Pétrographique

Pétrographique, or petrography in English, is the study of rocks through microscopic analysis of their mineralogy and textures. It focuses on identifying minerals, understanding their relationships, and interpreting the history and conditions of rock formation by examining thin sections prepared from rock samples.

The core method is optical petrography. A rock fragment is cut into a thin section about 30 micrometers thick and examined in a petrographic microscope under plane-polarized light and cross-polarized light. Observations include mineral identity, grain size and shape, fabric, textural relationships, and deformation features. Optical properties such as refractive index, pleochroism, birefringence, interference colors, extinction angles, and cleavage help distinguish minerals. Modal analysis and paragenetic relationships (the sequence of mineral growth) are often recorded to describe the rock comprehensively.

Applications span the three principal rock families. In igneous petrography, rocks are described by mineralogy and texture (e.g., felsic vs. mafic, holocrystalline or porphyritic). In sedimentary petrography, the focus is on quartz, feldspar, lithic fragments, and cement, with point counting used for classification schemes like QFL. In metamorphic petrography, mineral assemblages and textures reveal metamorphic conditions and history, including degree of deformation and metamorphic grade.

Modern petrography integrates digital imaging, image analysis, and supplementary techniques such as electron microprobe analysis and scanning electron microscopy to obtain chemical data on minerals. Petrographic study remains fundamental in geology for mineral exploration, academic research, and environmental assessments, providing detailed insight into rock origin and evolution.