Laurionite
Laurionite is PbCl(OH), an orthorhombic lead hydroxychloride from Laurium slag: hardness 3–3.5, density 6.241. How it differs from paralaurionite, worked.
Laurionite is lead hydroxychloride, PbCl(OH), on both the September 2026 IMA list and its Handbook sheet: orthorhombic, colourless to white, hardness 3–3.5, density 6.241, slightly soluble in cold water. Worked, it is 79.80% lead. Paralaurionite has the identical formula and is monoclinic. Laurionite is named for Laurium, Greece, where it forms from slag.
In short
- The species. The Handbook of Mineralogy sheet gives orthorhombic, pyramidally terminated prismatic crystals to 1 cm, tabular on {100}, cleavage {101} distinct, flexible, hardness 3–3.5, density 6.241, colourless or white, adamantine. It says laurionite is produced by the action of saline water on lead-bearing slag, and also occurs in the oxidized zone of lead deposits.
- One formula, two minerals. A script over the September 2026 IMA-CNMNC Master List finds 2 rows written PbCl(OH): laurionite (G, 1887, Greece) and paralaurionite (G, 1899, Greece). Each sheet says the two are dimorphous. Laurionite is orthorhombic, Pcmn; paralaurionite is monoclinic, C2/m.
- The dimorph cell check. Worked from the printed cells, laurionite’s 277.27 ų with Z = 4 gives a density of 6.22, against the 6.212 its sheet prints. Paralaurionite’s 279.90 ų gives 6.16, beside its measured 6.05–6.15, where the sheet prints 6.28 as calculated. The cells differ by 0.95% in volume per formula unit.
- Documented failure: the balance. A 20.00 g piece reads 16.80 g in water as laurionite and 16.69–16.75 g as paralaurionite, 0.05 to 0.10 g apart. The sheets separate them on cleavage, {101} distinct against {001} perfect, and twinning: almost all paralaurionite crystals are twinned on {100}.
- Where it is found. In slag from Laurium, Greece; Baratti Beach, Tuscany; Juliushütte, Harz; and the Argent mines, South Africa. In mineral deposits at Wheal Rose, Cornwall; the Wesley mine, Somerset; Nant-y-Cagl, Wales; Kara Elchi, Turkmenistan; Western Australia, New South Wales and Minnesota.
| Property | Laurionite | Paralaurionite | How obtained |
|---|---|---|---|
| IMA row | PbCl(OH); G; 1887; Greece | PbCl(OH); G; 1899; Greece | September 2026 IMA list, by script |
| System; space group | Orthorhombic; Pcmn | Monoclinic; C2/m | Sheets |
| Cell (Å); Z | a 9.6987, b 4.0203, c 7.1110; 4 | a 10.865, b 4.006, c 7.233, β 117.24°; 4 | Sheets |
| Cell volume per formula unit | 69.32 ų | 69.98 ų | Worked: abc ÷ 4; abc·sin β ÷ 4 |
| Density, calculated | 6.212 printed; 6.22 from the cell | 6.28 printed; 6.16 from the cell | Sheets; worked |
| Density, measured | 6.241 | 6.05–6.15 | Sheets |
| 20.00 g in water | 16.80 g | 16.69–16.75 g | Worked, measured densities |
| Hardness | 3–3.5 | Soft | Sheets |
| Cleavage | {101}, distinct | {001}, perfect | Sheets |
| Tenacity; twinning | Flexible; none printed | Flexible, due to twin gliding, but not elastic; almost all crystals twinned by contact on {100} | Sheets |
| Optics | Biaxial (−); α 2.08, β 2.12, γ 2.16 | Biaxial (−); α 2.05, β 2.15, γ 2.20 | Sheets |
| Crystal size | To 1 cm | To 3 cm | Sheets |
| Water | Slightly soluble in cold H2O, more so in hot | None printed | Sheets |
What is laurionite? A lead hydroxychloride that grows on slag
Laurionite is PbCl(OH), a colourless to white, adamantine lead hydroxychloride. The Handbook of Mineralogy laurionite sheet gives orthorhombic crystals, point group 2/m 2/m 2/m, pyramidally terminated and prismatic, to 1 cm, tabular on {100} and elongated along [010]; they may be striated along {100} ∥ {0-21}, forming chevrons, and 15 forms are recorded. Cleavage is distinct on {101}, the tenacity flexible, the hardness 3–3.5 and the density 6.241 measured. It is biaxial (−), α 2.08, β 2.12, γ 2.16, and pearly on {100}.
The formula, worked. PbCl(OH) has a molar mass of 259.66 g/mol on standard atomic weights: lead 79.80%, chlorine 13.65%. The sheet writes the hydroxyl as half a water molecule plus half an oxygen, and the arithmetic gives H2O 3.47% and O 3.08%, the figures its ideal column prints. Its Laurium analysis, Pb 79.38, O 3.17, Cl 13.77, H2O 3.68, works out at 1.01 chlorine per lead.
