New structural features of the high-pressure synthetic sheet-disilicate Phase-X, K(2–x)Mg2Si2O7Hx
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Mark D. Welch
,
Jürgen Konzett
Abstract
The structure of the synthetic high-pressure sheet-disilicate Phase-X (PhX), a possible host of H2O and K in the mantle, has been determined for a crystal synthesized at 16 GPa/1300 °C/23 h. The composition of the sample is close to K1.5Mg2Si2O7H0.5, which is 50% PhX/50% Anhydrous-PhX and has 25% of interlayer K sites vacant. The structures of four crystals were determined by singlecrystal X-ray diffraction and had very similar diffraction characteristics and structural results; the structure of one of the larger crystals is reported here. Reflection intensity statistics strongly indicate that PhX is centrosymmetric, space group P63/mcm, in contrast to other studies that have reported non-centrosymmetric space group P63cm. While it was possible to obtain good agreement indices for refinements in P63cm, there were strong correlations between atoms that are equivalent in P63/mcm, suggesting that the correct structure is centrosymmetric. Full anisotropic refinement in space group P63/mcm gave R1 = 0.036, wR2 = 0.079, GoF = 1.467. As with all previous studies of PhX, the H atom was not located. Difference-Fourier maps of the residual electron density indicated that the K atom is displaced from the 4c site lying on the sixfold axis on to three split 12j sites 0.2 Å away, each having ¼ occupancy, giving a total of 3 K atoms per unit cell and corresponding to 1.5 K apfu, in good agreement with the content derived from electron microprobe analysis. Diffraction patterns of all four crystals examined, reconstructed from the full-intensity data collection, consistently show the presence of a large hexagonal superstructure with dimensions 8asub × 8asub × csub, having Z = 128, compared with Z = 2 for the two-layer subcell. Complex arrays of superlattice reflections occur in layers with l = 2n, but are absent from l = 2n + 1 layers.
Unpolarized infrared spectra of single crystals of PhX were obtained that are similar to those reported previously in the literature. Spectra in the OH-stretching region consist of a major absorption band at 3595 cm-1 and three much weaker bands at 3690, 3560, and 3405 cm-1. Bond-valence analysis of PhX indicates that O1 is very over-bonded, whereas O2 is slightly under-bonded and a possible site for protonation. We present geometrical and crystal-chemical arguments that exclude O1 as a candidate for protonation, whereas a much better case can be made for O2. In PhX structures, H must be located at a partially occupied site with a multiplicity 4 ≤ m ≤ 24 in P63/mcm or 4 ≤ m ≤ 12 in P63cm. Such low occupancies for H sites are the likely reason for their invisibility to diffraction. We outline a model for the incorporation of H into PhX of composition K1.5Mg2Si2O7H0.5 that suggests a mechanism for ordering based upon avoidance of H and K, coupled with K-site vacancies. Such behavior may also be the origin of the superstructure. The P63/mcm structure and the presence of an underlying superstructure may well be characteristic of ordered intermediate compositions at or near PhX50/Anhydrous-PhX50. Identification of a new space group and recognition of a previously unobserved superstructure point to new possibilities for PhX and its derivatives that may bear significantly upon their stability at mantle conditions.
