THE EFFECT OF TSCHERMAK SUBSTITUTION ON ASSEMBLAGES IN ALUMINOUS DOLOMITES

A calculated petrogenetic grid for the system CaO-MgO-Al2O3-SiO2-CO2-H 2O (CaMASCH), incorporating Tschermak's substitutions in amphibole, ch lorite, talc and clinopyroxene, is used to examine phase relationships in aluminous marbles. A series of diagrams illustrating the effect up on stable mineral assemblages of increasing the aluminium content of a bulk composition is used to show the way aluminous minerals enter min eral assemblages in progressively more aluminous rocks. The effects of changing pressure and the incorporation of Fe into the bulk compositi on on the stable mineral assemblages are also examined. The calculated equilibria are shown to be in reasonable agreement with natural assem blages, and the incorporation of new experimental data on amphiboles i nto the existing dataset is shown to improve the agreement between obs erved and natural amphibole compositions.