High-pressure single-crystal X-ray diffraction and infrared spectroscopic studies of the C2/m-P2 ₁ /m phase transition in cummingtonite

Carnegie Institution of Washington, Geophysical Laboratory and Center for High Pressure Research, Washington, DC, United States

The structural changes associated with the C2/m-P2 ₁ /m phase transition in cummingtonite with (Fe+Mn)/(Fe+Mn+Mg) nearly equal 0.50 have been studied with single-crystal X-ray diffraction at various pressures up to 7.90 GPa and infrared spectroscopy up to 8.63 GPa. With increasing pressure, the crystal transforms from C2/m to P2 ₁ /m symmetry at approximately 1.21 GPa, as determined by the appearance of reflections violating the C2/m space group. Infrared spectra provide additional evidence for the phase transition: A distinct splitting of OH stretching bands results from an increase from one to two nonequivalent OH positions. The C2/m-P2 ₁ /m transition is of weakly displacive first-order or tricritical character with apparent slope changes in the plots of the axial ratios a/b and a/c as a function of pressure. The unit-cell compression is considerably anisotropic with the a dimension in both C2/m and P2 ₁ /m phases being the most compressible. Major structural changes for the C2/m-P2 ₁ /m transition include: (1) One crystallographically distinct silicate chain becomes two discontinuously, coupled by the splitting of the M4-05 bond, as well as M4-06, into two nonequivalent bonds, and (2) the M4-cation coordination increases from sixfold to sevenfold. More importantly, we observed a change in the sense of rotation for the A chain while the crystal structure maintains P2 ₁ /m symmetry: It is O rotated, as the B chain, at 1.32 GPa, but S-rotated at 2.97 GPa and higher pressures. As pressure increases from 1.32 to 7.90 GPa, there is a switching of the nearest bridging O atoms coordinated with the M4 cation: The M4-O5B distance contracts from 2.944 to 2.551 Aa, whereas the M4-O6B distance increases from 2.754 to 2.903 Aa. Compression mechanisms for the low-and high-pressure polymorphs appear to be slightly different. In the C2/m phase, the behavior of the A and M4 sites controls the compression of the structure, whereas the response of the M1, M2, and M3 octahedra to pressure also plays a role in determining the compression of the P2 ₁ /m structure. The phase transition is regarded as primarily driven by the differential compression between the M4 and T sites, and the symmetry breaking provides a necessary tighter coordination for the M4 site. Based on our data, the obvious changes in the hyperfine parameters of ⁵⁷ Fe in grunerite between 1.0 and 3.4 GPa, observed by Zhang and Hafner (1992), are likely to result from the C2/m-P2 ₁ /m structural transformation.

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