Tie lines and activities in the system NiO-MgO-SiO₂ at 1373 K

Abstract

The isothermal section of the phase diagram for the system NiO-MgO-SiO2 at 1373 is established. The tie lines between (Ni_xMg_1-x )O solid solution with rock salt structure and orthosilicate solid solution (Ni_yMg_1-y)Si_0.5O₂ and between orthosilicate and metasilicate (Ni_zMg_1-z)SiO₃ crystalline solutions are determined using electron probe microanalysis (EPMA) and lattice parameter measurement on equilibrated samples. Although the monoxides and orthosilicates of Ni and Mg form a continuous range of solid solutions, the metasilicate phase exists only for 0 < Z < 0.096. The activity of NiO in the rock salt solid solution is determined as a function of composition and temperature in the range of 1023 to 1377 using a solid state galvanic cell. The Gibbs energy of mixing of the monoxide solid solution can be expressed by a pseudo-subregular solution model: ΔG^ex = X(l - X)[(-2430 + 0.925T)X + (-5390 + 1.758T)(1 - X)] J/mol. The thermodynamic data for the rock salt phase are combined with information on interphase partitioning of Ni and Mg to generate the mixing properties for the orthosilicate and the metasilicate solid solutions. The regular solution model describes the orthosilicate and the metasilicate solid solutions at 1373 K within experimental uncertainties. The regular solution parameter ΔG^ex/Y(1-Y) is -820 (±70) J/mol for the orthosilicate solid solution. The corresponding value for the metasilicate solid solution is -220 (±150) J/mol. The derived activities for the orthosilicate solid solution are discussed in relation to the intracrystalline ion exchange equilibrium between M1 and M2 sites. The tie line information, in conjunction with the activity data for orthosilicate and metasilicate solid solutions, is used to calculate the Gibbs energy changes for the intercrystalline ion exchange reactions. Combining this with the known data for NiSi_0.5O₂, Gibbs energies of formation of MgSi_0.5O₂, MgSiO₃, and metastable NiSiO₃ are calculated. The Gibbs energy of formation of NiSiO₃, from its component oxides, is equal to 7.67 (±0.6)kJ/mol at 1373K.