The stability of clinopyroxene in the system CaO-MgO-SiO2-TiO2 (CMST)

To investigate the substitution of Ti4+ for Si4+ On tetrahedral sites in cl inopyroxene, a series of Ti-bearing clinopyroxene compositions was synthesi zed in the system CaO-MgO-SiO2-TiO2 (CMST) at pressures from 1 atm to 2 GPa and temperatures from 800 to 1200 degreesC. Along the join CaMgSi2O6(diops ide)-CaMgTiSiO6, clinopyroxene crystallizes, depending on coexisting phases , as binary solid solutions (diopside-CaMgTiSiO6) or ternary solid solution s (diopside-CaMgTiSiO6- enstatite). The solubility of Ti increases with inc reasing temperature, whereas it decreases with increasing pressure. The bin ary clinopyroxene has an invariant composition (CaMgSi2O6)(93)(CaMgTiSiO6)( 7) (mol%) corresponding to a Ti content of 2.6 wt% at 1000 degreesC and 1 a tm. Magnesium substitutes in the M2 site in ternary clinopyroxene and its s olubility mainly increases with increasing temperature. The substitution of Si4+ by the larger Ti4+ cation on tetrahedral sites causes a linear increa se in a, b, c, beta, and V by 0.0020 Angstrom, 0.0012 Angstrom, 0.0016 Angs trom, 0.01 degrees, and 0.28 Angstrom (3) per mol% CaMgTiSiO6, respectively .