Excess Silica in Stoichiometric and Non-Stoichiometric Clinopyroxene

Mulcahy, S R and Day, H W

The growth of quartz lamellae in omphacitic clinopyroxene has been interpreted as the result of exsolution of excess silica from a supersilicic host during decompression from high or ultrahigh pressure. The interpretation of mechanisms by which free silica might form in eclogite depends in part on which of several possible definitions of excess silica is adopted, but there is no agreement in the literature about how best to calculate the excess from chemical analyses. Our algebraic and graphical analysis shows that common methods of calculating excess silica yield compatible results and that four independent components are sufficient to describe the essential features of excess silica in clinopyroxene: CaMgSi₂O₆, (Di); CaAl₂SiO₆, (CaTs); NaAlSi₂O₆, (Jd); and either Ca0.5AlSi₂O₆, (CaEs), or SiO₂ (Qtz). Using two commonly chosen coordinate systems for Na-free pyroxene, Di-CaTs-Qtz and Di-CaTs-CaEs, we provide an internally consistent overview and reconciliation of the several definitions of excess silica and emphasize that recalculations of pyroxene chemical analyses yield no information not already in structural formulas. Excess silica in non-stoichiometric pyroxene with M-site vacancies is most commonly defined as positive Qtz or CaEs component relative to the Di-CaTs join. A common alternative definition using cations per formula unit, (Si+Ti-2Na) > (Ca+Mg+Fe+Ni+Mn), is shown to be equivalent to using Qtz as the measure of excess silica. Contours of total cation or vacancy content are parallel to the Di-CaTs join in both coordinate systems. Isopleths of Si per formula unit are parallel to the Di-CaEs join in both systems because both Di and CaEs component contain two Si per six oxygen anions. Compositions on the high silica side of the Di-CaEs join must contain six-coordinated silicon. Synthetic, stoichiometric Na-pyroxene is also known to contain excess six-coordinated Si and similar inclusions have been reported in diamond. We suggest that the term ``supersilicic'' be restricted to pyroxene compositions with Si > 2 per formula unit. The term ``silicic'' may be used for non-stoichiometric compositions with 2 > Si > Di-CaTs join. The distinction between excess silica in stoichiometric and non-stoichiometric pyroxene may require different mechanisms for exsolution of quartz lamellae and have different implications for HP versus UHP origins.

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