Euhedral and anhedral

Euhedral and anhedral are terms used to describe opposite properties in the formation of crystals.

Euhedral (also known as idiomorphic or automorphic) crystals are those that are well-formed, with sharp, easily recognised faces.

The opposite is anhedral (also known as xenomorphic or allotriomorphic), which describes rock with a microstructure composed of mineral grains that have no well-formed crystal faces or cross-section shape in thin section.

An intermediate texture with some crystal face-formation is termed subhedral (also known as hypidiomorphic or hypautomorphic).

The flat faces (also called facets) are oriented in a specific way relative to the underlying atomic arrangement of the crystal: They are planes of relatively low Miller index.

Euhedral pyrite crystals
A subhedral sample showing sharp to anhedral pyrargyrite crystals.
As a halite crystal is growing, new atoms can very easily attach to the parts of the surface with rough atomic-scale structure and many dangling bonds . Therefore, these parts of the crystal grow out very quickly (yellow arrows). Eventually, the whole surface consists of smooth, stable faces, where new atoms cannot as easily attach themselves.