Abstract
Rippite K2(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 (romanèchite ± 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 °C for the primary carbonatite paragenesis. Rippite is tetragonal: P4bm, a = 8.73885(16), c = 8.1277(2) Å, V = 620.69(2) Å3, Z = 2. It is closely identical in the structure and cell parameters to synthetic K2Nb2(Si4O12)O2 (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/cm3; density (calc.)—3.198 g/cm3; optically uniaxial (+); ω = 1.737-1.739; ε = 1.747 (589 nm). The empirical formula of the holotype rippite (mean of 120 analyses) is K2(Nb1.90Ti0.09Zr0.01)[Si4O12](O1.78OH0.12F0.10). Majority of rippite prismatic crystals are weakly zoned and show Ti-poor composition K2(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 K2(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]F2 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), K2(Nb,Ti)2(Si4O12)(O,OH)2∙4H2O.
Highlights
The natural analog of the tetragonal synthetic phase KNbSi2 O7, a new structural type among natural K-Nb-cyclosilicates, named rippite, was discovered in calciocarbonatites of the Chuktukon massif, Chadobets upland, SW Siberian Platform, Krasnoyarsk Territory, Russia [1,2].Synthetic K2 Nb2 (Si4 O12 )O2 is actively studied since 1990 [3]
The identification of all minerals in the Chuktukon calciocarbonatites was based on energy-dispersive spectra (EDS), back-scattered electron (BSE) images and elemental mapping (EDS system), using a TESCAN MIRA 3MLU scanning electron microscope equipped with an INCA Energy 450 XMax 80 microanalysis system (Oxford Instruments Ltd., Abingdon, UK) at the Institute of Geology and Mineralogy (IGM), Novosibirsk, Russia
In general the detailed studies for the Chuktukon calciocarbonatites (Chadobets upland, Krasnoyarsk Territory, Russia) gave the possibility of describing the chemical composition and crystal structure for a new mineral, rippite K2 (Nb,Ti)2 (Si4 O12 )(O,F)2, which belongs to a new structural type among the [Si4 O12 ]-cyclosilicates and is close to the labuntsovite supergroup
Summary
The natural analog of the tetragonal synthetic phase KNbSi2 O7 , a new structural type among natural K-Nb-cyclosilicates, named rippite, was discovered in calciocarbonatites of the Chuktukon massif, Chadobets upland, SW Siberian Platform, Krasnoyarsk Territory, Russia [1,2]. The mineral is named in the honor of German Samuilovich Ripp (1935 year of birth, Ulan-Ude, Buryatia, Russia) He is a leading expert on carbonatite petrogenesis, including carbonatite magmatic activity in Eastern Siberia, carbonatite mineralogy, geochemistry and stable isotope geochemistry and rare metal mineralization associated with carbonatites. He has performed detailed studies for carbonatite occurrences in Transbaikalia (Yuzhnoe, Khaluta, Arshan, Pogranichnoe, Veseloe), Eastern Sayan (Belaya Zima) and Mongolia (Mushugai-Khuduk) [17,18,19,20,21,22,23,24,25,26,27,28]. Some data on rippite-bearing calciocarbonatites of the Chuktukon massif were reported in the previous publications [1,2,29,30,31,32,33,34,35]
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