Petrographie

Petrographie, in English petrography, is the branch of geology that studies rocks through microscopic examination of prepared rock thin sections. It focuses on identifying minerals, determining their relative abundances, and describing textures and fabrics to interpret a rock’s origin and history. The field integrates mineralogy, petrology, and geochemistry to link mineral makeup and textures with formation processes.

Core methods center on optical petrography using a transmitted-light polarizing microscope. Thin sections about 30 micrometers thick are prepared and observed under cross-polarized and plane-polarized light. Minerals are identified by properties such as color under polarized light, interference colors, birefringence, pleochroism, cleavage, twinning, and extinction. Rocks are described in terms of mineral modes, grain size, zoning, porphyroblasts, foliation, fabric, and relationships between grain boundaries and mineral populations. Classification typically divides rocks into igneous, metamorphic, and sedimentary categories, with further subdivisions by mineralogy and texture.

In practice, petrography combines descriptive observation with quantitative and instrumental techniques. Modal analysis (point counting) estimates mineral abundances; X-ray diffraction determines bulk and mineral-specific phases; electron microprobe and SEM-EDS provide precise chemical compositions at small scales. Advanced methods such as cathodoluminescence and image analysis aid in revealing growth textures and diagenetic or metamorphic histories.

History and scope: Petrography developed in the 19th century with advances in thin-section preparation and optical mineralogy. Figures such as Henry Clifton Sorby contributed to its modern development, alongside earlier work on microscopes and mineral identification. Today, petrography remains essential in fields ranging from academic geology to mineral exploration, engineering geology, ceramics, and archaeology.