[SMP36-01] Synthesis and structural characterization of a new zinc silicate phase A (Zn7Si2O8(OH)6) by 1H and 29Si NMR and synchrotron powder X-ray diffraction
キーワード：結晶構造、含水ケイ酸塩相、高圧
Water is an important component determining the habitability of the surface environment and dynamics of the interior of the Earth, and may be stored in the deep Earth in the form of various hydrous phases and nominally anhydrous phases. Well known for the former are the various dense hydrous magnesium silicate (DHMS) phases (e.g. phase A, B, superhydrous B, D, E). It is also of crystal chemical interest to understand the stability and crystal structure of hydrous phases in silicate systems with other cations (e.g. Fe2+, Zn2+). Zinc is of particular interest, because although the ionic radii of Zn2+ is slightly larger than that of Mg2+, it has a tendency of occupying tetrahedral sites, rather than octahedral sites like Mg. P-T phase relations in the Zn2SiO4 and ZnSiO3 systems show different behavior from those of the analogous Mg silicates, although high-pressure phases of the same structure (e.g. M2SiO4 with modified spinel structure) for both systems are also known. A common natural hydrous zinc silicate mineral is hemimorphite (Zn4Si2O7(OH)2.H2O), which is known to be stable under ambient condition.
Here we report the synthesis of a new hydrous zinc silicate phase (Zn7Si2O8(OH)6) at 9 GPa and 500 °C, and show from the 1H and 29Si NMR and powder X-ray diffraction data that it has a crystal structure resembling that of phase A (Mg7Si2O8(OH)6) (denoted as Zn-phase A hereafter).
The sample was synthesized using a multi-anvil press at 9 GPa and 500 °C for 4 hours using natural hemimorphite as starting material. The recovered sample was characterized by micro-Raman, 1H and 29Si MAS NMR using a Bruker Avance NEO 400 spectrometer, and powder X-ray diffraction using a Bragg–Brentano–type diffractometer (SmartLab, Rigaku Co.). For structural refinement, synchrotron X-ray diffraction pattern was also acquired at BL5S2 of Aichi Synchrotron Radiation Center using a Debye-Scherrer type diffractometer equipped with four two-dimensional solid-state detectors. The wavelength was 0.69997 Å, as calibrated on CeO2 (NIST SRM 674b).
Micro-Raman measurement revealed that the sample contains ZnSiO3 clinopyroxene and a hydrous phase. The 1H MAS NMR spectra contain two peaks near 3.1 and 5.8 ppm with similar intensities, and 1H-29Si CPMAS NMR spectra revealed two 29Si peaks at -59.1 and -66.2 ppm with similar intensities. These suggest that the hydrous phase likely contains two tetrahedral Si sites and two H sites. For the powder X-ray diffraction pattern, about half of the observed peaks can be readily assigned to the diffraction peaks calculated from ZnSiO3 pyroxene (C2/c) structure. The remaining peaks can be indexed with a hexagonal cell and match well with the simulated diffraction pattern of phase A (Mg7Si2O8(OH)6 after replacing the Mg by Zn), which has two tetrahedral Si sites and two H sites, consistent with the NMR results. The refined lattice parameters for the Zn-phase A (Zn7Si2O8(OH)8) from Rietveld refinement of the synchrotron X-ray diffraction pattern are as follows: a =7.9837(3), c = 9.6452(3) Å. These values are slightly larger than those reported for the Mg-phase A, and are consistent with the slightly larger ionic size of Zn2+ than Mg2+. The M1 site of Zn-phase A is more distorted than M2 and M3, and may be considered as a (4+2) octahedron with four shorter M1-O bond distances of 1.98 - 2.13 Å and two longer distances of 2.42 and 2.65 Å. Similar behavior is also known for ZnSiO3 phases.
Acknowledgements: This study was supported by Kakenhi No. 17H01174 to X.X., and by a special budget for the Joint Use/Joint Research program by MEXT, which supported J.J. as a postdoc at IPM.
Here we report the synthesis of a new hydrous zinc silicate phase (Zn7Si2O8(OH)6) at 9 GPa and 500 °C, and show from the 1H and 29Si NMR and powder X-ray diffraction data that it has a crystal structure resembling that of phase A (Mg7Si2O8(OH)6) (denoted as Zn-phase A hereafter).
The sample was synthesized using a multi-anvil press at 9 GPa and 500 °C for 4 hours using natural hemimorphite as starting material. The recovered sample was characterized by micro-Raman, 1H and 29Si MAS NMR using a Bruker Avance NEO 400 spectrometer, and powder X-ray diffraction using a Bragg–Brentano–type diffractometer (SmartLab, Rigaku Co.). For structural refinement, synchrotron X-ray diffraction pattern was also acquired at BL5S2 of Aichi Synchrotron Radiation Center using a Debye-Scherrer type diffractometer equipped with four two-dimensional solid-state detectors. The wavelength was 0.69997 Å, as calibrated on CeO2 (NIST SRM 674b).
Micro-Raman measurement revealed that the sample contains ZnSiO3 clinopyroxene and a hydrous phase. The 1H MAS NMR spectra contain two peaks near 3.1 and 5.8 ppm with similar intensities, and 1H-29Si CPMAS NMR spectra revealed two 29Si peaks at -59.1 and -66.2 ppm with similar intensities. These suggest that the hydrous phase likely contains two tetrahedral Si sites and two H sites. For the powder X-ray diffraction pattern, about half of the observed peaks can be readily assigned to the diffraction peaks calculated from ZnSiO3 pyroxene (C2/c) structure. The remaining peaks can be indexed with a hexagonal cell and match well with the simulated diffraction pattern of phase A (Mg7Si2O8(OH)6 after replacing the Mg by Zn), which has two tetrahedral Si sites and two H sites, consistent with the NMR results. The refined lattice parameters for the Zn-phase A (Zn7Si2O8(OH)8) from Rietveld refinement of the synchrotron X-ray diffraction pattern are as follows: a =7.9837(3), c = 9.6452(3) Å. These values are slightly larger than those reported for the Mg-phase A, and are consistent with the slightly larger ionic size of Zn2+ than Mg2+. The M1 site of Zn-phase A is more distorted than M2 and M3, and may be considered as a (4+2) octahedron with four shorter M1-O bond distances of 1.98 - 2.13 Å and two longer distances of 2.42 and 2.65 Å. Similar behavior is also known for ZnSiO3 phases.
Acknowledgements: This study was supported by Kakenhi No. 17H01174 to X.X., and by a special budget for the Joint Use/Joint Research program by MEXT, which supported J.J. as a postdoc at IPM.