metamorphic rocks formed by contact metamorphism under conditions of high temperature and low pressure.
The products of metamorphism in contacts with extrusive and subvolcanic igneous rocks are classed with sanidinite faciès. Under these conditions, argillaceous schists are transformed into rocks with high-temperature potassium feldspar (sanidine) called sanidinites; calcareous rocks (marls and basic extrusives) become metamorphic forms composed of high-temperature, potassium-rich silicates, for example, larnite Ca2[SiO4] and wollastonite, and carbonate-silicates, such as spurrite Ca(SiO4)2CO3 and tilleyite Ca5(Si2O7[CO3]2).
Subfacies of the sanidinite facies are identified by associations of calcium-bearing minerals: spurrite or larnite (formed at the highest temperature, above 1,100°C), tilleyite (1,100°-1,000°C), calcite-wollastonite-monticellite or calcite-akerman-ite (1,000°-900°C), and diopside-calcite-wollastonite (below 900°C).
REFERENCESFatsii metamorfizma. Moscow, 1970.
Marakushev, A. A. Petrologiia metamorficheskikh gornykh porod. Moscow, 1973.
Eskola, P. “The Mineral Facies of Rocks.” Norsk Geologisk Tidsskrift, 1921, vol. 6.
A. A. MARAKUSHEV