Phosgenite
Phosgenite is Pb2(CO3)Cl2, molar mass 545.31 g/mol worked, 75.99% lead: tetragonal, sectile, hardness 2–3, density 6.12–6.15. Matlock, Monteponi, Tsumeb.
Phosgenite is lead chlorocarbonate, Pb2(CO3)Cl2 on the September 2026 IMA list. Its molar mass, worked from standard atomic weights, is 545.31 g/mol, 75.99% of it lead. It is tetragonal and sectile, hardness 2–3 varying with direction, density 6.12–6.15, and may fluoresce yellow under longwave UV. Fine crystals come from Matlock, Derbyshire, and Monteponi, Sardinia.
In short
- The species. The Handbook of Mineralogy sheet gives tetragonal, short prismatic crystals to 30 cm, cleavage {001} and {110} distinct, sectile and flexible ⊥ [001], hardness 2–3 varying with direction, density 6.12–6.15, white streak, adamantine lustre. The September 2026 IMA-CNMNC Master List writes Pb2(CO3)Cl2, status G, 1841, United Kingdom.
- Molar mass, worked. 2 × 207.2 + 12.011 + 3 × 15.999 + 2 × 35.45 = 545.31 g/mol. Lead is 75.99% of it, chlorine 13.00%, oxygen 8.80% and carbon 2.20%. cerussite, PbCO3, is 77.54% lead.
- The phosgene check. The name comes from phosgene, COCl2, and the arithmetic holds: COCl2 + 2 PbO = Pb2(CO3)Cl2, 98.91 + 446.40 = 545.31. The sheet’s Altai analysis works out at 1.01 carbon and 1.99 chlorine per 2 lead.
- Documented failure: the balance. A 20.00 g piece reads 16.73 to 16.75 g in water as phosgenite, 16.87 g as anglesite and 16.95 g as cerussite, 0.12 to 0.21 g away. Habit, sectility and uniaxial optics separate it; weight does not. Matlockite, also tetragonal, reads 17.19 g.
- Where it is found. The sheet names the Bage and other mines near Matlock, Derbyshire; mines in Cornwall; Monteponi and Montevecchio, Sardinia; slag at Laurium; the Touissit mine, Morocco; and large crystals at Tsumeb.
| Species | IMA formula | System | Hardness | Density (meas.) | 20.00 g in water | Molar mass (g/mol) | Pb by mass | Luminescence line |
|---|---|---|---|---|---|---|---|---|
| Phosgenite | Pb2(CO3)Cl2 | Tetragonal | 2–3, varying with direction | 6.12–6.15 | 16.73–16.75 g | 545.31 | 75.99% | May fluoresce yellow under LW UV, X-rays, electron beams |
| Cerussite | Pb(CO3) | Orthorhombic | 3–3.5 | 6.55 | 16.95 g | 267.21 | 77.54% | May fluoresce yellow under LW UV |
| Anglesite | Pb(SO4) | Orthorhombic | 2.5–3 | 6.38 | 16.87 g | 303.26 | 68.33% | May fluoresce yellow under UV |
| Matlockite | PbClF | Tetragonal | 2.5–3 | 7.12 | 17.19 g | 261.65 | 79.19% | None printed |
| Barstowite | Pb4(CO3)Cl6(H2O) | Monoclinic | 3 | 5.71 | 16.50 g | 1119.52 | 74.03% | None printed |
What is phosgenite? A lead chlorocarbonate that bends
Phosgenite is Pb2(CO3)Cl2, a soft, heavy, adamantine lead mineral with both carbonate and chloride in its formula. The Handbook of Mineralogy phosgenite sheet gives tetragonal crystals, point group 4/m 2/m 2/m, short prismatic to prismatic along [001], rarely thick tabular on {001}, commonly terminated by {001} or with large {111}, to 30 cm; also granular and massive. Cleavage is distinct on {001} and {110} and indistinct on {100}. The tenacity line reads sectile, flexible ⊥ [001], and the hardness 2–3, varying with direction. Density is 6.12–6.15 measured, 6.124 calculated. It is uniaxial (+), anomalously biaxial, ω 2.118 and ε 2.145.
How it forms. The sheet calls it uncommon, typically formed by alteration of galena in the oxidized zone of hydrothermal lead deposits, or by reaction of sea water with other lead minerals. Its association line is short: cerussite, anglesite, matlockite and laurionite. Its X-ray powder pattern was taken on material in a lead pot from Wookey Hole Cavern, England.
What the IMA list records. Phosgenite, Pb2(CO3)Cl2, status G (grandfathered), 1841, United Kingdom. The sheet marks no type locality, so this page names none; more on the term at what a type locality is.
