The crystal structure and cation ordering of Phase-X-(K1–x–n)2(Mg1–n[Al,Cr]n)2Si2O7H2x: A potential K- and H-bearing phase in the mantle
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Franco Mancini
,
George E. Harlow
Abstract
Phase-X, a potassium di-magnesium acid disilicate, is a high-pressure synthetic compound-a potential K-bearing silicate in the mantle-with space group P63cm (no. 185), a = b = 5.028(2) Å, c = 13.216(3) Å, V = 289.34 Å3, Z = 2. The structure has been determined with 1521 CCD measured intensities and refined by the least-square method to R = 0.0187. The structure is built up of octahedral MgO sheets and layers containing disilicate groups, Si2O7, (with distinct Si1 and Si2 tetrahedra linked by the apical O2 atom) alternating along the c axis. The octahedral sheet is based on a hexagonal closest-packed array of two layers of non-equivalent O atoms, O1 and O3; two-thirds of all edge-sharing M octahedra are filled. Within the framework of the Si2O7 groups are channel structures parallel to [100], [010], and [110] that contain K atoms disordered in the middle of a large trigonal cavity (the A site). The FTIR spectrum in the OH stretching region shows a sharp peak at 3602 cm-1 due to OH- ordered in one anion site; the position of hydrogen, which operates in a charge-balancing substitution for the partial occupancy of the A site (K1-x□x)A ↔ Hx□1-x)H, is undetermined. Densification in phase-X is affected by the greater compression of the empty octahedra in the octahedral layer and by constraining the trigonal A cavity containing the K atom to the size of the Si2O7 disilicate group. This dense packing contributes to the relatively high zero-pressure calculated density of 3.38 g/cm3.
© 2015 by Walter de Gruyter Berlin/Boston
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Articles in the same Issue
- Presidential Address. Scales of disequilibrium and rates of equilibration during metamorphism
- Characterization of coexisting NH4- and K-micas in very low-grade metapelites
- Effect of sonic and ultrasonic frequencies on the crystallization of basalt
- Sulfur saturation limits in silicate melts and their implications for core formation scenarios for terrestrial planets
- Molecular water in beryl, VIAl2 [Be3Si6O18]·nH2O, as a function of pressure and temperature: An experimental study
- Fluid-induced nucleation of (Y + REE)-phosphate minerals within apatite: Nature and experiment. Part I. Chlorapatite
- X-ray spectroscopic investigations of fluids in the hydrothermal diamond anvil cell: The hydration structure of aqueous La3+ up to 300 °C and 1600 bars
- Fibrous nanoinclusions in massive rose quartz: HRTEM and AEM investigations
- Polysomatism in högbomite: The crystal structures of 10T, 12H, 14T, and 24R polysomes
- Ordering of hydroxyl defects in hydrous wadsleyite (β-Mg2SiO4)
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- Raman spectroscopic study of garnet inclusions in diamonds from the mantle transition zone
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- Study of the (010) olivine surface by Rutherford backscattering spectrometry in channeling geometry
- Equation of state of dense hydrous magnesium silicate phase A, Mg7Si2O8(OH)6
- Si-F bonding in aluminosilicate glasses: Inferences from ab initio NMR calculations
- Monoclinic nearly stoichiometric wüstite at low temperatures
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- Letters. Chloride ion sites in silicate and aluminosilicate glasses: A preliminary study by 35Cl solid-state NMR
- Nuclear magnetic resonance and infrared spectroscopic study of excess-boron olenite from Koralpe, Styria, Austria
