petrographic

Petrographic is an adjective relating to petrography, the branch of geology that describes rocks through microscopic analysis and textural study. The core of petrographic work is optical petrography, conducted with thin sections about 30 micrometers thick examined under a petrographic microscope with polarized light. Minerals are identified by optical properties such as color, relief, birefringence, pleochroism, extinction, and crystal morphology. The technique allows determination of mineral assemblages, modal proportions, textures, and relationships between minerals and surrounding rock.

Petrographic analysis provides detailed information about rock formation and history. In igneous petrology, it helps reveal crystallization sequence and cooling history by identifying minerals such as quartz, feldspars, pyroxenes, olivine, and micas and by analyzing textures like grain boundaries and zoning. In sedimentary petrography, it supports rock classification and diagenetic interpretation through the study of framework grains, matrix, cement, and porosity, often using point counting to estimate mineral abundances. In metamorphic geology, petrography aids in recognizing metamorphic minerals and textures that indicate pressure–temperature conditions and deformation histories.

Modern petrography extends beyond traditional optical work to include electron microscopic techniques, such as scanning electron microscopy and electron microprobe analysis, as well as cathodoluminescence imaging and automated image analysis. These methods enable quantitative mineral analyses and microstructural investigations that complement thin-section studies. Petrographic data underpin the classification, interpretation, and modeling of rocks, contributing to fields ranging from resource exploration to environmental geology.