Chalcoalumite
Chalcoalumite is CuAl4(SO4)(OH)12·3H2O, hardness 2.5, density 2.29, in turquoise-green crusts: one copper to four aluminium, where cyanotrichite has two to one.
Chalcoalumite is a hydrated copper aluminium sulphate, CuAl4(SO4)(OH)12·3H2O, monoclinic, hardness 2.5 and measured density 2.29, turquoise-green to Nile blue with a white streak. It is a rare secondary mineral of oxidised copper deposits, usually as botryoidal crusts; crystals reach only 0.5 mm. It carries one copper to four aluminium, where cyanotrichite carries two to one.
In short
- One copper to four aluminium. Chalcoalumite is CuAl4; cyanotrichite, the copper aluminium sulphate collectors know better, is Cu4Al2. The Cu:Al ratio differs eightfold, and by mass chalcoalumite is 12.09% copper against cyanotrichite's 39.45%.
- Density separates the two cleanly: 2.29 for chalcoalumite against 2.74 to 2.95 for cyanotrichite. The trap is chalcanthite, at the same hardness of 2.5 and a density of 2.286 — but chalcanthite is Berlin blue to sky-blue and soluble in water.
- Crystals reach only 0.5 mm, and the twins are the reason to look. The Handbook records equilateral triangular tablets, diamond-shaped tablets, swallowtail composites, fourlings and eightlings. Chalcoalumite is a microscope species.
- 31 of the 176 sheets in this site's reference set name Bisbee, and chalcoalumite is one: its analysis and its powder pattern are both of Bisbee material.
- Three British localities are on the sheet: the Britannia mine in Snowdonia, South Wheal Crebor near Tavistock in Devon, and the Penberthy Croft mine at St Hilary in Cornwall.
| Species | Formula | Hardness | Density | Cu by mass | Colour on the sheet |
|---|---|---|---|---|---|
| Chalcoalumite | CuAl4(SO4)(OH)12·3H2O | 2.5 | 2.29 | 12.09% | Turquoise-green to Nile blue, pale grey to nearly colourless |
| Cyanotrichite | Cu4Al2(SO4)(OH)12·2H2O | Not determined | 2.74 to 2.95 | 39.45% | Sky-blue to azure-blue |
| Chalcanthite | CuSO4·5H2O | 2.5 | 2.286 | 25.45% | Berlin blue to sky-blue, greenish blue |
| Langite | Cu4(SO4)(OH)6·2H2O | 2.5 to 3 | 3.28 to 3.34 | 52.05% | Deep blue, greenish blue, blue-green |
| Brochantite | Cu4(SO4)(OH)6 | 3.5 to 4 | 3.97 | 56.20% | Emerald-green, blackish green, pale green |
| Turquoise | CuAl6(PO4)4(OH)8·4H2O | 5 to 6 | 2.86 | 7.81% | Sky-blue, bluish green, apple-green, greenish grey |
| Hydrobasaluminite | Al4(SO4)(OH)10·12–36H2O | Not determined | 1.86 | 0% | Colourless to pale yellow |
What is chalcoalumite? One copper to four aluminium
The name says it: from the Greek chalkos, copper, and the Latin alum, for aluminium sulphate. The formula on the Handbook of Mineralogy sheet and the September 2026 IMA-CNMNC Master List is CuAl4(SO4)(OH)12·3H2O. The IMA list records it as a grandfathered species first described in 1925 from the USA.
A worked calculation. On standard atomic weights the formula mass is 525.66. Copper, 63.546, is therefore 12.09% by mass; aluminium, four at 26.982, is 20.53%. The same method gives cyanotrichite, Cu4Al2(SO4)(OH)12·2H2O, 39.45% copper and 8.38% aluminium. The Handbook's ideal-composition columns agree: 15.13% CuO for chalcoalumite and 49.38% for cyanotrichite, which convert to the same copper figures.
