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Oxana Karimova, Institute of Geology of Ore Deposits RAS (Russian Federation)
Alexander Zadov, OOO ?NPP Thermochem? (Russian Federation)
Victor Gazeyev, Institute of Geology of Ore Deposits RAS (Russian Federation)
Anna Ivanova, Institute of Crystallography RAS (Russian Federation)
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Reinhardbraunsite Ca5(SiO4)2(OH,F)2 - hydroxyl dominant calico-chondrodite, was first discovered in a skarned xenolithes at Ettringer Bellerberg volcano (Eifel district, Germany) in association with ellestadite, cuspidine, gehlenite, brownmillerite, mayenite and periclase and later at Hatrurim (Israel). Chemical composition of mineral from both localities is characterized by equal amount of OH groups and F.
In 2007 we found reinhardbraunsite in skarn xenolithe within ignimbrites (2,5-3 millions years) near mountain Lakargi (Kabardino-Balkaria, North Caucasus, Russia). It is rock forming mineral at the locality and is formed over spurrite and larnite.
Crystals with the most high content of OH groups (up to 0,5 apfu) Ca5(Si0.99Ti0.01O4)2(OH1.3F0.7) were found at the Lakargi locality in a spurrite-calcite-hydrokalumite-brusite skarn (which was initial spurrite-calcite-vadalite-periclase one) on a boundary of spherical grains (resulted in melting), which have 2 cm in diameter and consist of spurrit-rondorfite. Mineral forms transparent light-pink grains (up to 2-3 mm) with cleavage perfect in one dimension. Studied crystals are characterized by optical properties: 2V=-80(10)°, nα=1,620(1), nβ=1,623(2), nγ=1,625(2). Differences in optical properties from golotype can be caused by increase OH content.
X-ray diffraction study of that crystal was performed using XCaliburS diffractometer equipped with a CCD detector (λMoKα=0.7107Å, graphite monochromator). Refined unit cell parameters are a=8.879(1) Å, b=5.059(1) Å, c=11.452(2) Å, α=90°, β=108.77(1)°, γ=90°, V=487.1(1) Å3, sp. gr. P21/c, Z=2, dcalc=2.8580(8) g/cm3. Crystal structure solved by direct methods and refined to R=0.0416 for 2979 unique reflections with Fo>4sig(Fo) using SHEXL-97 software [Sheldrick G.M. 2008].
Crystal structure of reinhardbrausite belongs to chondrodite type. Three crystallography independent Ca octahedra form framework in the structure. Single silicon tetrahedra reinforce the framework. Mean Si-O distances in tetrahedra is 1.6394 Å. Mean M-O distances in CaO6, CaO5(OH,F) and CaO4(OH,F)2 octahedra are equal to 2.370 Å, 2.3841 Å and 2.3554 Å, respectively. They are slightly larger than in [Kirfel A. et. al. 1983]. Increase of hydroxyl content in the structure leads to increase of mean M-O distances in octahedra, especially Ca-(OH,F) distances, because of intensifying of proton-proton interaction.
It is the most hydroxyl member among structurally studied natural chondrodites. Increase of OH content in the structure cause increase of cell volume and b and a constants, and decrease of c constant. This correlation was shown in [Kirfel A. et. al. 1983] and our data fit in the picture.
Present study was supported by RFBR grant 08-05-00181.
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