Cotunnite
Cotunnite, PbCl2: orthorhombic lead chloride, hardness 2.5, density 5.80, a volcanic sublimate and galena alteration, somewhat soluble in water. Sheet data.
Cotunnite is lead chloride, PbCl2 on the IMA list (grandfathered, 1825, Italy): orthorhombic, colourless to white, adamantine, hardness 2.5, density 5.80 measured on synthetic material, with a perfect {010} cleavage. Its Handbook sheet says it is somewhat soluble in water and records it as a volcanic sublimate and an alteration of galena in saline settings.
In short
- The species. The September 2026 IMA-CNMNC Master List lists cotunnite as PbCl2, status G, 1825, Italy. The Handbook of Mineralogy sheet gives orthorhombic Pnam, prismatic crystals to about 2 mm, perfect {010} cleavage, slightly sectile, hardness 2.5, density 5.80, biaxial (+) with indices 2.199 to 2.260. Worked, PbCl2 is 74.51% lead and 25.49% chlorine.
- The two-chlorine check. The Kolar Gold Fields, India, microprobe analysis, Pb 71.96 and Cl 25.32, works out at 2.06 chlorine per lead; Tolbachik volcano, Russia, PbO 77.04 and Cl 26.9, at 2.20. The cell, a 7.6222, b 9.0448, c 4.5348 Å with Z = 4, gives 5.908, the sheet’s bracketed [5.91].
- Documented failure: the Tolbachik oxygen line. For 26.9% chlorine, the oxygen equivalent to subtract (−O = Cl2) works out at 6.07; the sheet prints 4.07, and its total of 101.16 uses 4.07. With 6.07 the total is 99.16. This page reports the arithmetic and does not correct the sheet.
- Where and how it forms. The sheet calls it a volcanic sublimate or an alteration of galena in saline environments, and also an alteration product on leaden archaeological objects or slag in seawater. It lists Vesuvius, Tolbachik, three slag localities including Laurium (Lavrion), and Leadhills and Wanlockhead in Scotland.
- Its lead–chlorine neighbours. Matlockite (PbClF) and laurionite (PbCl(OH)) carry one chlorine per lead, 13.55% and 13.65% by mass; cotunnite carries two. The cotunnite sheet names matlockite as an associate at Caracoles, Chile.
| Source on the sheet | Lead, wt% | Chlorine, wt% | Cl per Pb, worked | Printed total | Worked check | Reading |
|---|---|---|---|---|---|---|
| PbCl2, ideal column (3) | 74.50 (worked 74.51) | 25.50 (worked 25.49) | 2.00 | 100.00 | Reproduces | The formula |
| Kolar Gold Fields, India, analysis (2), electron microprobe | 71.96 | 25.32 | 2.06 | 97.28 | Sum 97.28: reproduces | Close to 2 |
| Tolbachik volcano, Russia, analysis (1); also CuO 0.11, ZnO 0.10, H2O 1.08 | 71.52 (from PbO 77.04) | 26.9 | 2.20 | 101.16, with −O = Cl2 printed 4.07 | −O = Cl2 works out at 6.07; total 99.16 | Chlorine high; one printed correction does not reproduce |
| Cell, Pnam, Z = 4 (V 312.63 ų worked) | — | — | — | D(calc.) [5.91] | 5.908: reproduces | Measured 5.80 on synthetic material; 20.00 g reads 16.55 g in water |
What is cotunnite? Lead chloride, PbCl2
Cotunnite is PbCl2 on the September 2026 IMA list, status G, grandfathered, 1825, with Italy as the country. The first reference is Prodromo della mineralogia vesuviana (Da’ Torchi del Tramater, Napoli, 1825); the second, Soviet Physics – Crystallography 21 (1976), 38. A field scan of the IMA formula column finds one row written PbCl2, cotunnite. The Handbook of Mineralogy cotunnite sheet gives the same formula.
What the sheet records. Prismatic crystals, elongated along [001] and flattened on {010}, to about 2 mm; scepter-shaped or skeletal crystals; doubly terminated crystals may show unequally developed equivalent faces; radiating sprays; also granular, as crusts or pseudomorphs, and massive. Cleavage perfect on {010}; fracture subconchoidal; tenacity slightly sectile; hardness 2.5. Colourless to white, pale green or pale yellow, colourless in thin section; adamantine, silky to pearly.
Water. The sheet’s physical properties end: “Somewhat soluble in H2O.” This page quotes that line and gives no cleaning or storage method. The laurionite page quotes the laurionite sheet’s own solubility line, which is worded differently.
