The type of metamorphic rock formed depends on protolith. During the metamorphic process, minerals are subjected to temperatures and pressures unlike those in which they formed. The minerals become unstable and, in the attempt to become stable, elements are released and reused to form new minerals (neocrystallization).
Some protoliths yield specific rocks. There are four broad compositional classes.
- Pelitic - these rocks have a shale protolith. Aluminum-rich clay minerals, abundant in shale rocks, transform into micas. The rock type form depends on grade of metamorphism:
(low grade) slate → phyllite → schist → gneiss (high grade)
- Basic (or Mafic) - These rocks have a basalt or gabbro protolith, which are rich in Fe and Mg and poor in Si, Al, Na, and K. These protoliths will turn into biotite- and amphibole-dominated rocks.
- Calcareous - Carbonate protolith (rocks rich in Ca (limestones) and Ca and Mg (dolostones)). Rocks composed of a single mineral - like calcite in limestones - do not change much during the metamorphic process. The minerals will recrystallize into calcite and dolomite marbles
- Quartzo-feldspathic - originate from a granitic protolith. Quartz + feldspar in granites are stable under metamorphism. They will recrystallize and become foliated into granitic gneisses.