= Petrology
<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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