Ni2SiO4-enthalphy of the olivine-spinel transition by solution calorimetry at 713°C

The high pressure spinel polymorph of Ni₂SiO₄ persists metastably at 713°C and atmospheric pressure. The enthalpy of the olivine-spinel transition was obtained by measuring the heats of solution of both polymorphs in a molten oxide solvent, 2PbO · B₂O₃, at that temperature. For Ni₂SiO₄(ol)→Ni₂SiO₄, ΔH₉₈₆⁰ = +1.4 ± 0.7kcal/mol. The heat content increments, H₉₈₆ ? H₂₉₇, were found to be: olivine, 25.73 ± 0.42kcal/mol, and spinel, 25.39 ± 0.20kcal/mol. The measured enthalpy of the transformation is consistent with the low slope of the phase boundary, ?P/?T = ～ 12b/deg, observed by Akimoto and others. The entropy of the olivine-spinel transition in Ni₂SiO₄ is accordingly about a factor of three smaller in magnitude (ΔS = ～ ?1cal/deg mol) than that for Co₂SiO₄,Fe₂SiO₄,Mg₂SiO₄or Mg₂GeO₄ (ΔS = ?3to?3.5cal/deg mol).