Experiments have been conducted in the P-T range 2.5-15 GPa and 850-1,500 degrees C using bulk compositions in the systems SiO2-TiO2-Al2O3-Fe2O3-FeO-MnO-MgO-CaO-Na2O-K2O-P2O5 and SiO2-TiO2-Al2O3-MgO-CaO-Na2O to investigate the Ca-Eskola (CaEs Ca-0.5 square 0.5AlSi2O6) content of clinopyroxene in eclogitic assemblages containing garnet + clinopyroxene + SiO2 +/- TiO2 +/- kyanite as a function of P, T, and bulk composition. The results show that CaEsss in clinopyroxene increases with increasing T and is strongly bulk composition dependent whereby high CaEs-contents are favoured by bulk compositions with high normative anorthite and low diopside contents. In this study, a maximum of 18 mol% CaEsss was found at 6 GPa and 1,350 degrees C in a kyanite-eclogite assemblage garnet + clinopyroxene + kyanite + rutile + coesite. By comparison, no significant increase in CaEsss with increasing P could be observed. If the formation of oriented SiO2-rods frequently observed in eclogititc clinopyroxenes is due to the retrogressive breakdown of a CaEs-component then these textures are a cooling rather than a decompression phenomenon and are most likely to be found in kyanite-bearing eclogites cooled from temperatures >= 750 degrees C. The presence of clinopyroxene with approx. 4 mol% CaEsss in an experiment conducted at 2.5 GPa/850 degrees C confirms earlier suggestions based on field data that vacancy-rich clinopyroxenes are not necessarily restricted to ultrahigh pressure metamorphic conditions.
Konzett, J., Frost, D. J., Proyer, A., & Ulmer, P. (2008). The Ca-Eskola component in eclogitic clinopyroxene as a function of pressure, temperature and bulk composition: an experimental study to 15 GPa with possible implications for the formation of oriented SiO2-inclusions in omphacite. Contributions to Mineralogy and Petrology, 155(2), 215-228. https://doi.org/10.1007/s00410-007-0238-0