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

Institut für Mineralogie, Petrologie und Geochemie, Ludwig-Maximilians-Universität München, Theresienstrasse 41, 80333 München, Germany

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, ß, and V by 0.0020 Å, 0.0012 Å, 0.0016 Å, 0.01°, and 0.28 ų per mol% CaMgTiSiO₆, respectively.

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