Single crystal hydrostatic compression of (Mg,Mn,Fe,Co)₂SiO₄ 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 β_a, β_b, β_c 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 K_T0 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 K_T0 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.