 Talc carbonate mineral assemblages are controlled by temperature and pressure of metamorphism and the partial pressure of carbon dioxide within metamorphic fluids, as well as by the composition of the host rock.In a general sense, talc carbonate metamorphic assemblages are diagnostic of the magnesium content of the ultramafic protolith.Thus, the MgO content of a metamorphosed ultramafic rock can be estimated roughly by understanding the mineral assemblage of the rock.