Abstract

To investigate the substitution of Ti4+ for Si4+ on tetrahedral sites in clinopyroxene, a series of Ti-bearing clinopyroxene compositions was synthesized in the system CaO-MgO-SiO2-TiO2 (CMST) at pressures from 1 atm to 2 GPa and temperatures from 800 to 1200 °C. Along the join CaMgSi2O6(diopside)–CaMgTiSiO6, clinopyroxene crystallizes, depending on coexisting phases, as binary solid solutions (diopside-CaMgTiSiO6) or ternary solid solutions (diopside-CaMgTiSiO6-enstatite). The solubility of Ti increases with increasing temperature, whereas it decreases with increasing pressure. The binary clinopyroxene has an invariant composition (CaMgSi2O6)93(CaMgTiSiO6)7 (mol%) corresponding to a Ti content of 2.6 wt% at 1000 °C and 1 atm. Magnesium substitutes in the M2 site in ternary clinopyroxene and its solubility mainly increases with increasing temperature. The substitution of Si4+ by the larger Ti4+ cation on tetrahedral sites causes a linear increase in a, b, c, β, and V by 0.0020 Å, 0.0012 Å, 0.0016 Å, 0.01°, and 0.28 Å3 per mol% CaMgTiSiO6, respectively.

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