SINGLE-CRYSTAL X-RAY DIFFRACTION STUDY OF THE METASTABLE COMPRESSION OF C2/M TREMOLITE UP TO 40 GPA

The hydrated calcic amphibole mineral tremolite (Ca₂Mg₅Si₈O₂₂(OH)₂) is a common constituent of altered oceanic crust and metamorphic rocks in subduction zones, and an important contributor of water to metasomatic processes and flux melting in the deep earth. While the thermodynamic stability of amphiboles under equilibrium conditions has been well-constrained, metastably preserved tremolite—recently proposed by researchers to figure prominently in intermediate depth earthquakes and conductivity anomalies—remains largely unstudied. We explore the structural response of a natural tremolite sample with near end-member composition to pressures ~40 GPa by single-crystal X-ray diffraction using synchrotron radiation. Tremolite metastably persists with C2/m symmetry to the highest pressures of this study with no evidence of a phase transition—in marked contrast to the behavior observed in the ferromagnesian clinoamphiboles of the cummingtonite-grunerite series. Slope changes of the a/b and a/c axial ratios and the F_E-f_E­ plot indicate changes in the compressional mechanism at pressures near 5 and 22 GPa. Data in the three compressional regimes were fit by third-degree Birch-Murnaghan EOS to determine the axial compressibilities of tremolite, revealing a significant reduction in the anisotropy of the axial compressibilities in the high-pressure compression regimes, which differs from the increase in the elastic anisotropy of the high-pressure phases of cummingtonite and grunerite. The persistence of C2/m tremolite to 40 GPa in this study, the largest documented metastability range of an amphibole, implies that phase stability in the clinoamphiboles can be correlated with the size of the cation occupant of the M4 crystallographic site. The stiffening of the axial compressibilities in compression regime III suggest that tremolite may be approaching a structural instability or amorphization, thus the limit of its metastability range may lie near 40 GPa at ambient temperatures.