Petrografiska

Petrografiska, or petrography, is the branch of geology concerned with describing rocks by microscopic examination of thin sections to determine their mineral composition and textural relationships. It relies on optical mineralogy using a petrographic microscope under plane-polarized and crossed-polarized light to identify minerals by properties such as refractive index, birefringence, pleochroism, and extinction angles. Thin sections (about 30 micrometers) are prepared from rock samples to reveal grains, boundaries, and fabric.

Petrography covers all rock classes: igneous, sedimentary, and metamorphic. In igneous rocks, petrography helps distinguish felsic from mafic, and porphyritic from equigranular textures. In sedimentary rocks, textures like grain size, sorting, cement, and porosity are documented. In metamorphic rocks, foliation, lineation, and mineral assemblages indicate metamorphic conditions.

Laboratory methods include optical microscopy; in addition, electron microprobe, X-ray diffraction, and scanning electron microscopy extend mineral chemistry and microtextures.

Typical outputs of petrografiska work include rock-name classification, modal mineralogy, textural descriptions, metamorphic grade, and interpretations of formation environment. Petrography informs exploration geology, petrology, engineering geology, and environmental geology.

Limitations include sample representativeness and diagenetic alteration; interpretation integrates field context and geochemical data. The term petrografiska is commonly used in Swedish-speaking contexts to denote petrographic analyses and related descriptive work in geosciences.