Pressure dependence of crystal structures in the system MgOAl 2O 3SiO 2H 2O at pressures up to 30 kilobars

Abstract

The stability relations, as determined by experiment, of the following quaternary and anhydrous ternary phases of the system MgOAl 2O 3SiO 2H 2O are reviewed in the light of their crystal structures: Cordierite, gedrite, aluminous enstatite, boron-free kornerupine, sapphirine, yoderite, Mg-staurolite, pyrope, aluminous talc, and chlorite. With the exception of the last two phases, there is a consistent relationship between density, oxygen packing and coordination of Al on one side, and stability with respect to pressure on the other side: Densely packed phases with only octahedral Al like pyrope and Mg-staurolite are only stable at high pressures, whereas loosely packed cordierite with only tetrahedral Al is only stable at low pressures. The phases gedrite, aluminous enstatite, boron-free kornerupine, sapphirine, and yoderite exhibiting intermediate degrees of packing and (except yoderite) both tetrahedral and octahedral Al are stable at intermediate pressures. The phyllosilicates talc and chlorite are pressure-indifferent within the range considered, although they have low densities and contain Al both in tetrahedral and octahedral coordination.