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 ceruue 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 { 11o1, producing six-rayed stellate groups (figs . , and 2) with the individual crystals intercrossing at angles of nearly 6o° . Twinning on the faces of the prism r { 1301, ; the angles of which are also nearly 6o°, 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 3-32; 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 FIG . 2 .

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% of lead . (L . J .