Clinopyroxene Solid Solutions in the CaMgSi2O6 – Ca0,5AlSi2O6 Cross-Section at High P-T Parameters

Abstract

Clinopyroxenes (Cpx) are one of the main rock-forming minerals, but stoichiometry of their compositions was called into question. In particular, an idea of hypothetical calcium molecule Eskola (CaEs, $C{a}_{0,5}AlS{i}_2{O}_6$) existence was expressed. This minal has structure vacancy and silica excess. Numerous experimental investigations in CMAS-system ($CaO-MgO-A{l}_2{O}_3-Si{O}_2$) have showed that the question of non-stoichiometric Cpx existence remains open. This paper presents the results of an experimental study of the diopside Di ($CaMgS{i}_2{O}_6$) – calcium molecule Eskola CaEs ($C{a}_{0,5}AlS{i}_2{O}_6$) cross-section in the CMAS-system. The experiments were carried out in the following pressure and temperature range: $P={10}^{-4} – 3,0$ GPa; $T=966 – {1525}^0$C. Experiments at atmospheric pressure (${10}^{-4}$ GPa) were performed on a vertical shaft electric resistance furnace; high-pressure ones were performed on a "piston-cylinder" type apparatus. Samples obtained were analyzed using electron microprobe (EMP), scanning electron microscope (SEM) and Raman spectrometer. Depending on the P-T conditions, the samples contain the following phases: anorthite An, garnet Grt, diopside Di, clinopyroxene Cpx, quartz Qtz (tridymite Tr – for experiments at atmospheric pressure), and glass L. The data array on the composition of clinopyroxenes crystallized in this cross-section with diopside in various associations is generalized and supplemented. Clinopyroxenes were found to form quaternary solid solutions of diopside Di ($CaMgS{i}_2{O}_6$) – enstatite En ($M{g}_{2}S{i}_2{O}_6$) – calcium molecule Tschermak CaTs ($CaA{l}_{2}Si{O}_6$) – calcium molecule Eskola CaEs ($C{a}_{0,5}AlS{i}_2{O}_6$). The CaTs and CaEs minals contents are positively correlated with the amount of aluminum in clinopyroxene, and this relationship is particularly pronounced for CaTs. It is confirmed that clinopyroxenes in this cross-section can contain an excess of silica at both atmospheric and high pressures. Apparently, the cation vacancy that exists in pyroxene structure can participate in ordering processes. As a result the pyroxenes of another structure (not diopside – C2/c-symmetry) can be crystallized from total compositions in the Di-CaEs cross-section. Additional research is needed to support this hypothesis. Besides, at present investigation it was not possible to establish an unambiguous relationship between the Cpx composition and P-T-parameters, since it is also associated with both the mixture initial composition and the mineral association. Further experiments are required to justify any geothermobarometric dependence.