Single crystal hydrostatic compression of (Mg,Mn,Fe,Co) 2 SiO 4 olivines

Abstract

Four synthetic endmember olivines (Mg,Mn, Fe,Co)2SiO4 with space group Pbnm were loaded together in one diamond cell mount. Their unit-cell parameters were determined by single crystal X-ray diffraction to 10 GPa. The linear compressibilities βa, βb, βc were 1.53, 2.90, 2.32; 1.45, 3.48, 1.98; 1.35, 3.29, 1.76; and 1.25, 2.82, 2.01×10−3 GPa−1 for Mg2SiO4, Mn2SiO4, Fe2SiO4 and Co2SiO4, respectively. The b axis is the most compressible direction in all crystals studied. Bulk modulus KT0 and its first pressure derivative were simultaneously determined for Mg2SiO4, Fe2SiO4 and Co2SiO4 crystals respectively by fitting volume data to a third order Birch-Murnaghan equation of state. They are 127(4) and 4.2(8), 136(3) and 4.1(7), and 144(2) and 4.1(5). The KT0 and could not simultaneously be determined unambiguously for Mn2SiO4. Direct comparisons of unit-cell volumes at high pressure among pairs of olivines reveal anomalous compression behavior of the Mg2SiO4 crystal regarding the bulk modulus-volume relationship. This behavior, however, could not be observed in the transition metal olivines (Mn,Fe,Co)2SiO4. The distinct electronic configurations of Mg2+ and the transition metal cations Mn2+, Fe2+, and Co2+ result in the different compression behaviors of Mg2SiO4 and (Mn,Fe,Co)2SiO4.