HEMIMORPHITE  , a

mineral consisting of hydrous zinc silicate, H2Zn2SiO5, 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 terminated at the two ends . In the figure, the faces at the upper end of the crystal are the basal plane k and the domes o, p, 1, m, whilst at the lower end there are only the four faces of the pyramid P . 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 opposite end . There are perfect cleavages parallel to the prism faces (d in the figure) . 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 hydrochloric acid with gelatinization; this property affords a ready means of distinguishing hemimorphite from calamine (zinc carbonate), these two minerals being, when not crystallized, very like each other in appearance . The water contained in hemimorphite is expelled only at a red heat, and the mineral must therefore be considered as a basic metasilicate, (ZnOH)2SiOa . The name hemimorphite was given by G . A .

Kenngott in 1853 because of the typical hemimorphic development of the crystals . The mineral had long been confused with calamine (q.v.) and even now this name is often applied to it . On account of its pyroelectric properties, it was called electric calamine by J . Smithson in 1803 . Hemimorphite occurs with other ores of zinc (calamine and blende), forming veins and beds in sedimentry limestones . British localities are Matlock, Alston, Mendip Hills and Lead-hills; at Roughten Gill, Caldbeck Fells, Cumberland, it occurs as mammillated incrustations of a sky-blue colour . Well-crystallized specimens have been found in the zinc mines at Altenberg near Aachen in Rhenish Prussia, Nerchinsk mining district in Siberia, and Elkhorn in Montana . (L . J .