petrography

Petrography is the branch of geology that describes and interprets rocks through their mineral content and textures, primarily by microscopic examination of thin sections. It combines optical mineralogy with systematic description to infer the origin and history of rocks in igneous, sedimentary, and metamorphic categories.

Methods in petrography include preparing thin sections about 30 micrometers thick and analyzing them with a petrographic microscope under plane-polarized and cross-polarized light to identify minerals and textures. Reflected-light microscopy is used for opaque minerals. Modern petrography also employs electron microprobe analysis, scanning electron microscopy, and cathodoluminescence to obtain quantitative mineralogy and textural details. Modal analysis, often by point counting or image analysis, yields mineral proportions; normative calculations such as the CIPW norm assist igneous rock naming. Classification schemes include QAPF diagrams for felsic rocks and QFL or other mineralogical plots for sedimentary rocks.

Applications of petrography are broad. It supports rock classification and interpretation of formation processes, deformation, metamorphism, and post-formational histories. In sedimentary geology, it aids provenance studies and fabric analysis; in metamorphic petrology, it helps determine grade and metamorphic conditions; and in petroleum geology, it characterizes reservoir rocks and diagenetic alteration. Petrography also provides essential context for geochemical and geochronological work, linking mineralogy and textures to tectonic and climatic histories.

The term derives from Greek petra, rock, and graphein, to write. The modern method emerged in the 19th century, with significant advances in thin-section technique and optical analysis that established its foundational role in rock interpretation.