Two copper aluminium sulphates, and the Cu:Al ratio differs eightfold — 1:4 against 2:1. Chalcoalumite is a dilute copper mineral, a hydrated aluminium sulphate carrying some copper, and its sheet colours are the paler ones: turquoise-green to Nile blue, grading to pale grey and nearly colourless. The Handbook's Bisbee analysis, at 14.56% CuO, is 11.63% copper, a little below the ideal.
How to tell chalcoalumite from cyanotrichite, turquoise and chalcanthite
Against cyanotrichite, use density and habit. Chalcoalumite measures 2.29; cyanotrichite 2.74 to 2.95. Cyanotrichite forms acicular crystals to 3 cm in radially fibrous, plush-like tufts and is sky-blue to azure; chalcoalumite is most commonly fibrous to massive botryoidal crusts, paler, with a white streak where cyanotrichite's is pale blue. Both occur at Bisbee and in the Grandview mine in the Grand Canyon.
Against turquoise, use hardness. Turquoise, CuAl6(PO4)4(OH)8·4H2O, is the other pale copper aluminium mineral — a phosphate, not a sulphate — and at 5 to 6 it is 2.5 to 3.5 Mohs points harder than chalcoalumite's 2.5. It is also denser, 2.86 against 2.29. See how to tell real turquoise from fake.
Against chalcanthite, density fails and colour and solubility decide. Chalcanthite is hardness 2.5 and density 2.286: identical to chalcoalumite in hardness and within 0.004 in density. But it is Berlin blue to sky-blue, and its sheet records it as soluble in H2O and dehydrating in dry air; chalcoalumite's records neither. Do not test with water on a specimen you want to keep. Langite at 3.28 to 3.34 and brochantite at 3.97 are far denser.
Crystals under half a millimetre, and the twins worth a microscope
The Handbook records chalcoalumite as thin tabular crystals with an equilateral triangular outline, to 0.5 mm, as spherical aggregates, and most commonly as fibrous to massive botryoidal crusts. It is monoclinic, point group 2, with perfect {100} cleavage, and it is sectile rather than brittle.
The twinning is unusually varied for such a small crystal. The sheet lists three laws: twinning on {100}, typical, which gives composites that closely resemble thick untwinned individuals; on {010}, forming diamond-shaped tablets; and on {136}, producing a swallowtail composite with a deep re-entrant, or fourlings and eightlings. Diamond-shaped eightlings combining all three twin laws have been noted.
None of this is visible without magnification, which is why chalcoalumite is a micromount species rather than a cabinet one. See what is a micromount and mineral twinning explained.
Where chalcoalumite is found, including three British mines
The sheet calls chalcoalumite a rare secondary mineral in the oxidised portions of copper deposits. At Bisbee it occurs with cuprite, malachite, azurite and goethite; in the Mbobo Mkulu Cave in South Africa with mbobomkulite and hydrombobomkulite.
Bisbee is the reference locality in practice. Across the 176 Handbook sheets in this site's reference set (method: each sheet converted with pdftotext -layout and searched for the string), 31 name Bisbee. Chalcoalumite's analysis and X-ray powder pattern are both of Bisbee material.
The full distribution: Bisbee in Cochise County and the Grandview mine in the Grand Canyon, Arizona; the Titusville mine in San Juan County, Colorado; the Lovelock mine in Churchill County, Nevada; the Spatsum claim near Ashcroft Manor, British Columbia; the Britannia mine, Snowdonia, Gwynedd, Wales; South Wheal Crebor, Tavistock, Devon; and the Penberthy Croft mine, St Hilary, Cornwall; Ottré in Belgium; Lľubietová in Slovakia; the Baccu Locci mine in Sardinia; the Sjö mine near Grythyttan in Sweden; and the Mbobo Mkulu Cave near Ngodwana, Eastern Transvaal, South Africa.
The copper sulphates are indexed at the species index. We hold no catalogue and nothing here is offered for sale; the wanted list is the only commercial route, and British chalcoalumite with a named mine is on it.