What is the molar mass of phosgenite? 545.31 g/mol, worked
Phosgenite’s molar mass is 545.31 g/mol on standard atomic weights. Two lead at 207.2 make 414.40; one carbon, 12.011; three oxygen at 15.999, 47.997; two chlorine at 35.45, 70.90. The sum is 545.308. By mass that is lead 75.99%, chlorine 13.00%, oxygen 8.80% and carbon 2.20%.
The phosgene check. The sheet says the name is from phosgene, COCl2, composed of elements in the mineral’s composition. The formula splits exactly that way: COCl2, 98.91, plus two PbO, 446.40, is 545.31. Written as oxides, that is PbO 81.86%, CO2 8.07% and Cl 13.00%, which over-counts oxygen by the 2.93% that one chlorine pair replaces. The sheet’s ideal column prints those four numbers, −O = Cl2 included, to a total of 100.00. Its Altai district analysis, PbO 81.78, CO2 8.13, Cl 12.91, works out at 2 lead, 1.01 carbon and 1.99 chlorine.
Its chemical neighbours. A script over the September 2026 IMA list for formulas containing lead, a carbonate group and chlorine, checked against a raw line scan, finds 7 rows: ashburtonite, barstowite, hagstromite, mereheadite, philolithite, phosgenite and thorneite. Barstowite, Pb4(CO3)Cl6(H2O), is the one besides phosgenite built from lead, carbonate, chlorine and water alone; the others add copper, silicon, tellurium, boron, manganese or sulfate.
Phosgenite or cerussite? What separates them and what does not
Phosgenite and cerussite are both pale, adamantine, heavy and soft, and both may glow yellow under longwave UV; the sheets separate them on crystal system, tenacity and optics. Phosgenite is tetragonal, sectile and uniaxial (+); cerussite is orthorhombic, very brittle and biaxial (−), with hardness 3–3.5 against 2–3. anglesite is orthorhombic and brittle. Why the shared yellow glow settles nothing is on is cerussite fluorescent.
Documented failure: the balance. Worked as 20 − 20/D, a 20.00 g piece reads 16.73 to 16.75 g in water as phosgenite, 16.87 g as anglesite and 16.95 g as cerussite. The gaps are 0.12 to 0.13 g and 0.20 to 0.21 g, small enough that a crack, a matrix fleck or a drop of trapped air closes them. Barstowite, 16.50 g, sits below. Matlockite, PbClF, on phosgenite’s association line and also tetragonal, reads 17.19 g, 0.44 to 0.46 g heavier; its sheet gives tabular crystals flattened on {001}, perfect {001} cleavage and a brittle tenacity. Method: how to measure specific gravity at home.
Matlock on two labels. The matlockite sheet names that mineral for its first occurrence near Matlock, gives large crystals from the Bage and Wallclose mines, and lists phosgenite among its associates at Cromford. The phosgenite sheet names the Bage and other mines near Matlock for fine crystals but marks no type locality. The lead oxychloride mendipite, Pb3O2Cl2, is named for the Mendip Hills and is a different species again.
Where is phosgenite found? Matlock, Monteponi and Tsumeb
The phosgenite sheet’s Distribution field names England first. Fine crystals from the Bage and other mines near Matlock, Derbyshire, and from a number of mines in Cornwall. On Sardinia, fine crystals from Monteponi and Montevecchio near Iglesias, and from Gibbas near Cagliari. In slag from Laurium, Greece. Tarnowitz, Poland; the Syzjanov mine, Altai district, Siberia; large fine crystals from the Touissit mine near Oujda, Morocco; Sidi Amor ben Salem, Tunisia; large crystals from Tsumeb, Namibia; Broken Hill, New South Wales, and the Comet mine, Dundas, Tasmania; Boleo, near Santa Rosalia, Mexico; and in the USA large masses from the Terrible mine, Custer Co., Colorado, the Mammoth mine at Tiger, Arizona, and the Stevenson-Bennett mine, New Mexico.
Gem and lamp questions. The International Gem Society phosgenite article calls phosgenite too soft for jewelry wear and gives it strong yellowish fluorescence in UV and X-rays; the Handbook’s wording is may fluoresce yellow under LW UV. Lamp choice is on best UV lamp for mineral collecting.
Phosgenite is 75.99% lead by mass; lead is covered on toxic minerals in a collection and is cerussite toxic, and this page is not health or safety advice. Related species are at the species index. We hold no catalogue and nothing here is offered for sale; the wanted list is the only commercial route.