HEMIMORPHITE, a mineral consisting of hydrous zinc silicate, of importance as an ore of the metal, of which it contains 54-4%. It is interesting crystallographically by reason of the hemimorphic development of its orthorhombic crystals; these are prismatic in habit and are differently termi nated at the two ends. Connected with this polarity of the crystals is their pyroelectric character—when a crystal is subjected to changes of temperature it becomes positively electrified at one end and negatively at the other. There are perfect cleavages par allel to the prism faces. Crystals are usually colourless, some times yellowish or greenish, and transparent ; they have vitreous lustre. The hardness is 5, and the specific gravity 3.45. The mineral also occurs as stalactitic or botryoidal masses with a fibrous structure, or in a massive, cellular or granular condition intermixed with calamine and clay. It is decomposed by hydro chloric acid with gelatinization ; this property affords a ready means of distinguishing hemimorphite from calamine (zinc car bonate), these two minerals being, when not crystallized, very like each other in appearance.
Hemimorphite occurs with other ores of zinc (calamine and blende) , forming veins and beds in sedimentary limestones. British localities are Matlock, Alston, Mendip hills and Lead hills ; at Roughten Gill, Caldbeck Fells, Cumberland, it occurs as mammillated sky-blue incrustations. Well-crystallized specimens have been found in the zinc mines at Altenberg (Vieille Montagne) in Belgium, Nerchinsk, Siberia and Elkhorn, Montana.