1911 Encyclopædia Britannica/Cerussite

EB1911 Cerussite Fig. 1.jpg

Fig. 1.

CERUSSITE, a mineral consisting of lead carbonate (PbCO3), and an important ore of lead. The name (sometimes erroneously spelt cerusite) is from the Lat. cerussa, “white lead.” “Cerussa nativa” was mentioned by K. Gesner in 1565, and in 1832 F. S. Beudant applied the name céruse to the mineral, whilst the present form, cerussite, is due to W. Haidinger (1845). Popular names in early use were lead-spar and white-lead-ore.

Cerussite crystallizes in the orthorhombic system and is isomorphous with aragonite. Like aragonite it is very frequently twinned, the compound crystals being pseudo-hexagonal in form. Three crystals are usually twinned together on two faces of the prism m{110}, producing six-rayed stellate groups (figs, 1 and 2) with the individual crystals intercrossing at angles of nearly 60°. Twinning on the faces of the prism r{130}, the angles of which are also nearly 60°, produces a similar kind of grouping, but is much less common. Crystals are of frequent occurrence, and they usually have very bright and smooth faces. The mineral also occurs in compact granular masses, and sometimes in fibrous forms. It is usually colourless or white, sometimes grey or greenish in tint; it varies from transparent to translucent, and has an adamantine lustre. It is very brittle, and has a conchoidal fracture. Hardness 3-31/2; sp. gr. 6.5. A variety containing 7% of zinc carbonate, replacing lead carbonate, is known as iglesiasite, from Iglesias in Sardinia, where it is found.

The mineral may be readily recognized by its characteristic twinning, in conjunction with the adamantine lustre and high specific gravity. It dissolves with effervescence in dilute nitric acid. Before the blow-pipe it fuses very readily, and gives reactions for lead. Cerussite occurs in metalliferous veins in association with galena, and has been formed by the action of carbonated waters on the galena; it is therefore found in the upper parts of the lodes together with other secondary minerals, such as limonite. Finely crystallized specimens have been obtained from the Friedrichssegen mine near Ems in Nassau, Johanngeorgenstadt in Saxony, Mies in Bohemia, Phenixville in Pennsylvania, Broken Hill in New South Wales, and several other localities. Delicate acicular crystals of considerable length were found long ago in the Pentire Glaze mine near St Minver in Cornwall. It is often found in considerable quantities, and contains as much as 771/2% of lead.  (L. J. S.) 

EB1911 Cerussite Fig. 2.jpg

Fig. 2.