subhedral

Subhedral is a term used in mineralogy and petrology to describe a crystal habit in which part of the crystal shows well-formed, recognizable faces, while other faces are absent, eroded, or irregular. It denotes a crystal that lies between euhedral (fully bounded by a complete set of crystal faces) and anhedral (lacking visible faces). The partial development of faces reflects the growth environment and history, not the mineral’s composition.

Crystals become subhedral when growth is partially constrained or interrupted. Causes include limited space within a rock, rapid crystallization, partial dissolution, or interaction with neighboring minerals during growth or subsequent alteration. As a result, some surfaces may display clear, angular faces, whereas other surfaces remain smooth, rough, or unoriented.

In petrography, subhedral habit helps infer the crystallization conditions of minerals in igneous and metamorphic rocks. For example, feldspars or pyroxenes may appear subhedral in crowded assemblages, while more open spaces can yield euhedral forms. The description is descriptive rather than absolute; a single crystal can exhibit subhedral characteristics that vary across its facets.

Subhedral is distinct from hypidiomorphic forms, which describe partially developed crystals with many faces present but not a complete set, and from anhedral, where no crystal faces are evident. Understanding crystal habit, including subhedral shapes, contributes to interpretations of growth environment, texture, and history in geological samples.