Abstract
The high pressure spinel polymorph of Ni 2SiO 4 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 · B 2O 3, at that temperature. For Ni 2SiO 4(ol)→Ni 2SiO 4, ΔH 986 0 = +1.4 ± 0.7kcal/mol . The heat content increments, H 986 − H 297 , 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, ϖP/ϖT = ∼ 12b/deg, observed by Akimoto and others. The entropy of the olivine-spinel transition in Ni 2SiO 4 is accordingly about a factor of three smaller in magnitude (ΔS = ∼ −1cal/deg mol) than that for Co 2SiO 4,Fe 2SiO 4,Mg 2SiO 4or Mg 2GeO 4 (ΔS = −3to−3.5cal/deg mol) .
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