Thermodynamic aspects of phase boundary among α-, β-, and γ-Mg2SiO4
1987; American Geophysical Union; Linguagem: Inglês
10.1029/gm039p0269
ISSN2328-8779
AutoresToshifumi Ashida, Shōichi Kume, Eiji Ito,
Tópico(s)Crystal Structures and Properties
ResumoMolar heat capacities of three polymorphs (α, β, γ) of Mg2SiO4 were measured in the temperature range of 180–700 K by means of differential scanning calorimetry (DSC). The results were extrapolated towards low and high temperatures, and the entropy of each phase was evaluated in the range of 0–2000 K. The phase boundaries for α-β and for α-γ were determined by thermodynamic calculations using these entropies and additional reported data on enthalpies of formations, compressibilities, and thermal expansions. The boundaries of α-β ana α-γ above 1000 K were roughly approximated by the following linear relations $$\eqalign{ & P = 11.68 + 0.0025\,T,\,{\rm for}\,{\rm \alpha } - \beta \cr & P = 11.08 + 0.0048\,T,\,{\rm for}\,\beta - \gamma \cr}$$ where P is in GPa and T is in degrees K. The triple point, where the three phases coexist, was found to be located at 13±0.5 GPa and 400–600 K. The results for the boundaries were compared with those obtained by two synthesis experiments and found to be in agreement with the results reported by Suito (1977) for above 1000 K.
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