Thermodynamic equilibrium of Al 2 SiO 5 polymorphs in the triple point as a function of rock and fluid composition

Abstract

Abstract For decades the geologists have discussed whether the Al2SiO5 polymorphs (andalusite, kyanite and sillimanite) can co-exists in thermodynamic equilibrium in a real rock at P–T conditions of the triple point or not. A triple-point rock sample was not so far identified reliably in nature. The thermodynamic-potentials minimization technique allows us to determine the bulk rock and fluid composition, other minerals in the assemblage, and other factors that could influence the stability of aluminosilicates. All three polymorphs were predicted to occur simultaneously in a mineral assemblage at P–T conditions of the triple point. Simple mineral assemblages consisting of plagioclase ± quartz ± paragonite ± graphite, biotite ± muscovite ± quartz ± magnetite ± graphite were studied in order to determine the conditions of the stable coexistence of the Al2SiO5 polymorphs. The study demonstrates how the hydrogen-, oxygen- and carbon-content in fluid can define the mineral assemblage and the coexistence of the Al2SiO5 polymorphs in it. The effect of such elements as Na, Fe, Mg, Mn, Ti and P was studied using bulk compositions of natural rock samples. A series of restrictions on the bulk composition of the host rock was identified.