MELTING OF ASSOCIATIONS WITH a-DIOPSIDE IN THE CaMgSi<sub>2</sub>O<sub>6</sub>-Ca<sub>05</sub>AlSi<sub>2</sub>O<sub>6</sub> SECTION AT ATMOSPHERIC PRESSURE
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 a-diopside (aDi) in section CaMgSi2O6 (Di) - Ca0 5AlSi2O6 (CaEs). Also recent experiments have shown the absence of non-stechiometric 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 Es-kola at atmospheric pressure. It was found that at atmospheric pressure in this section there is a phase a-diopside (aDi). 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+aDi+An+Tr. The melting happens at the eutectic reaction L=Cpx+aDi+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