HEMIMORPHITE

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, l, 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, sometimes 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)2sio3. [illustration] 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 sedimentary limestones. british localities are matlock, alston, mendip hills and leadhills; 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. s.)