N. A. Yamnova

Crystal structure of larnite β-Ca2SiO4 and specific features of polymorphic transitions in dicalcium orthosilicate

The crystal structure of larnite, a natural analog of synthetic β-Ca2SiO4, has been determined: a = 5.5051(3) Å, b = 6.7551(3) Å, c = 9.3108(5) Å, β = 94.513(4)o, sp. gr. P21/n, Z = 4, and R1 = 0.0532 for 1071 reflections with I > 2σ (I). Larnite was found in skarn xenoliths (Lakargi, Kabardino-Balkaria, Russia). The mineral structure is based on a heteropolyhedral glaserite-like framework of interconnected Ca polyhedra and isolated [SiO4] tetrahedra. Based on an analysis of the layer-by-layer packing of atoms in the structures of larnite and other Ca2SiO4 polymorphs, the structural features and mechanisms of transitions from high-temperature (α, α′L, and α′H) to low-temperature (β and γ) Ca2SiO4 modifications, as well as their relationship with natural glaserite-like orthosilicates (merwinite Ca3Mg[SiO4]2 and bredigite Ca7Mg[SiO4]4), have been considered. The most likely atomic arrangement in hypothetical Ca2SiO4 models has been calculated by the method of atomistic potentials.

Calcio-olivine γ-Ca2SiO4: I. Rietveld refinement of the crystal structure

AbstractThe structure of the natural mineral calcio-olivine (γ-Ca2SiO4) found in skarn xenoliths in the region of the Lakargi Mountain (North Caucasus, Kabardino-Balkaria, Russia) is refined by the Rietveld method [a = 5.07389(7) Å, b = 11.21128(14) Å, c = 6.75340(9) Å, V = 384.170(5) Å3, Z = 4, ρcalcd = 2.98 g/cm3, space group Pbnm]. The X-ray diffraction pattern of a powdered sample is recorded on a STOE STADI MP diffractometer [λCuKα1; Ge(111) primary monochromator; 6.00° < 2θ < 100.88°; step width, 2.5° in 2θ; number of reflections, 224]. All calculations are performed with the WYRIET (version 3.3) software package. The structural model is refined in the anisotropic approximation to Rp = 6.44, Rwp = 8.52, Rexp = 5.85, RB = 4.98, RF = 6.90, and s = 1.46. It is shown that the sample under investigation is a mixture of several mineral phases, among which calcio-olivine (the natural analogue of the γ-Ca2SiO4 compound) (83%), hillebrandite (13%), and wadalite (4%) are dominant. Only the scale factors and the unit cell parameters are refined for hillebrandite Ca2SiO3(OH)2 [a = 3.63472(16) Å, b = 16.4140(10) Å, c = 11.7914(8) Å, space group Cmc21, Z = 6] and wadalite Ca6Al5Si2O16Cl3 (a = 12.0088 Å, space group, I

Calcioolivine, γ-Ca2SiO4, an old and New Mineral species

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Crystal structure of new synthetic calcium pentaborate Ca[B5O8(OH)] · H2O and its relation to pentaborates with similar boron-oxygen radicals

3dZ = 4). The results of the structure refinement of the main component of the sample confirm that the mineral calcio-olivine is isostructural to the synthetic compound γ-Ca2SiO4. The structure of this compound is formed by the heteropolyhedral framework composed of Ca octahedra joined together into olivine-like ribbons and isolated Si tetrahedra.

Crystal structure of litvinskite: A new natural representative of the lovozerite group

AbstractThe structure of Na,Ca,Zr-silicate lovozerite, Na2CaZr[Si6O12(OH,O)6] · H2O, from the Khibiny alkaline massif (the Kola Peninsula) was refined by single-crystal X-ray diffraction analysis (Syntex

Crystal structure of new synthetic Ca,Na carbonate-borate Ca2Na(NaxCa0.5 − x)[B3tB2δO8(OH)(O1 − xOHx)](CO3)

P\bar 1