When rocks are metamorphosed their bulk chemistry remains about the same, except for water and CO2, which are fairly mobile. The mineral assemblages constantly change but the chemistry remains the same. Thus, the temperature and pressure of metamorphism control the mineral assemblages in metamorphic rocks. In 1915, a famous petrologist named Pentti Eskola presented the concept of meta-morphic facies, stating that different assemblages of metamorphic minerals that reach equilibrium during metamorphism within a specific range of physical conditions belong to the same metamorphic facies.
Eskola studied rocks of basaltic composition, so he named his facies according to the metamorphic names for basaltic rocks. His classification, shown in the upper figure on page 541, stands to this day.
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