An experimental investigation of thermodynamic mixing properties and unit-cell parameters of forsterite–monticellite solid solution

Olivines have been synthesized along the forsterite–monticellite join in the temperature range 1100 to 1490°C, and the pressure range 1 bar to 10 kbar. Previous X-ray determinative curves have been corrected. Unit-cell parameters, when plotted on an a–b diagram using the procedure of Lumpkin et al. (1983), indicate the possibility of a small degree of Ca–Mg disorder on M1 and M2, especially in forsterite-rich compositions. The apparent disorder appears to start at temperatures of about 1200°C; however, all the results are nearly within the stated error limits of complete order.

The two phase region of the forsterite–monticellite join was investigated at 5 and 10 kbar in the temperature range 1100 to 1490°C. Reversals of the solvi to within 1 mole% reveal the following features:

1. The miscibility gap is asymmetric, with the monticellite limb exhibiting a greater degree of solution than the forsterite limb.

2. Pressure has no measurable effect on either solvus limb in the pressure range investigated.

3. A single-site asymmetric solution model adequately fits the two-phase miscibility data with W_GFo = 43.1 kJ/mole − T(6.01 J/deg-mole), and W_GMo = 61.2 kJ/mole − T(15.2 J/deg-mole).