© 2015 by Walter de Gruyter Berlin/Boston
Articles in the same Issue
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- CO2 solubility in primitive martian basalts similar to Yamato 980459, the effect of composition on CO2 solubility of basalts, and the evolution of the martian atmosphere
- New structural features of the high-pressure synthetic sheet-disilicate Phase-X, K(2–x)Mg2Si2O7Hx
- Crystal structure refinements of borate dimorphs inderite and kurnakovite using 11B and 25Mg nuclear magnetic resonance and DFT calculations
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- In situ dehydration behavior of zeolite-like cavansite: A single-crystal X-ray study
- LREE-redistribution among fluorapatite, monazite, and allanite at high pressures and temperatures
- A single-crystal neutron diffraction study of hambergite, Be2BO3(OH,F)
- Analyzing water contents in unexposed glass inclusions in quartz crystals
- The atomic structure of deuterated boyleite ZnSO4·4D2O, ilesite MnSO4·4D2O, and bianchite ZnSO4·6D2O
- Synthesis and crystal chemistry of Fe3+-bearing (Mg,Fe3+)(Si,Fe3+)O3 perovskite
- Mixed-layered structure formation during trans-vacant Al-rich illite partial dehydroxylation
- Structural regularities in 2M1 dioctahedral micas: The structure modeling approach
- Incorporation of Fe and Al in MgSiO3 perovskite: An investigation by 27Al and 29Si NMR spectroscopy
- In situ high-temperature Raman and FTIR spectroscopy of the phase transformation of lizardite
- Packing systematics and structural relationships of the new copper molybdate markascherite and related minerals
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Articles in the same Issue
- Cation ordering over short-range and long-range scales in the MgAl2O4-CuAl2O4 series
- Blue spinel crystals in the MgAl2O4-CoAl2O4 series: Part I. Flux growth and chemical characterization
- Blue spinel crystals in the MgAl2O4-CoAl2O4 series: Part II. Cation ordering over short-range and long-range scales
- CO2 solubility in primitive martian basalts similar to Yamato 980459, the effect of composition on CO2 solubility of basalts, and the evolution of the martian atmosphere
- New structural features of the high-pressure synthetic sheet-disilicate Phase-X, K(2–x)Mg2Si2O7Hx
- Crystal structure refinements of borate dimorphs inderite and kurnakovite using 11B and 25Mg nuclear magnetic resonance and DFT calculations
- Hydrogen-bond system and dehydration behavior of the natural zeolite parthéite
- In situ dehydration behavior of zeolite-like cavansite: A single-crystal X-ray study
- LREE-redistribution among fluorapatite, monazite, and allanite at high pressures and temperatures
- A single-crystal neutron diffraction study of hambergite, Be2BO3(OH,F)
- Analyzing water contents in unexposed glass inclusions in quartz crystals
- The atomic structure of deuterated boyleite ZnSO4·4D2O, ilesite MnSO4·4D2O, and bianchite ZnSO4·6D2O
- Synthesis and crystal chemistry of Fe3+-bearing (Mg,Fe3+)(Si,Fe3+)O3 perovskite
- Mixed-layered structure formation during trans-vacant Al-rich illite partial dehydroxylation
- Structural regularities in 2M1 dioctahedral micas: The structure modeling approach
- Incorporation of Fe and Al in MgSiO3 perovskite: An investigation by 27Al and 29Si NMR spectroscopy
- In situ high-temperature Raman and FTIR spectroscopy of the phase transformation of lizardite
- Packing systematics and structural relationships of the new copper molybdate markascherite and related minerals
- On the origin of sellaite (MgF2)-rich deposits in Mg-poor environments
- Edgrewite Ca9(SiO4)4F2-hydroxyledgrewite Ca9(SiO4)4(OH)2, a new series of calcium humite-group minerals from altered xenoliths in the ignimbrite of Upper Chegem caldera, Northern Caucasus, Kabardino-Balkaria, Russia
- Whelanite, Cu2Ca6[Si6O17(OH)](CO3)(OH)3(H2O)2, an (old) new mineral from the Bawana mine, Milford, Utah
- 2M1-phlogopite from Black Hills (South Australia): The first case of configurational polytype in micas
- Oxy-chromium-dravite, NaCr3(Cr4Mg2)(Si6O18)(BO3)3(OH)3O, a new mineral species of the tourmaline supergroup
- First-principles study of self-diffusion and viscous flow in diopside (CaMgSi2O6) liquid
- Browneite, MnS, a new sphalerite-group mineral from the Zakłodzie meteorite
- Mineralogical variation of silica induced by Al and Na in hydrothermal solutions
