1911 Encyclopædia Britannica/Pyromorphite

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PYROMORPHITE, a mineral species composed of lead chlorophosphate (PbCl)Pb4(PO4)3, sometimes occurring in sufficient abundance to be mined as an ore of lead.

Crystals are common, and have the form of a hexagonal prism terminated by the basal planes, sometimes combined with narrow faces of a hexagonal pyramid. Crystals with a barrel-like curvature are not uncommon. Globular and reniform masses are also found. As proved by the etched figures on the faces, crystals possess the same parallel-faced hemihedrism as apatite, with which mineral pyromorphite and also mimetite are isomorphous. Between pyromorphite and the corresponding chloro-arsenate (mimetite, (q.v.) the resemblance in external characters is so close that, as a rule, it is only possible to distinguish between them by chemical tests: and they were formerly confused under the names “ green lead ore ” and “ brown lead ore ” (German, Grünbleierz and Braunbleierz). The phosphate was first distinguished chemically by M. H. Klaproth, in 1784, and it was named pyromorphite by J. F. L. Hausmann in 1813, being so named from the Gr. πῦρ (fire) and μορφή (form), because when a fragment of the mineral is fused the globule assumes a faceted form on solidifying. The colour of the mineral is usually some bright shade of green, yellow or brown, and the lustre is resinous. The hardness is 3½ and the specific gravity 6.5–7.1. Owing to isomorphous replacement of the phosphorus by arsenic there may be a gradual passage from pyromorphite to mimetite. Varieties containing calcium isomorphously replacing lead are lower in density (specific gravity 5.9–6.5) and usually lighter in colour; they bear the names "polysphaerite" (because of the globular form), "' miesite ” from Mies in Bohemia, “ nussierite ” from Nussière near Beaujeu, Rhône, France, and "c heroine" from Cherokee county in Georgia. Pyromorphite has resulted from the alteration of galena in the oxidized portions of metalliferous veins, and is frequently met with in the upper levels of lead mines. Finely crystallized specimens have been found at Braubach and Ems in Nassau, Wheal Alfred in Cornwall, Roughten Gill in Cumberland, Leadhills in Scotland, Phoenixville in Pennsylvania, Huelgoat in Finistère, Brittany, &c. At the last-named locality, as well as at Wheal Hope, near Truro in Cornwall, there were formerly found curious pseudomorphs of galena after pyromorphite, known as “ blue lead ore.” (L. J. S.)