β(Mg_0.9, Fe_0.1)₂SiO₄: Single crystal structure, cation distribution, and properties of coordination polyhedra

The synthesis boundaries of the phase transformation; a+ß?ß?ß+??? in (Mg0.9, Fe0.1)SiO4, have been clarified at temperatures to 2000° C and pressures up to 20 GPa in order to synthesize single crystals of high quality. A single crystal of ß (Mg0.9, Fe0.1)2SiO4 was grown successfully to a size of 500 µm. The crystal structure has been refined from single-crystal X-ray intensities. The ferrous ions prefer M1 and M3 sites to over the larger M2 site. The volume change of all the occupied polyhedra does not contribute to the decrease of total volume in the a?ß transformation; rather it tends to increase the bulk volume through the expansion of occupied tetrahedra. The volume reduction in the phase transformations is accounted for by unoccupied polyhedra, with the octahedra contributory 60% and the tetrahedra 40% to the ?V of the a?ß transition. The volume change in the ß?? transformation is caused also partly by the volume decrease of MO6 (25%), partly the unoccupied tetrahedra (45%) and octahedra (30%).