The measured cation distributions in Fe2SiO4, Co2SiO4, and Ni2SiO4 (Yagi et al., 1974; Morimoto et al, 1974) are used to calculate interchange enthalpies (Navrotsky and Kleppa, 1967) of +28.8, +16.5, and +37.5 kcal, respectively, for the reaction: M2+oct + Si4+tet=M2+tet + Si4+oct. The octahedral site preference (Navrotsky and Kleppa, 1967) of Si4+ in spinels is estimated to be +24.3 ± 5 kcal (strong tetrahedral preference). The interchange enthalpy for Mg2SiO4 spinel is +23.3 kcal, leading to 5% inversion at 1500°C. The small degree of disorder in Fe2SiO4 and Mg2SiO4 is sufficient to account quantitatively for the curvature above 1000°C of the olivine-spinel phase boundary in Fe2SiO4 (Akimoto and Komada, 1967) and to cause positive values of ∂P/∂T above 1500°C for the decomposition of Mg2SiO4 to its component oxides. The volume change is estimated to be less than 0.5 cm3/mole for M2SiO4 (normal → inverse) leading to a very weak pressure dependence of the cation distribution in silicate spinels.
ASJC Scopus subject areas
- Geochemistry and Petrology
- Earth and Planetary Sciences (miscellaneous)
- Space and Planetary Science