What is the structure of cotunnite?
Orthorhombic, space group Pnam, four PbCl2 units per cell. The sheet’s cell, measured on synthetic material, is a = 7.6222, b = 9.0448, c = 4.5348 Å, Z = 4. Worked, that is 312.63 cubic ångströms, and with a formula mass of 278.10 it gives a density of 5.908, the sheet’s calculated [5.91]. The measured density, 5.80, is also on synthetic material, so a 20.00 g piece reads 16.55 g in water at 5.80 and 16.62 g at 5.91.
Optics. Biaxial (+), with X = b, Y = a and Z = c; α 2.199, β 2.217, γ 2.260; 2V not determined, 67°12′ calculated. The structure references the sheet cites are Sass, Brackett and Brackett (1963), The crystal structure of lead chloride, and Nozik, Fykin and Muradyan (1976), who refined cotunnite by neutron diffraction. This page states the sheet’s cell and leaves the atomic arrangement to those papers.
Does cotunnite have two chlorines per lead? The analyses
Near enough at Kolar, high at Tolbachik. The Kolar Gold Fields analysis, by electron microprobe, gives Pb 71.96 and Cl 25.32, a total of 97.28: 0.3473 moles of lead and 0.7143 of chlorine per 100 g, or 2.06 chlorine per lead. The Tolbachik analysis is written as oxides: CuO 0.11, ZnO 0.10, PbO 77.04, Cl 26.9, H2O 1.08, less 4.07 for oxygen equivalent to chlorine, total 101.16. PbO 77.04 is 71.52% lead, which gives 2.20 chlorine per lead.
Documented failure: 4.07 against 6.07. When chlorine is reported alongside oxides, the oxygen it replaces is subtracted at 15.999 / (2 × 35.45) of the chlorine figure. For 26.9% chlorine that is 6.07, not the printed 4.07, and the total becomes 99.16 instead of 101.16. A printed 4.07 would correspond to about 18.0% chlorine. The table sets the three columns side by side; this page does not guess which figure in the original is in error.
Where is cotunnite found? Volcanoes, slag and galena
Occurrence, as printed. A volcanic sublimate or an alteration of galena in saline environments; also an alteration product on leaden archaeological objects or slag immersed in seawater. Associates: galena, cerussite, anglesite and matlockite at Caracoles, Chile; tenorite, ponomarevite, sofiite, burnsite, ilinskite, georgbokite, chloromenite, halite, sylvite and gold at Tolbachik.
Distribution, as printed. In Italy, on Vesuvius, Campania; the Niccioleta mine near Siena; and along Baratti Beach, Tuscany, in slag. In Germany, the Christian-Levin mine near Essen, and Richelsdorf, Hesse, in slag. At Laurium, Greece, in slag (locality page: Lavrion). From Leadhills and Wanlockhead, Scotland. The Tolbachik fissure volcano, Kamchatka; the Kolar Gold Fields, India; Caracoles and Challacollo, Chile; the La Pamapa district, Peru, as the sheet spells it; and in the USA the Grand Gulch mine, Arizona, Bonanza, Colorado, Tintic, Utah, the Loudville mine, Massachusetts, and the Minnamax deposit, Minnesota.
Type. The sheet marks no type locality and lists no type material; the IMA list gives Italy and 1825, and the first reference is a work on the mineralogy of Vesuvius. The name honours Domenico Cotugno (Cotunnius), 1736–1822, Professor of Anatomy at Naples. The term is explained at what is a type locality.
Cotunnite, matlockite, laurionite and mendipite
Same elements, different ratios. Worked from the IMA formulas: cotunnite PbCl2 is 74.51% lead and 25.49% chlorine; matlockite PbClF 79.19% and 13.55%; laurionite PbCl(OH) 79.80% and 13.65%; mendipite Pb3O2Cl2 85.80% and 9.79%. Of the four, cotunnite carries the most chlorine, and the laurionite page’s 74.51% for cotunnite agrees. The cotunnite sheet names matlockite as an associate at Caracoles, Chile.
Halide family. As a simple metal chloride cotunnite sits beside halite and sylvite; the chloride rows of the IMA list are counted on is sylvite a halide. Cotunnite is 74.51% lead: handling and hazard questions go to toxic minerals in a collection, and this page is not safety advice. More species are in the species index. We hold no catalogue and nothing here is offered for sale; the wanted list is the only commercial route.