Ludlockite
Ludlockite is PbFe4As10O22, a ferric iron arsenite from Tsumeb: red to orange-brown hairlike laths to 4 cm, hardness 1.5 to 2, very flexible and sectile.
Ludlockite is a lead ferric iron arsenite, PbFe3+4As3+10O22, approved by the IMA in 1969 (1969-046) and described from Tsumeb, Namibia. It forms red to orange-brown hairlike laths to 4 cm, triclinic, hardness 1.5 to 2, density 4.33 to 4.40, very flexible and sectile. Its arsenic, 48.91% of the mass, is trivalent: an arsenite, not an arsenate.
In short
- Ludlockite is an arsenite, not an arsenate. The IMA formula writes the arsenic as As3+, and the Handbook of Mineralogy records that both the ferric iron and the trivalent arsenic were confirmed by crystal-structure analysis. Its 1996 structure paper is titled for pentameric arsenite groups and orange hair.
- The valence changes the numbers by more than ten points. Ideal ludlockite is 64.58% As2O3. The same arsenic written as As2O5 would read 75.02%. The first Tsumeb analysis reported its arsenic as As2O5, at 70.82%; the later microprobe work reports As2O3, at 60.67%.
- Very flexible is unusual. Against the 176 sheets in this site's reference set, 18 record any flexibility (ligature-aware search; the literal search finds 14), and stibnite's highly flexible is the only one of the 18 with an intensifier. Ludlockite, whose sheet is not in the set, reads very flexible and sectile.
- It is a Tsumeb mineral from the sulphide ore, not the oxidised zone. The associates are zincian siderite, tennantite, chalcocite, pyrite, bornite, germanite and quartz, with leiteite, reniérite, stolzite and schneiderhöhnite. See Tsumeb.
- The name joins two mineral dealers: Fredrick LUDlow Smith III and Charles LOCKe Key, then of New Jersey, who supplied the first specimens for study.
| Species | Formula | Hardness | Density | Tenacity | Colour | Streak |
|---|---|---|---|---|---|---|
| Ludlockite | PbFe3+4As3+10O22 | 1.5 to 2 | 4.33 to 4.40 | Very flexible and sectile | Red, red-brown, orange-brown, yellow | Pale brown |
| Crocoite | PbCrO4 | 2.5 to 3 | 6.0 to 6.1 | Sectile | Hyacinth-red, red-orange, orange | Yellow-orange |
| Realgar | AsS | 1.5 to 2 | 3.56 | Sectile, also slightly brittle | Red to yellow-orange | Red-orange to red |
| Orpiment | As2S3 | 1.5 to 2 | 3.49 | Sectile; cleavage lamellae flexible | Lemon-yellow to golden or brownish yellow | Pale lemon-yellow |
| Cuprite (hairlike form) | Cu2O | 3.5 to 4 | 6.14 | Brittle | Cochineal-red, ruby-red, purplish red | Brownish red, shining |
| Stibnite | Sb2S3 | 2 | 4.63 | Highly flexible but not elastic; slightly sectile | Lead-grey | Lead-grey |
What is ludlockite? A lead iron arsenite from Tsumeb
Ludlockite is PbFe3+4As3+10O22 on both the September 2026 IMA-CNMNC Master List and its Handbook sheet: one lead, four ferric iron and ten trivalent arsenic to twenty-two oxygen, with no water and no hydroxyl. The IMA list records it as approved under number 1969-046 with Namibia as the type country, and the Handbook's first reference is its 1977 description in the Mineralogical Record, Ludlockite: a new mineral from Tsumeb. Type material is at the Natural History Museum in London and at Harvard.
By mass, on standard atomic weights, ideal ludlockite is 48.91% arsenic, 14.58% iron and 13.53% lead (formula mass 1531.78). Despite the lead in its formula, it is chiefly an iron arsenite. The Handbook's ideal column agrees: PbO 14.57%, Fe2O3 20.85%, As2O3 64.58%.
It is triclinic, forming simply terminated laths elongated along [100] and flattened on {011}, less often prisms with nearly equant cross-sections, to 4 cm, sometimes slightly twisted, in divergent bundles of hairlike crystals. Lamellar twinning on {011} is common.
Arsenite, not arsenate: a worked calculation
Arsenates such as scorodite and beudantite carry As5+ in AsO4 groups; ludlockite's arsenic is As3+. The Handbook records that its Tsumeb microprobe analysis gave total iron as Fe2O3 and total arsenic as As2O3, both confirmed by crystal-structure analysis, and the IMA formula writes both valences into the name.
Why it matters on paper. An analysis reports arsenic as an oxide, and the oxide chosen changes the number. As2O3 has a formula mass of 197.84 and As2O5 of 229.84, so the same arsenic weighs 229.84 / 197.84 = 1.162 times as much when booked as As2O5. Ideal ludlockite is 64.58% As2O3; booked as As2O5 it would read 75.02%. The reverse conversion takes the first Tsumeb analysis, which reported total As as As2O5 at 70.82%, down to 60.96% As2O3, close to the microprobe's 60.67%.
The quotable version: ludlockite's arsenic is about 49% of its mass, whichever oxide it is reported as; only the oxygen bookkeeping changes. If you are comparing its analysis with an arsenate's, convert first.
Is ludlockite flexible? How it compares with the 176 sheets
Yes: its Handbook sheet reads very flexible and sectile, with a perfect micaceous cleavage on {011} and a second perfect cleavage on {021}, and a hardness of 1.5 to 2.
Method. All 176 sheets in this site's reference set were converted with pdftotext -layout and searched case-insensitively for flexib, matching either the letters fl or the stray glyph that replaces the fl ligature on the older sheets. 18 sheets record some flexibility; the literal search finds only 14. Positive controls: muscovite and talc, which print °exible and must match; they do, along with biotite and phlogopite. Ludlockite's own sheet is not in the set and is not counted.
Nine of the 18 restrict it to thin pieces: leaves, laminae, lamellae, plates or flakes, as in covellite, molybdenite, orpiment, muscovite and phlogopite. Stibnite's highly flexible is the only one of the 18 with an intensifier, and pyrostilpnite's somewhat flexible the only one with a softener. So ludlockite's very flexible, applied to the crystals themselves, puts it in stibnite's company rather than muscovite's. Treat it as a micromount-scale, handle-by-the-matrix species: see handling and storage and what is a micromount.
Where ludlockite occurs
The Handbook gives one occurrence: in sulphide ore from a complex polymetallic hydrothermal ore deposit, at Tsumeb, Namibia. That is the primary ore, not the oxidised zone that produced Tsumeb's azurite and cerussite, and the associates say so: zincian siderite, tennantite, chalcocite, pyrite, bornite, germanite and quartz, with the rarer leiteite, reniérite, stolzite and schneiderhöhnite. The locality is covered at Tsumeb.
The sheet's full distribution is short: Tsumeb; Laurium, Greece, in slag; one kilometre north of Campiglia, Tuscany, Italy, also in slag; and Zarehehuran, near Takap, in the Takht-e-Suleiman massif, which the sheet places in Azerbaijan. No British locality is recorded.
The rest of this reference section is indexed at the species index. We hold no catalogue and nothing here is offered for sale; the wanted list is the only commercial route.