How it forms, and the name. The sheet says it is produced by the action of saline water on lead-bearing slag, and occurs in the oxidized zone of lead-bearing mineral deposits, with paralaurionite, penfieldite, fiedlerite, phosgenite, cerussite and anglesite. It is named for the occurrence at Laurium, Greece. The sheet marks no type locality, so this page names none; the Laurium page sets out what a 2021 review says about the district.
Laurionite or paralaurionite? Same formula, two structures
Laurionite and paralaurionite are dimorphs: the same formula, PbCl(OH), in two crystal structures. A script over the September 2026 IMA list, cross-checked by a raw line scan, finds exactly 2 rows with that formula, laurionite (G, 1887) and paralaurionite (G, 1899), both with Greece as the country. The paralaurionite sheet says the name is from the Greek for near, for the dimorphous relation to laurionite, and both sheets print “Dimorphous with” the other. Laurionite is orthorhombic, space group Pcmn; paralaurionite is monoclinic, C2/m, with β = 117.24°.
The dimorph cell check. Laurionite’s printed cell, 9.6987 × 4.0203 × 7.1110 Å, is 277.27 ų; four formula units of 259.66 g/mol in it give 6.22, against the 6.212 printed. Paralaurionite’s, with the sin β term, is 279.90 ų and gives 6.16, 0.01 above the top of its measured 6.05–6.15, but its sheet prints 6.28 as calculated. This page reports both figures and does not choose. Per formula unit the paralaurionite cell is 0.95% larger.
Documented failure: the balance. Worked as 20 − 20/D, a 20.00 g piece reads 16.80 g in water as laurionite and 16.69–16.75 g as paralaurionite, a gap of 0.05 to 0.10 g. That is too small to name a specimen. The sheets separate them on other lines: paralaurionite’s cleavage is perfect on {001}, almost all its crystals are twinned by contact on {100}, its tenacity is flexible due to twin gliding but not elastic, its hardness is given only as soft, and its colours run to pale greenish, yellow-orange and rarely violet. Method: how to measure specific gravity at home.
Which lead chlorides have only lead, chlorine, oxygen and hydrogen?
Six IMA rows are built from lead, chlorine, oxygen and hydrogen alone, and laurionite sits in the middle of their chlorine-per-lead ladder. A script over the September 2026 list for formulas whose element set is exactly Pb, Cl, O and H finds: penfieldite, Pb2Cl3(OH), 1.5 chlorine per lead; laurionite and paralaurionite, PbCl(OH), 1; yeomanite, Pb2O(OH)Cl, 0.5; blixite, Pb8O5(OH)2Cl4, 0.5; and damaraite, Pb3O2(OH)Cl, 0.33. Worked, their lead contents run from 77.06% in penfieldite to 88.04% in damaraite, with laurionite at 79.80%.
Their neighbours. Cotunnite, PbCl2, is lead and chlorine only, 74.51% lead and 2 chlorine per lead; mendipite, Pb3O2Cl2, has no hydrogen, 85.80% lead; fiedlerite, Pb3Cl4F(OH)(H2O), adds fluorine; matlockite, PbClF, swaps hydroxyl for fluorine at the same 1 chlorine per lead. The laurionite sheet names penfieldite, fiedlerite and paralaurionite as associates.
Is laurionite water soluble? The sheet’s physical properties end with “Slightly soluble in cold H2O, more so in hot H2O.” The paralaurionite sheet prints no solubility line; the cotunnite sheet prints “Somewhat soluble in H2O.” This page quotes those lines and gives no cleaning or storage method.
Where is laurionite found? Slag first, then lead deposits
The laurionite sheet calls it probably more widespread than the known localities indicate. As an alteration product of slag from Laurium, Greece; along Baratti Beach, Tuscany, Italy; from Juliushütte, Astfeld, Harz Mountains, Germany; and from the Argent Pb–Zn mines, about 100 km east of Johannesburg, South Africa. In mineral deposits at Wheal Rose, near Sithney, Cornwall, and the Wesley mine, near Higher Pitts, Somerset, which the paralaurionite sheet places in the Mendip Hills; the Nant-y-Cagl (Eaglebrook) mine, Ceulanymaesmawr, Dyfed, Wales; the Kara Elchi mercury deposit, Turkmenistan; the Anticline prospect, Capricorn Range, Western Australia; the Elura deposit near Cobar, New South Wales; and the Minnamax Cu–Ni sulfide deposit near Hibbing, Minnesota.
Shared with matlockite and paralaurionite. Laurium slag, Baratti Beach and the Argent mines appear on the matlockite sheet too, and paralaurionite’s sheet adds the Meadowfoot smelter near Wanlockhead, the Merehead quarry, Somerset, and exceptional crystals from Tiger, Arizona.
Laurionite is 79.80% lead by mass; lead is covered on toxic minerals in a collection, 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.