Single-crystal X-ray diffraction of grunerite up to 25.6 GPa: a new high-pressure clinoamphibole polymorph
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High-pressure single-crystal X-ray diffraction experiments were conducted on natural grunerite crystals with composition (Fe5.237Mg1.646Ca0.061Mn0.051Na0.015Ti0.002Cr0.001K0.001)(Si7.932Al0.083)O22(OH)2, using a synchrotron X-ray source. Grunerite has C2/m symmetry at ambient conditions. The samples were compressed at 298 K in a diamond-anvil cell to a maximum pressure of 25.6(5) GPa. We observe a previously described phase transition from C2/m (α) to P21/m (β) to take place at 7.4(1) GPa, as well as a further transition from P21/m (β) to C2/m (γ) at 19.2(3) GPa. The second-order Birch–Murnaghan equation of state fit to our compressional data, yielded the values V0 = 914.7(7) Å3 and K0 = 78(1) GPa for α-grunerite, V0 = 926(5) Å3 and K0 = 66(4) GPa for β-grunerite and V0 = 925(27) Å3 and K0 = 66(13) GPa for γ-grunerite. The β–γ phase transition produces a greater degree of kinking in the double silicate chains of tetrahedra accompanied by a discontinuous change in the a and c unit cell parameters and the monoclinic β angle. At 22.8(4) GPa the O5–O6–O5 kinking angle of the new high-pressure C2/m phase is 137.5(4)°, which is the lowest reported for any monoclinic amphibole. This study is the first structural report to show the existence of three polymorphs within an amphibole group mineral. The high-pressure γ-phase illustrates the parallel structural relations and phase transformation behavior of both monoclinic single and double chain silicates.
KeywordsAmphibole Phase transition High pressure Single-crystal X-ray diffraction Diamond anvil cell Synchrotron source
The project was supported by the National Science Foundation Division of Earth Sciences Geophysics Grant No. 1722969. Portions of the X-ray diffraction work were conducted using the X-ray Atlas instrument at the University of Hawaii, funded by NSF EAR Instrumentation and Facilities Grant 1541516. Portions of this work were performed at GeoSoilEnviroCARS (Sector 13), Advanced Photon Source (APS), and Argonne National Laboratory. GeoSoilEnviroCARS is supported by the National Science Foundation—Earth Sciences (EAR-1128799) and Department of Energy—Geosciences (DE-FG02-94ER14466). Use of the Advanced Photon Source was supported by the US Department of Energy, Office of Science, Office of Basic Energy Sciences, under Contract No. DE-AC02-06CH11357.
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