Aurichalcite
Aurichalcite is (Zn,Cu)5(CO3)2(OH)6: sky-blue silky tufts, hardness 1 to 2, the softest of the 15 CO3 sheets in our set. Its Utah analysis is Zn3.63Cu1.37.
Aurichalcite is a zinc copper carbonate, (Zn,Cu)5(CO3)2(OH)6, grandfathered by the IMA (1839, Russia). It forms sky-blue to pale green tufts of silky laths to 3 cm: hardness 1 to 2, density 3.96, the softest of 15 CO3-bearing sheets in this site's reference set. Its Utah analysis is Zn3.63Cu1.37.
In short
- The identity card: (Zn,Cu)5(CO3)2(OH)6, monoclinic, hardness 1 to 2, density 3.96, silky to pearly. The Handbook of Mineralogy sheet describes tufted divergent sprays or spherical aggregates of acicular to lathlike crystals to 3 cm.
- The softest CO3 sheet in the set. Of the 15 sheets in this site's reference set with CO3 in the header formula, aurichalcite is the softest at 1 to 2. Hydrozincite follows at 2 to 2.5, then phosgenite at 2 to 3.
- The only “Fragile” in 176 sheets. The tenacity is printed in quotation marks as “Fragile”. No other sheet in the set uses the word.
- More zinc than copper, worked. The Utah analysis recalculates to Zn3.63Cu1.37, as the sheet prints. That is 72.7% of the metal atoms zinc, and 43.4% zinc against 15.9% copper by mass.
- Density will not separate it from its companions: 3.96 against hydrozincite's 4.00 and rosasite's 4.0 to 4.2. Hardness, colour and habit do, as the table shows.
| Species | Formula (Handbook) | Hardness | Density (measured) | Colour on the sheet | Lustre | Fluorescence on the sheet |
|---|---|---|---|---|---|---|
| Aurichalcite | (Zn,Cu)5(CO3)2(OH)6 | 1 to 2 | 3.96 | Pale green, greenish blue, sky-blue | Silky to pearly | None recorded |
| Hydrozincite | Zn5(CO3)2(OH)6 | 2 to 2.5 | 4.00 | Colourless, white, grey, pale shades | Pearly, silky to dull or earthy | Pale blue to lilac under UV |
| Rosasite | (Cu,Zn)2(CO3)(OH)2 | 4.5 | 4.0 to 4.2 | Green, bluish green, sky-blue | Not printed | None recorded |
| Smithsonite | ZnCO3 | 4 to 4.5 | 4.43 | White, grey, brown, many pale shades | Vitreous, may be pearly | May fluoresce pale green or pale blue |
| Hemimorphite | Zn4Si2O7(OH)2·H2O | 4.5 to 5 | 3.475 | Colourless, white; pale blue, pale green | Vitreous, subpearly | May fluoresce bluish under SW UV |
| Malachite | Cu2(CO3)(OH)2 | 3.5 to 4 | 4.05 | Bright to blackish green, commonly banded | Adamantine to vitreous; silky if fibrous | None recorded |
| Azurite | Cu3(CO3)2(OH)2 | 3.5 to 4 | 3.773 | Azure-blue to pale blue | Vitreous to subadamantine | None recorded |
What is aurichalcite?
Aurichalcite is (Zn,Cu)5(CO3)2(OH)6, a zinc copper carbonate with hydroxyl. The Handbook and the September 2026 IMA-CNMNC Master List write the same formula; the IMA list records it as grandfathered, 1839, Russia. The Handbook's first locality is the Loktevskii mine in the west Altai Mountains, Russia; the sheet does not mark it as the type locality, and we do not either. Type material is at the Natural History Museum, London, number 31118. The name is from the Latin for golden copper, brass, for the copper and zinc content of the mineral.
It is monoclinic, pseudo-orthorhombic by twinning, point group 2/m and space group P21/m. The crystals are acicular to lathlike with prominent {010}, commonly striated parallel to [001], with wedgelike terminations, to 3 cm. They grow typically in tufted divergent sprays or spherical aggregates, and may form thick crusts. Cleavage is perfect on {010} and {100}. The colour is pale green, greenish blue or sky-blue, and the lustre silky to pearly.
