| Annotation |
Natural clinopyroxenes are usually composed of stoichiometric end-members, but in some cases, chemical analyses show some excess of SiO2 that is explained by the possible existence of end-member Ca0,5AlSi2O6. However, in recent experimental works have shown the existence of additional, previously undiagnosed phase α-diopside (αDi) in section CaMgSi2O6 (Di) – Ca0,5AlSi2O6 (CaEs). Also recent experiments have shown the absence of nonstechiometric of clinopyroxenes at high pressures. However, there remained the probability of nonstoichiometric clinopyroxenes at lower pressures, in particular at atmospheric pressure.
In this regard, a study was made of phase relations in the cross-section of the diopside – calcium molecule Eskola at atmospheric pressure. It was found that at atmospheric pressure in this section there is a phase α-diopside (αDi). This phase is close in composition to diopside, does not contain alumina, and does not belong to solid solutions of clinopyroxene series. At atmospheric pressure in a solidus of section CaMgSi2O6 – Ca0,5AlSi2O6 the following phase volumes are observed: Cpx+An+Tr and Cpx+αDi+An+Tr. The melting happens at the eutectic reaction L=Cpx+αDi+An+Tr at temperatures 1137…1208 °C. The analysis of the compositions of the coexisting phases have shown the absence of excess silica content in clinopyroxenes
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