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 ± 0.7 kcal/mol. The heat content increments, H₉₈₆ - H₂₉₇, were found to be: olivine, 25.73 ± 0.42 kcal/mol, and spinel, 25.39 ± 0.20 kcal/mol. The measured enthalpy of the transformation is consistent with the low slope of the phase boundary, π{variant}P/π{variant}T = ∼ 12 b/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 = ∼ -1 cal/deg mol) than that for Co₂SiO₄, Fe₂SiO₄, Mg₂SiO₄ or Mg₂GeO₄ (ΔS = -3 to -3.5 cal/deg mol).