rocktexture

Rock texture is the overall appearance and arrangement of mineral grains or crystals within a rock. It includes grain size, shape, boundaries between grains, sorting, and any fabric or layering. Texture reflects the rock’s formation history and is distinct from mineral composition, which defines rock type. Texture can be assessed on hand specimens or in thin sections viewed under a microscope.

Igneous textures record cooling and crystallization. Aphanitic textures have crystals too small to see with the naked eye, as in basalt or rhyolite. Phaneritic textures have visible grains, as in granite. Porphyritic textures contain larger crystals, or phenocrysts, set in a finer groundmass. Glassy textures form when lava cools so rapidly that crystals do not form, as in obsidian. Vesicular textures show gas-bubble holes, common in some volcanic rocks.

Sedimentary textures arise from grain size, shape, and how grains are bonded. Clastic textures show transported particles with varying sorting and rounding (for example, sandstone and shale). Bioclastic textures come from shell fragments or other organic matter. Crystalline or chemical sedimentary rocks, such as limestone and rock salt, consist of interlocking grains that precipitated from solution. Diagenetic cementation binds grains into solid rock.

Metamorphic textures develop under pressure and temperature during metamorphism. Foliated textures display planar mineral alignment, producing layers (slate, schist, gneiss). Non-foliated textures are granular and lack a preferred orientation (marble, quartzite). Porphyroblasts are conspicuous large crystals that grow within a finer metamorphic matrix.

Texture analysis aids in identifying rock types and inferring formation conditions and history, and is studied in the field and in petrographic work.