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HEMIMORPHITE

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Originally appearing in Volume V13, Page 258 of the 1911 Encyclopedia Britannica.
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HEMIMORPHITE , a See also:

mineral consisting of hydrous See also:zinc silicate, H2Zn2SiO5, of importance as an ore of the See also:metal, of which it contains 54.4%. It is interesting crystallographically by See also:reason of the See also:hemimorphic development of its orthorhombic crystals; these are prismatic in See also:habit and are differently terminated at the two ends. In the figure, the faces at the upper end of the crystal are the basal See also:plane k and the domes o, p, 1, m, whilst at the See also:lower end there are only the four faces of the See also:pyramid P. Connected with this See also:polarity of the crystals is their pyroelectric See also: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 See also: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 See also:condition intermixed with See also:calamine and See also:clay. It is decomposed by hydrochloric See also:acid with gelatinization; this See also:property affords a ready means of distinguishing hemimorphite from calamine (zinc carbonate), these two minerals being, when not crystallized, very like each other in See also:appearance. The See also:water contained in hemimorphite is expelled only at a red See also:heat, and the mineral must therefore be considered as a basic metasilicate, (ZnOH)2SiOa. The name hemimorphite was given by G. A.

See also:

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

End of Article: HEMIMORPHITE

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