euhedral

Euhedral is a descriptive term used in mineralogy and geology to characterize crystals that display well-formed, recognizable faces with smooth, regular edges and corners. The term implies that the crystal has achieved its characteristic external form, or habit, with minimal obstruction during growth.

Euhedral crystals typically form in open spaces where there is room for facets to develop unimpeded, such as in crystal-rich cavities, vesicles, or open fractures in igneous and metamorphic rocks, or within pegmatites and hydrothermal veins. Growth conditions often include stable chemistry, adequate time, and low interference from surrounding minerals, allowing a crystal to reach a form close to its ideal lattice-facing geometry.

This habit contrasts with subhedral or anhedral crystals. Subhedral crystals show partially developed faces, indicating some obstruction or rapid growth, while anhedral crystals lack visible crystal faces, often due to rapid growth in crowded environments. Faces of euhedral crystals reveal the internal symmetry and lattice orientation of the mineral.

Common minerals that can exhibit euhedral forms include quartz, feldspars, pyrite, and magnetite, though the degree of euhedra depends on the growth environment. In hand samples and petrographic analysis, the presence of euhedral crystals can provide clues about crystallization conditions, space availability, and the history of the rock.