Crystal shape (igneous rocks)
Igneous rocks derive from the solidification of magma or lava in crystals and, eventually, volcanic glass. When they crystallize from a melt, growing crystals compete for space and may get reabsorbed if their chemical composition is in disequilibrium with the surrounding melt. Consequently, the shape of crystals is an important textural hint of the crystallization history of an igneous rock.
Euhedral, subhedral, anhedral
Crystals in rocks are classified as euhedral, subhedral, and anhedral based on their crystal shape. These three terms comes from the same Greek root, hedron, meaning ‘face’ (of a solid figure). Eu means ‘good’, hence euhedral ‘good face’ (i.e. well-formed face). The term ‘anhedral’ contains the alpha privative and it means the opposite (no faces). Subhedral has an intermediate meaning between the two (some faces).
Crystals are said to be euhedral (synonyms: idiomorphic, automorphic) when they are bounded by their characteristic crystal faces, in other words when they show their crystal habit. In igneous rocks, crystals are able to be euhedral only when they have been able to grow undisturbed within the magma, not interacting or interfering with neighboring crystals, and without being corroded by the melt. In general, euhedral crystals represent the earliest products of crystallization from the melt.
Anhedral crystals (synonyms: xenomorphic, allotriomorphic) are crystals that completely lack their characteristic crystal shape. This includes crystals with no-well defined shape, for example embayed crystals with irregular outline, but also interstitial grains that grew in the space left between other, pre-existing crystals. In this latter sense, anhedral crystals may be bounded by sharp crystal faces but are never their crystal faces but the faces of surrounding crystals.
Subhedral crystals (synonym; ipidiomorphic) are bound by only some of their characteristic crystal faces. This means that were able to crystallize with their characteristic habit only in part, due to the presence of neighboring crystals that partly constrained their growth.