Stability relations of K₂Mg₅Si₁₂O₃₀, and end member of the merrihueite-roedderite group of meteoritic minerals

Abstract

The phase K₂Mg₅Si₁₂O₃₀ was synthesized both hydrothermally and dry under a variety of pressures and temperatures, and its stability relations were determined. Under hydrothermal conditions it exhibits a lower stability limit lying at 595°C, 1 kb, and 650°C, 2 kb, due to its breakdown into the hydrous assemblage quartz+KMg_2.5Si₄O₁₀(OH)₂ (a mica phase). Its upper temperature stability under hydrothermal conditions is given by its incongruent melting to MgSiO₃+liquid. Near 820° C at a fluid pressure of approximately 6.5 kb the two univariant curves for these breakdown reactions intersect thus limiting the stability field to lower fluid pressures. — Under anhydrous conditions K₂Mg₅Si₁₂O₃₀ becomes unstable at pressures between approximately 7 and 32.5 kb due to its incongruent melting to the assemblage MgSiO₃+quartz (or coesite)+liquid; this melting curve has a pronounced negative slope. No subsolidus breakdown assemblage was encountered at 32.5 kb down to temperatures as low as 750°C. This behavior is probably due to the instability of other ternary compounds in the system K₂O-MgO-SiO₂ at high pressures and thus to the existence of very low-temperature eutectics involving only binary and unary solid phases plus liquid.

It is likely that these stability relations provide a model for those of the natural minerals merrihueite and roedderite which contain Na and Fe₊₂ partly substituting for K and Mg and which were encountered in several meteorites. Therefore, the cosmic events leading to the formation of these minerals must have taken place at relatively low pressures and high temperatures, especially when water was present. The bulk compositions of these minerals appear to be incompatible with average chondritic matter under equilibrium conditions. Hence merrihueite and roedderite are not likely to be found in equilibrated chondrites which contain feldspars instead.