Cerussite

Cerussite
Cerussite from Les Farges mine, France
General
Category	Carbonate mineral
Formula (repeating unit)	Lead carbonate: PbCO3
Strunz classification	5.AB.15
Crystal system	Orthorhombic
Crystal class	Dipyramidal (mmm) H-M symbol: (2/m 2/m 2/m)
Space group	Pmcn
Identification
Color	Colorless, white, gray, blue, or green
Crystal habit	Massive granular, reticulate, tabular to equant crystals
Twinning	Simple or cyclic contact twins
Cleavage	Good [110] and [021]
Fracture	Brittle conchoidal
Mohs scale hardness	3 to 3.5
Luster	Adamantine, vitreous, resinous
Streak	White
Diaphaneity	Transparent to translucent
Specific gravity	6.53–6.57
Optical properties	Biaxial (-)
Refractive index	nα = 1.803, nβ = 2.074, nγ = 2.076
Birefringence	δ = 0.273
Other characteristics	May fluoresce yellow under LW UV
References

Cerussite (also known as lead carbonate or white lead ore) is a mineral consisting of lead carbonate (PbCO₃), and an important ore of lead. The name is from the Latin cerussa, white lead. Cerussa nativa was mentioned by Conrad Gessner in 1565, and in 1832 F. S. Beudant applied the name cruise to the mineral, whilst the present form, cerussite, is due to W. Haidinger (1845). Miners' 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, producing six-rayed stellate groups with the individual crystals intercrossing at angles of nearly 60°. 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. The mineral is usually colorless or white, sometimes grey or greenish in tint and varies from transparent to translucent with an adamantine lustre. It is very brittle, and has a conchoidal fracture. It has a Mohs hardness of 3 to 3.75 and a specific gravity of 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. A blowpipe test will cause it to fuse very readily, and gives indications for lead.

Finely crystallized specimens have been obtained from the Friedrichssegen mine in Lahnstein in Rhineland-Palatinate, Johanngeorgenstadt in Saxony, Stříbro in the Czech Republic, Phoenixville 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. Cerussite is often found in considerable quantities, and has a lead content of up to 77.5%.