| Metamorphic rocks form when pre-existing rocks (igneous, sedimentary, or metamorphic) are exposed to high temperatures and pressures under the Earth's surface. Some pre-existing minerals, unstable at the higher temperature and pressure conditions, transform into new minerals. Others recrystallize and grow larger.
Metamorphic rocks tend to be dominated by minerals you already know: feldspar, quartz, muscovite, biotite, amphibole, and calcite/dolomite. However, a few minerals are found exclusively or mainly in metamorphic rocks:
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