Cornubite
Cornubite, Cu5(AsO4)2(OH)4: triclinic copper arsenate, apple-green, hardness about 4, density 4.64; dimorphous with cornwallite, named from Cornubia, Cornwall.
Cornubite is a copper arsenate hydroxide, Cu5(AsO4)2(OH)4 on the IMA list (A, 1962 s.p., United Kingdom): triclinic, apple-green, hardness about 4, density 4.64 on fibrous material. It has exactly cornwallite’s formula, 2.5 copper per arsenic, between olivenite’s 2 and clinoclase’s 3; the sheet’s Wheal Carpenter analysis works out at 2.47.
In short
- The copper-to-arsenic ladder. A script over the September 2026 IMA-CNMNC Master List finds 28 rows made only of copper, arsenic, oxygen and hydrogen. Seven are plain arsenate hydroxides, with (AsO4) and (OH), water aside, and no other anion or cation: olivenite, euchroite and strashimirite at 2 copper per arsenic; cornubite and cornwallite at 2.5; clinoclase and gilmarite at 3.
- Worked against the sheet. The Handbook of Mineralogy cornubite sheet prints two analyses. Converted from oxide weights, Wheal Carpenter, Cornwall, gives 2.47 copper per arsenic and Reichenbach, Germany, 2.64; both are nearer 2.5 than 2 or 3. On standard atomic weights the ideal formula is 47.88% copper and 22.58% arsenic.
- Documented failure: the ladder cannot split the dimorphs. Cornubite and cornwallite share Cu5(AsO4)2(OH)4. The sheets separate them by structure: cornubite triclinic, about 4, apple-green, α 1.87; cornwallite monoclinic, 4.5, verdigris-green to blackish green, α 1.81 to 1.820.
- Documented failure: the 4.64 coincidence. Cornubite’s measured density on fibrous material, 4.64, is exactly cornwallite’s calculated density. A 20.00 g piece reading 15.69 g in water fits either; measured cornwallite reads 15.20 g and calculated cornubite 15.88 g.
- Where. The Distribution field gives Wheal Carpenter, Gwinear, Wheals Gorland, Unity and Muttrell, Gwennap, and Wheal Phoenix, Linkinhorne, Cornwall; the Bedford United mines, Tavistock, Devon; the Potts Gill mine, Caldbeck Fells; Southwick Cliffs, Scotland; Germany, France, Slovakia, the USA and Congo.
| Species | Formula (IMA) | IMA status | Cu per As | Cu by mass | Water by mass | System (sheet) | Hardness | Density | 20.00 g piece in water | Colour (sheet) |
|---|---|---|---|---|---|---|---|---|---|---|
| Olivenite | Cu2(AsO4)(OH) | G, 1820, United Kingdom | 2 | 44.91% | 3.18% | Monoclinic, pseudo-orthorhombic | 3 | 4.46 | 15.52 g | Olive-green, greenish brown, brown and others |
| Euchroite | Cu2(AsO4)(OH)(H2O)3 | G, 1823, Slovakia | 2 | 37.71% | 18.71% | Orthorhombic | 3.5 to 4 | 3.44 | 14.19 g | Bright emerald-green or leek-green |
| Strashimirite | Cu4(AsO4)2(OH)2·2.5H2O | A, 1967-025, Bulgaria | 2 | 41.60% | 10.32% | Monoclinic | n.d. | n.d.; calc. 3.67 | 14.55 g (calc.) | White to pale green |
| Cornubite | Cu5(AsO4)2(OH)4 | A, 1962 s.p., United Kingdom | 2.5 | 47.88% | 5.43% | Triclinic | About 4 | 4.64 (fibrous); calc. 4.85 | 15.69 g; 15.88 g (calc.) | Apple-green, pale to dark green |
| Cornwallite | Cu5(AsO4)2(OH)4 | G, 1847, United Kingdom | 2.5 | 47.88% | 5.43% | Monoclinic | 4.5 | 4.17; calc. 4.64 | 15.20 g; 15.69 g (calc.) | Verdigris-green to blackish green, pale green |
| Clinoclase | Cu3(AsO4)(OH)3 | G, 1830, United Kingdom | 3 | 50.09% | 7.10% | Monoclinic | 2.5 to 3 | 4.38 | 15.43 g | Dark greenish blue to greenish black |
| Gilmarite | Cu2+3(AsO4)(OH)3 | A, 1996-017, France | 3 | 50.09% | 7.10% | Triclinic | About 3 | 4.2 | 15.24 g | Blue-green |
What is cornubite? A copper arsenate named for Cornwall
Cornubite is Cu5(AsO4)2(OH)4, on the September 2026 IMA list and on the Handbook of Mineralogy cornubite sheet alike. The list gives status A, 1962 s.p., the United Kingdom as the country of first description, and Mineralogical Magazine 32 (1959), 1 as the first reference; the sheet cites that paper as Claringbull, Hey and Davis, Cornubite, a new mineral dimorphous with cornwallite. The name is from Cornubia, the Latin name for Cornwall, source of the first specimens. Type material is at the Natural History Museum, London; the sheet marks no type locality.
On the sheet. Triclinic, point group −1 and space group P−1 (overbars read from the rendered sheet). Crystals are very rare, to 5 mm, tabular parallel to {2−11}, in aggregates; it is generally fibrous, botryoidal, globular or porcelaneous massive. Two cleavages are observed, both perpendicular to {2−11}, meeting at about 70°. Hardness about 4; measured density 4.64 for fibrous material, calculated 4.85. Translucent, apple-green or pale to dark green, streak pale green, vitreous; biaxial (−), α 1.87, γ about 1.90, 2V very large.
