonatites. The calcite-dolomite coexistence assumes crystallization temperature near 837 degrees C for the primary carbonatite paragenesis. Rippite is tetragonal: P4bm, a = 8.73885(16), c = 8.1277(2) angstrom, V = 620.69(2) angstrom(3), Z = 2. It is closely identical in the structure and cell parameters to synthetic K2Nb2(Si4O12)O-2 (or KNbSi2O7). Similar to synthetic phase, the mineral has nonlinear properties. Some optical and physical properties for rippite are: colorless; Mohs' hardness-4-5; cleavage-(001) very perfect, (100) perfect to distinct; density (meas.)-3.17(2) g/cm(3); density (calc.)-3.198 g/cm(3); optically uniaxial (+); omega = 1.737-1.739; epsilon = 1.747 (589 nm). The empirical formula of the holotype rippite (mean of 120 analyses) is K-2(Nb1.90Ti0.09Zr0.01)[Si4O12](O1.78OH0.12F0.10). Majority of rippite prismatic crystals are weakly zoned and show Ti-poor composition K-2(Nb1.93Ti0.05Zr0.02)[Si4O12](O1.93F0.07). Raman and IR spectroscopy, and SIMS data indicate very low H2O content (0.09-0.23 wt %). Some grains may contain an outermost zone, which is enriched in Ti (+Zr) and F, up to K-2(Nb1.67Ti0.32Zr0.01)[Si4O12](O1.67F0.33). It strongly suggests the incorporation of (Ti,Zr) and F in the structure of rippite via the isomorphism Nb5+ + O2- -> (Ti,Zr)(4+) + F1-. The content of a hypothetical end-member K2Ti2[Si4O12]F-2 may be up to 17 mol. %. Rippite represents a new structural type among [Si4O12]-cyclosilicates because of specific type of connection of the octahedral chains and [Si4O12](8-) rings. In structural and chemical aspects it seems to be in close with the labuntsovite-supergroup minerals, namely with vuoriyarvite-(K), K-2(Nb,Ti)(2)(Si4O12)(O,OH)(2)center dot 4H(2)O.