The crystal structure of forsterite Mg₂SiO₄ under high pressure up to 149 kb

Using a miniature diamond-anvil pressure cell and by means of single-crystal four-circle diffractometry, the crystal structure of synthetic forsterite has been studied up to 149 kb. The results are: (1) the distortion of the oxygen lattice from that of an ideal hexagonal closest packing (hcp) decreases with an increase of pressure, the extrapolation of the variation mode showing that the oxygen lattice would be ideally of the hcp type at around 160 kb. (2) While the mean Mg(2) – O bond shows a linear decrease with an increase of pressure, the mean Mg(1) – O distance ceases decreasing at around 80 kb. The value at this pressure is kept constant up to the experimental limit of 149 kb. At this extreme pressure both mean values are 2.05 Å. (3) The bulk modulus for the mean Si – O bond shows a value of 1.9 Mb.