The High-Pressure Structural Evolution of Olivine along the Forsterite-Fayalite Join

Structural refinements from single-crystal X-ray diffraction data are reported for olivine with a composition of Fo(100) (forsterite Mg2SiO4, synthetic), Fo(80) and Fo(62) (Mg1.6Fe0.4SiO4 and Mg1.24Fe0.76SiO4, both natural) at room temperature and high pressure to 8 GPa. The new results, along with data from the literature on Fo(0) (fayalite Fe2SiO4), were used to investigate the previously reported structural mechanisms which caused small variations of olivine bulk modulus with increasing Fe content. For all the investigated compositions, the M2 crystallographic site, with its bonding configuration and its larger polyhedral volume, was observed to control the compression mechanisms in olivine. From Fo(100) to Fo(0), the compression rates for M2-O and M1-O bond lengths were observed to control the relative polyhedral volumes, resulting in a less-compressible M1O(6) polyhedral volume, likely causing the slight increase in bulk modulus with increasing Fe content.