Abstract
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) A3 and K0 = 78(1) GPa for α-grunerite, V0 = 926(5) A3 and K0 = 66(4) GPa for β-grunerite and V0 = 925(27) A3 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.
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