Its chemistry, worked. On standard atomic weights the formula mass is 663.59, giving CuO 59.93%, As2O5 34.64% and H2O 5.43%, the ideal column on the sheet. Arsenic is 22.58% by mass; arsenic minerals are covered on toxic minerals in a collection and is orpiment toxic, and nothing here is health or safety advice.
Where does cornubite sit among the copper arsenates? The copper-to-arsenic ladder
Plain copper arsenate hydroxides come at three ratios on the IMA list: 2, 2.5 and 3 copper per arsenic. Cornubite and cornwallite are the pair at 2.5.
Method. A row parser over the September 2026 list (pdftotext -layout; it reads 6150 rows against the 6239 species the list states, so a raw line scan was run as a cross-check) kept the 28 rows whose formula contains only Cu, As, O and H. Of those, seven contain (AsO4) and (OH) with no acid arsenate group and no free oxide oxygen: olivenite, euchroite, strashimirite, cornubite, cornwallite, clinoclase and gilmarite. A raw scan for lines with copper, (AsO4) and (OH) found 15; the other eight each carry a further cation (calcium, lead or a rare earth), for example conichalcite and duftite. Gilmarite’s IMA formula, Cu2+3(AsO4)(OH)3, carries a valence mark that the script strips before counting.
What the ladder shows. Two ratios hold a dimorph pair on the sheets: cornubite with cornwallite at 2.5, and gilmarite with clinoclase at 3. At 2, olivenite differs from euchroite and strashimirite by water: 3.18% against 18.71% and 10.32%. The ladder places a copper arsenate by analysis; it does not replace one.
Does the sheet analysis fit 2.5? Worked
Yes, within the scatter the sheet itself prints. Converting oxide weights to atoms (CuO 79.545, As2O5 229.839 on standard atomic weights): the Wheal Carpenter, Gwinear, analysis, CuO 59.86 and As2O5 35.07, gives 0.7525 copper to 0.3052 arsenic, a ratio of 2.47. The sheet writes the same analysis as Cu4.8(AsO4)1.95(OH)3.6, which is 2.46. The Reichenbach, Germany, microprobe analysis, CuO 58.60 and As2O5 32.01, gives 2.64; there phosphorus, silicon, iron and aluminium share the arsenic site, and copper per site is 2.52.
Against the neighbours. Olivenite’s ratio is 2 and clinoclase’s 3, so both cornubite analyses sit closer to 2.5 than to either. The cornwallite sheet’s two analyses give 2.84 and 2.56 by the same arithmetic; the first carries 2.71 P2O5, and counting phosphorus with arsenic brings it to 2.48. Analyses with phosphorus need the phosphorus counted before a ratio means anything; counted the same way, Reichenbach gives 2.61.
Is it cornubite or cornwallite? Where the ladder fails
The ladder cannot separate them, because their IMA formulas are identical: Cu5(AsO4)2(OH)4. The sheets separate them by structure and its consequences. Cornubite is triclinic, hardness about 4, apple-green to dark green, α 1.87 and γ about 1.90. Cornwallite is monoclinic, hardness 4.5, verdigris-green to blackish green or pale green, in microcrystalline or radial fibrous botryoidal crusts to 1 mm thick, α 1.81 to 1.820 and γ 1.85 to 1.88. Cornubite’s α sits about 0.05 above cornwallite’s, but only a grain mount shows it.
The 4.64 coincidence. Cornubite’s measured density on fibrous material is 4.64; cornwallite’s calculated density is also 4.64. Measured cornwallite is 4.17, 10.13% below its own calculated figure; its sheet describes microcrystalline or radial fibrous crusts. Worked as 20 − 20/D, 15.69 g in water for 20.00 g fits fibrous cornubite and ideal cornwallite alike; 15.20 g fits measured cornwallite and 15.88 g calculated cornubite. The cornubite sheet’s X-ray pattern is from the Potts Gill mine; the cornwallite sheet’s from Wheal Carpenter. An X-ray powder pattern settles it; a balance does not. The method and its porous-material caveat are on how to measure specific gravity at home.
Where is cornubite found, and what with?
The Distribution field names, in England, Wheal Carpenter, Gwinear; Wheals Gorland, Unity and Muttrell, Gwennap; Wheal Phoenix, Linkinhorne; and a number of other mines in Cornwall; the Bedford United mines, Tavistock, Devon; and the Potts Gill mine, Caldbeck Fells, Cumbria. Southwick Cliffs near Dalbeattie, Kirkcudbrightshire, Scotland. In Germany, Neubulach and the Clara mine near Oberwolfach, Black Forest, and Reichenbach near Bensheim, Hesse. The Cap Garonne mine, Var, and the Roua copper mines, Alpes Maritimes, France. L’ubietová, Slovakia. In the USA, the Centennial Eureka mine, Tintic, and Gold Hill, Utah; the Majuba Hill mine, Nevada; near the Humboldt and Marshall mines, Arizona; Sylvanite and the Buckhorn mine, New Mexico. Mindigi, Katanga Province, Congo.
Its company. The association field lists cornwallite, chalcophyllite, olivenite, liroconite, chenevixite, clinoclase, pseudomalachite, bayldonite, parnauite, tyrolite, azurite, malachite, cuprite, chrysocolla and quartz. Pseudomalachite, Cu5(PO4)2(OH)4 on the IMA list, is cornubite’s formula with phosphate for arsenate. How liroconite and chalcophyllite are told from this company is on their own pages. The rest of the reference section is at the species index. We hold no catalogue and nothing here is offered for sale; the wanted list is the only commercial route.