Rosasite
Rosasite is a copper zinc carbonate, hardness 4.5 and density 4.0 to 4.2, in blue-green botryoidal crusts. The IMA redefined its formula in 2017 and sheets lag.
Rosasite is a secondary copper zinc carbonate of the oxidised zone, hardness 4.5 and measured density 4.0 to 4.2, forming sky-blue to green botryoidal crusts and radial sprays of acicular crystals to 0.5 mm. Its type locality is the Rosas mine in Sardinia, described in 1908. The IMA redefined its formula to an ordered CuZn(CO3)(OH)2 in 2017, and most printed sheets still carry the old one.
In short
- The formula on your reference sheet is superseded. The Handbook of Mineralogy prints (Cu,Zn)2(CO3)(OH)2, a disordered solid solution. The September 2026 IMA-CNMNC Master List prints the ordered CuZn(CO3)(OH)2, citing a 2017 redefinition in the Canadian Mineralogist. Both describe the same mineral; only one is current.
- And the analysis that defined the species does not match the formula that now defines it. The Handbook's own Sardinian analysis gives CuO 47.53% and ZnO 24.60%, which works out at Cu:Zn = 1.98 to 1, or (Cu1.33Zn0.67). The end-member is exactly 1 to 1. Type-region rosasite is two-thirds copper.
- It is a Caldbeck Fells and Cornwall mineral as well as a Tsumeb and Ojuela one. The Handbook names the Caldbeck Fells in Cumbria and mines in Cornwall directly after the Sardinian type locality, alongside Tsumeb, the Kelly mine in New Mexico, Bisbee and the 79 mine in Arizona, and Ojuela at Mapimí.
- Some rosasite is younger than the mine it came from. The Handbook records that it may be of post-mine origin, meaning crystals that grew on dump and stope surfaces after mining exposed them. That is a real provenance fact, not a defect, and it belongs on the label.
- Hardness 4.5 is the separator from the blue-green crusts it grows with. Aurichalcite is 1 to 2, malachite 3.5 to 4, smithsonite 4 to 4.5, hemimorphite 4.5 to 5. Rosasite is hard enough that a steel point will not mark it easily, which aurichalcite in the same cavity will not survive.
| Species | Formula | Hardness | Density | How it differs |
|---|---|---|---|---|
| Rosasite | CuZn(CO3)(OH)2 | 4.5 | 4.0 to 4.2 | Sky-blue to green botryoidal crusts of fibrous acicular crystals; strongly pleochroic |
| Aurichalcite | (Zn,Cu)5(CO3)2(OH)6 | 1 to 2 | 3.96 | Far softer, pale blue-green, in delicate feathery blades rather than tight crusts |
| Malachite | Cu2(CO3)(OH)2 | 3.5 to 4 | 4.05 | Bright to blackish green, never sky-blue; no zinc |
| Smithsonite | ZnCO3 | 4 to 4.5 | 4.43 | Denser, and the blue-green kinds are botryoidal masses rather than fibrous sprays |
| Hemimorphite | Zn4Si2O7(OH)2·H2O | 4.5 to 5 | 3.475 | A silicate, not a carbonate; lighter, and it forms bladed crystals and crusts |
| Chrysocolla | Cu2H2Si2O5(OH)4 | 2 to 4 | 1.93 to 2.4 | Half the density; amorphous, and it takes a conchoidal fracture rather than a fibrous one |
Two formulae, one mineral, and why it matters on a label
The Handbook of Mineralogy sheet for rosasite heads the page (Cu,Zn)2(CO3)(OH)2 and dates from 2001 to 2005. The September 2026 IMA-CNMNC Master List heads its entry CuZn(CO3)(OH)2 and cites the Canadian Mineralogist volume 55 (2017), page 1027.
The brackets are the whole difference. (Cu,Zn)2 says copper and zinc share two crystallographic sites in whatever proportion the fluid supplied. CuZn says the structure has two distinct sites, one taken by copper and one by zinc, and that order is part of the definition of the species. That is the redefinition: rosasite went from being a composition range to being an ordered end-member.
Now put the Handbook's own analysis against it. Analysis (1) on the sheet, from Sardinia, gives CO2 19.54%, CuO 47.53%, ZnO 24.60% and H2O 8.33%. Convert to moles and the copper-to-zinc ratio is 1.98 to 1 — about (Cu1.33Zn0.67), two-thirds copper and one-third zinc. The Handbook's column (2), the theoretical one, is 1 to 1.
So material from the region the species is named for sits a third of the way off the end-member that now defines it. That is not an error in either source; it is what happens when a name coined in 1908 for a natural crust is later pinned to an idealised structure. The practical consequence is small and real: a rosasite label is a group-and-locality statement, and anything more precise needs an analysis.
Where it grows, and what post-mine origin means
The Handbook describes rosasite as an uncommon secondary mineral in the oxidised zone of Cu-Zn-bearing deposits, typically formed by zinc-bearing solutions on primary copper minerals; may be of post-mine origin. Every clause of that is useful.
Oxidised zone. Rosasite belongs to the same part of an orebody as malachite, aurichalcite, smithsonite, cerussite, hydrozincite and hemimorphite, which are exactly the species the sheet lists as its associates. How that zone forms is covered on how mineral specimens form.
Zinc solutions on copper minerals. The mineral records a sequence, not a single event: a copper mineral was there first and zinc-bearing water arrived later. That is why rosasite so often sits directly on or in malachite.
Post-mine origin. Some rosasite crystallised after the workings were opened, on surfaces that mining exposed. A specimen of that kind is perfectly genuine and perfectly collectable, and it is also, strictly, younger than the mine. It is worth knowing before you write a geological age on a label, and it is worth knowing that a fresh-looking crust on old dump material may be exactly that rather than a sign of restoration. The question of altered and repaired material is covered on how to tell if a specimen is repaired.
British rosasite, and the localities the sheet names first
The Handbook's distribution entry opens at the Rosas mine in Sulcis, Sardinia, and its second sentence is: At a number of places in England, as Caldbeck Fells, Cumbria, and mines in Cornwall. Britain gets named before Tsumeb.
Then the list runs Tsumeb in Namibia for fine examples; the Kelly mine at Magdalena, Socorro County, New Mexico; the Silver Bill mine at Gleeson, Bisbee, and the 79 mine in Arizona; the Mohawk mine in California; Leadville in Colorado; Majuba Hill in Nevada; the Hidden Treasure mine in Utah; and abundant specimens from the Ojuela mine at Mapimí, Durango, Mexico, which is where most rosasite in circulation comes from.
The type material is held at the Muséum national d'Histoire naturelle in Paris under numbers 109.327, 109.328 and 139.61. For Caldbeck Fells and Cornwall material, the constraint is size rather than quality: rosasite crystals reach only 0.5 mm, so British examples are micromount and thumbnail objects, and what a micromount actually is is worth reading first.
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, and Caldbeck Fells and Ojuela rosasite are both on it.