hypidiomorphic

Hypidiomorphic is a term used in crystallography and mineralogy to describe crystals that show partial development of the mineral’s characteristic external form. A hypidiomorphic crystal has some faces that are well developed and conform to the ideal crystal form, while other faces are poorly developed, missing, or irregular. The word combines hypo- meaning under or less with idiomorphic, meaning having the same form as the crystal’s ideal shape.

This habit lies between idiomorphic, where all faces of the external form are fully developed, and allotriomorphic, where the crystal lacks most of the faces typical of the mineral’s external form. Hypidiomorphic development occurs when growth conditions favor the formation of only some faces, or when later processes such as dissolution or overgrowth by other forms alter portions of the crystal.

Causes of hypidiomorphic habit include nonuniform growth rates along different crystallographic directions, changes in chemical environment during growth, or partial replacement of faces by secondary growth. The resulting crystals exhibit a mixed appearance: portions of the crystal outline resemble the true form, while other portions appear truncated, irregular, or less defined.

In practice, recognizing hypidiomorphic crystals requires noting which faces are well formed and which are not, and understanding that the partial development reflects the history of the crystal’s growth conditions. See also idiomorphic, allotriomorphic, and related terms in crystal morphology.