Solid solutions and phase transitions in (Ca,M2+)M2+Si2O6 pyroxenes (M2+ = Co, Fe, Mg)

The effect of the substitution of Ca with Co, on the phase transition and on the extension of the the extension of the miscibility gap, was studied to model the general mechanism of phase transitions and solid solutions in (Ca,M2+)M2+Si2O6 pyroxenes. Eleven pyroxenes with composition Ca1-xCo1+xSi2O6, (0 <= x <= 1) were therefore synthesized by piston cylinder at P = 3 GPa, and T between 1100 and 1350 degrees C. The samples were characterized by SEM-EDS, XRD powder diffraction, and TEM. The results were compared with those of Ca-Fe and Ca-Mg pyroxenes. The phase diagram of Ca-Co pyroxenes is similar to that of Ca-Fe and Ca-Mg ones, with a wide asymmetric miscibility gap, and higher solubility in the Ca-rich side of the gap. The solubility on the Ca-rich side of the gap is related to the radius of the cation substituting.