Crystal structures and infrared spectra of two Fe-bearing hydrous magnesium silicates synthesized at high temperature and pressure

抄録

Two Fe-bearing hydrous magnesium silicates, Fe-bearing phase E with the composition Mg_2.22Fe_0.52Si_0.98O6H_2.08, and phase E' with the composition Mg_2.13Fe_0.59Si_0.87O6H_2.52, were synthesized at 14 GPa and 1000°C and studied with single-crystal X-ray diffraction and infrared spectroscopy. The space groups and unit-cell parameters are R3m, a = 2.981(1) Å, c = 13.898(3) Å for Fe-bearing phase E, and P6₃/mmc, a = 2.953(1) Å, c = 14.170(1) Å for phase E'. In addition to the similar compositions and unit-cell parameters, phase E' possesses many structural features characteristic of those of Fe-bearing phase E. Most remarkably, both structures are built of close-packed oxygen atoms along the c axis, with all Si occupying the interstitial tetrahedral sites and M (= Mg + Fe) cations occupying the octahedral sites. All cation sites are partially occupied in both structures. The implausible configuration of M octahedra sharing faces with Si tetrahedra observed in Fe-bearing phase E exists in phase E' as well. The structures of Fe-bearing phase E and phase E', on the other hand, exhibit obvious differences in the stacking sequence of close-packed oxygen atoms, the distributions of M and Si cations between layers of close packed oxygen atoms, and the degree of the M cation ordering. The synthesis and characterization of Fe-bearing phase E and phase E' demonstrate that the phase E-type structures can be rather compliant and complex, and that as we further explore the temperature-pressure-composition space, other types of structures that are similar to or related to the structure of phase E may be discovered.