Structural behavior of C2/m tremolite to 40 GPa: A high-pressure single-crystal X-ray diffraction study

Abstract

The high-pressure structure and stability of the calcic amphibole tremolite [Ca2Mg5Si8O22(OH)2] was investigated to ~40 GPa at 300 K by single-crystal X-ray diffraction using synchrotron radiation. C2/m symmetry tremolite displays a broader metastability range than previously studied clinoamphiboles, exhibiting no first-order phase transition up to 40 GPa. Axial parameter ratios a/b and a/c, in conjunction with finite strain vs. normalized pressure trends, indicate that changes in compressional behavior occur at pressures of ~5 and ~20 GPa. An analysis of the finite strain trends, using third-order Birch-Murnaghan equations of state, resulted in bulk moduli (K0T) of 72(7), 77(2), and 61(1) GPa for the compressional regimes from 0–5 GPa (regime I), 5–20 GPa (II), and above 20 GPa (III), respectively, and accompanying pressure-derivatives of the bulk moduli (K0T′) of 8.6(42), 6.0(3), and 10.0(2). The results are consistent with first-principle theoretical calculations of tremolite elasticity. The axial compressibility ratios of tremolite, determined as βa:βb:βc = 2.22:1.0:0.78 (regime I), 2.12:1.0:0.96 (II), and 1.03:1.0:0.75 (III), demonstrate a substantial reduction of the compressional anisotropy of tremolite at high pressures, which is a notable contrast with the increasingly anisotropic compressibility observed in the high-pressure polymorphs of the clinoamphibole grunerite. The shift in compression-regime at 5 GPa (I–II) transition is ascribed to stiffening along the crystallographic a-axis corresponding to closure of the vacant A-site in the structure, and a shift in the topology of the a-oriented surfaces of the structural I-beam from concave to convex. The II–III regime shift at 20 GPa corresponds to an increasing rate of compaction of the Ca-polyhedra and increased distortion of the Mg-octahedral sites, processes which dictate compaction in both high-pressure compression-regimes. Bond-valence analyses of the tremolite structure under pressure show dramatic overbonding of the Ca-cations (75% at 30 GPa), with significant Mg-cation overbonding as well (40%). These imply that tremolite’s notable metastability range hinges on the calcium cation’s bonding environment. The eightfold-coordinated Ca-polyhedron accommodates significant compaction under pressure, while the geometry of the Ca-O polyhedron becomes increasingly regular and inhibits the reorientation of the tetrahedral chains that generate phase transitions observed in other clinoamphiboles. Peak/background ratio of diffraction data collected above 40 GPa and our equation of state determination of bulk moduli and compressibilities of tremolite in regime III, in concert with the results of our previous Raman study, suggest that C2/m tremolite may be approaching the limit of its metastability above 40 GPa. Our results have relevance for both the metastable compaction of tremolite during impact events, and for possible metastable persistence of tremolite within cold subduction zones within the Earth.

High P‐T Sound Velocities of Amphiboles: Implications for Low‐Velocity Anomalies in Metasomatized Upper Mantle

“…We also calculated the linear compressibilities of tremolite based on the ambient C ij s: β a (0.005 GPa −1 ) > β b (0.0027 GPa −1 ) > β c (0.0021 GPa −1 ) (Table S9 in Supporting Information ). This general relationship is consistent with what was found in previous XRD experiments (Comodi et al., 1991; Ott et al., 2023), although β a and β c obtained in this study are smaller than the values given in Ott et al. (2023).…”

“…This general relationship is consistent with what was found in previous XRD experiments (Comodi et al., 1991; Ott et al., 2023), although β a and β c obtained in this study are smaller than the values given in Ott et al. (2023). Peng and Mookherjee (2020) and Brown and Abramson (2016) both suggested β a (0.0063 GPa −1 ) > β c (0.0027 GPa −1 ) > β b (0.0026 GPa −1 ).…”

“…Compared with previous static compression XRD studies, the K s 0 of 98 (5) GPa and (∂ K s/∂ P ) T0 of 3.5 (1) obtained in this study are significantly higher and lower than the K T0 of 72 (7) GPa and (∂ K T0 /∂ P ) T0 of 8.6 (42) obtained in Ott et al. (2023), which cannot be explained by the difference between isothermal and adiabatic elasticity. Instead, it is related to the higher F content in the tremolite sample used in Ott et al.…”

“…obtained in this study are significantly higher and lower than the K T0 of 72 (7) GPa and (∂K T0 /∂P) T0 of 8.6 (42) obtained in Ott et al (2023), which cannot be explained by the difference between isothermal and adiabatic elasticity. Instead, it is related to the higher F content in the tremolite sample used in Ott et al (2023), which can soften the bulk and shear modulus of amphiboles (e.g., glaucophane, Mookherjee & Bezacier, 2012), as well as the well-known tradeoffs between K T0 and (∂K T0 /∂P) T0 in static compression XRD studies.…”

“…In terms of aggregate elastic properties, within the P-T range we studied, no abrupt sound velocity changes were observed as a result of the compression mechanism change at ∼5 GPa suggested by Ott et al (2023). The best-fit aggregate elastic properties are: Ks 0 = 98 (5) GPa, G 0 = 62 (3) GPa, (∂Ks/∂P) T0 = 3.5 (1), (∂G/∂P) T0 = 1.15 (5), (∂Ks/∂T ) P0 = 0.004 (2) GPa/K, and (∂G/∂T ) P0 = 0.011 (1) GPa/K.…”