OICC Press Iranian Journal of Earth Sciences 2228-785X 2026 07 14 Experimental study of Di-CaEs solid solution at pressures from 1 atm to 3.0 GPa 10.57647/j.ijes.2025.17558 EN Sofja Viktorovna Banushkina V.S. Sobolev Institute of Geology and Mineralogy, Siberian Branch, Russian Academy of Sciences, Novosibirsk, Russia Zoia Friedrichovna Golitsyna V.S. Sobolev Institute of Geology and Mineralogy, Siberian Branch, Russian Academy of Sciences, Novosibirsk, Russia Alexey Anatolyevich Kirdyashkin V.S. Sobolev Institute of Geology and Mineralogy, Siberian Branch, Russian Academy of Sciences, Novosibirsk, Russia Journal Article 2026 07 14 The possibility of ‘excess’ silica dissolution in clinopyroxene (Cpx) as the calcium molecule of Eskola (CaEs) end member has been known for a long time. Currently, attempts to determine the dependence of non-stoichiometric Cpx compositions on (P, T)-parameters are ongoing. This article presents data on the analysis of Cpx compositions and the structural and textural features of a phase association obtained from an experimental study of the Di-CaEs cross-section in a ‘dry’ CMAS system. At atmospheric pressure, the temperature range is 960-1240±10 oC. At pressures of 1.0-3.0 GPa, the temperature range is 1170-1550±10 oC. Experiments with synthetic compositions were carried out in a vertical shaft electric resistance furnace with silicon carbide heaters (at atmospheric pressure) and by the quenching method on a piston-cylinder apparatus (at high pressure). It is noted that in the investigated cross-section, two independent stable pyroxene phases are formed: an aluminum-bearing Cpx and a high-magnesian aluminum-free diopside (Di). The composition of clinopyroxene solid solution Cpx(ss) is represented by a quad series of diopside (Di), enstatite (En), calcium molecule of Tschermak (CaTs), calcium molecule of Eskola (CaEs) end members. The diopside phase belongs to the Di-En series. The influence of P-T parameters and bulk chemical composition on the content of CaEs in Cpx is considered. The trend of eutectic equilibria exists in the temperature range from 1137±10 оС (at 1 atm) to 1300±10 оС (at 3.0 GPa). The experimental data obtained can be used in the future to build a physicochemical model of the evolution of quartz-normative rocks.