Experimental constraints on mixing properties of forsterite-monticellite olivine under atmospheric pressure

Abstract

A series of experiments provided data on the forsterite-monticellite solvus under a pressure of 1 atm and in the temperature range of 1100 to 1450°C. The forsterite-monticellite solvus is asymmetric as shown in the previous study. Margules type of the excess energy is expressed by
      G^ex=X_Mg^M2X_Ca^M2(X_Ca^M2W_MgCa+X_Mg^M2W_CaMg),
where the asymmetric Ca-Mg mixing model yielded as W_CaMg=36.32±0.34 (unit in kJ), W_MgCa=32.72±0.32, whereas the symmetric Ca-Mg mixing model resulted as W_CaMg=W_MgCa=34.05±0.22. The asymmetric Ca-Mg mixing model fits well with the data of the forsterite-monticellite solvus. The present results are consistent with the previous analyses of miscibility gaps in Ca-Mg olivines, and the deviation from the ideality of Ca-Mg mixing is slightly smaller than the previous estimates. Compared with the previous data, the solvus becomes narrow with the increase of pressure. The negative excess volume of Ca-Mg olivine solid solution causes this character, but the pressure effect on the solvus is small.