It is widespread in small amounts in the oxidised zones of copper and zinc deposits. Its associates on the sheet are rosasite, smithsonite, hemimorphite, hydrozincite, malachite and azurite.
How soft is aurichalcite? Hardness 1 to 2
Method. The header formulas of the 176 sheets in this site's reference set were searched for CO3; 15 match, including meionite, where the carbonate is a minor component. Their printed hardnesses run from aurichalcite 1 to 2 up to meionite's 5 to 6. The next softest are hydrozincite 2 to 2.5 and phosgenite 2 to 3. The rest start at 3 (calcite, cerussite) or above.
The tenacity line is unique in the set. A case-insensitive search of all 176 sheets for fragile matches one: aurichalcite’s Tenacity: “Fragile”, printed in quotation marks. The word has no ligature. Of the 132 sheets with a Tenacity field, 102 contain brittle; hydrozincite prints very brittle.
In practice: the tufts are soft laths with two perfect cleavages. The sheet gives no handling instructions and neither does this page. General specimen care is on handling and storage, and the microscope side of a 3 cm spray on what is a micromount.
What is aurichalcite made of? A worked composition
The sheet's one analysis is from Utah: ZnO 54.01%, CuO 19.87%, CO2 16.22%, H2O 9.93% and CaO 0.36%, total 100.39%. The sheet says it corresponds to (Zn3.63Cu1.37)Σ=5.00(CO3)2(OH)6. On standard atomic weights, with zinc plus copper set to 5, we get Zn3.633 and Cu1.367, with CO3 2.017 and OH 6.034. That reproduces the sheet.
Zinc is 72.7% of the metal atoms, a zinc-to-copper ratio of 2.66 to 1. By mass the ideal Zn3.63Cu1.37(CO3)2(OH)6 is 43.4% zinc and 15.9% copper. The name says golden copper, but the analysed material is mostly zinc.
The zinc end is hydrozincite, Zn5(CO3)2(OH)6, the same formula with no copper: 59.55% zinc. On paper, the Utah aurichalcite is the hydrozincite formula with 27% of the zinc sites holding copper. The sheets do not say whether copper is required for aurichalcite's structure, and we do not claim it. Rosasite, CuZn(CO3)(OH)2 on the IMA list, is the other zinc copper carbonate, with a different formula type and hardness 4.5.
How to tell aurichalcite from rosasite, hydrozincite and smithsonite
Hydrozincite is the nearest in formula, hardness (2 to 2.5) and density (4.00 against 3.96). It is colourless, white or grey rather than blue-green, and its sheet says it fluoresces pale blue to lilac under UV. Aurichalcite's sheet records no fluorescence. Each sheet names the other as an associate.
Rosasite shares the colours, green, bluish green and sky-blue, but it is hardness 4.5, and its sheet describes acicular crystals only to 0.5 mm, in globular and botryoidal crusts with warty surfaces. Aurichalcite forms silky laths to 3 cm. Smithsonite, 4 to 4.5, and hemimorphite, 4.5 to 5, are much harder and vitreous; their mutual test is on smithsonite vs hemimorphite.
Aurichalcite is named in the Association field of 4 other sheets in the set: hemimorphite, hydrozincite, rosasite and smithsonite, the zinc side of its company. Malachite and azurite are on aurichalcite's own line, but their sheets do not name it.
Where aurichalcite is found
The sheet's localities noteworthy for good specimens, in order: the Loktevskii mine, west Altai, Russia; Monteponi and the Rosas mine, Sardinia; Laurium, Greece; Chessy, France. In Arizona, Bisbee, the Silver Hill mine and the 79 mine; Cerro Gordo, California; the Kelly and Graphic mines, Magdalena, New Mexico. In Utah, the Tintic district, the Hidden Treasure mine, the Apex mine near St George and Big Cottonwood Canyon; Leadville, Colorado. Rich specimens from the Ojuela mine, Mapimí, Durango, Mexico; the Tchah Khuni mine, Iran; Tsumeb, Namibia; and Kipushi, Congo.
The locality pages are Ojuela and Tsumeb. The sheet names no British locality. The other carbonates 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.