Petrology studies how rocks form and change, using their composition, texture and mineral assemblages. Crystals and surrounding material record processes such as partial melting, fractional crystallization, deformation and fluid reaction. These observations can be compared with laboratory phase equilibrium measurements and thermal models.
An assemblage constrains a range of conditions rather than a complete history by itself. Minerals can retain older cores, grow new rims or fail to equilibrate fully during a short event. Linking textures with mineral chemistry and dates helps distinguish the original formation of a rock from later metamorphism.
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