Progressive Metamorphism of Siliceous Limestone and Dolomite

The metamorphism of siliceous dolomitic limestone is considered with the aid of a composition tetrahedron which suggests that the changes may be referred to thirteen steps or grades of increasing decarbonation, taking place at successively higher temperatures at any given pressure. At the temperature appropriate to each step a certain phase assemblage becomes unstable and each step is characterized by the disappearance of its appropriate phase assemblage. The survival of that phase assemblage in a rock is rigorously indicative of the fact that the step in question has not been attained. In most of the steps, to wit ten, the disappearance of its phase assemblage is accompanied by a more conspicuous phenomenon, the appearance of a new phase, a meta-morphic mineral, but this appearance of a new phase cannot be regarded as rigorously indicative of the accomplishment of the step since the new phase can be produced otherwise. Nevertheless, if certain additional conditions are fulfilled, the new phase will appear only when the temperature of the appropriate step is attained and the metamorphic minerals then become indicators of grade of metamorphism. In the order of the rising temperature steps at which they are produced the ten minerals are tremolite, forsterite, diopside, periclase, wollastonite, monticellite, akermanite, spurrite, merwinite, and larnite. Examination of their natural occurrence suggests that they are for the most part produced under conditions which permit their use as temperature indicators.