Pharmacosiderite
Pharmacosiderite is a cubic iron arsenate, hardness 2.5 and density 2.797 — one of eleven species covered here that were first described in the United Kingdom.
Pharmacosiderite is a hydrated potassium iron arsenate, cubic, hardness 2.5 and density 2.797, forming diagonally striated cubes to 2 cm. It was first described in 1813 from Cornwall, and is one of eleven species this site covers with a United Kingdom type locality — more than any other named country in that set.
In short
- Eleven of the 65 species this site covers have a United Kingdom type locality. That is more than any other named country — Germany has six, Italy five — and second only to the seventeen whose country of first description is recorded as unknown.
- All eleven were described in a single 60-year window, 1805 to 1865. Bournonite 1805, pharmacosiderite 1813, chalcosiderite 1814, vivianite 1817, olivenite 1820, clinoclase 1830, cassiterite and smithsonite 1832, mendipite 1839, langite 1864, botallackite 1865.
- Pharmacosiderite is cubic, which almost no arsenate is. Diagonally striated cubes to 2 cm in space group P-43m. An olive-green cube of hardness 2.5 from a Cornish or Cumbrian mine has few alternatives.
- The water content is variable, 6 to 7 molecules per formula unit, and the species dehydrates and rehydrates with humidity. That is a reason to keep it out of a desiccated cabinet, not a reason to seal it.
- The name is a warning label. From the Greek for poison, for the arsenic, and iron for the composition — the mineral was named for what it contains. It is an oxidation product of arsenic-bearing sulphides and sits with scorodite, beudantite, carminite and arseniosiderite.
| Species | Formula | First described | Type area |
|---|---|---|---|
| Bournonite | CuPbSbS3 | 1805 | Cornwall |
| Pharmacosiderite | KFe3+4(AsO4)3(OH)4·6-7H2O | 1813 | Cornwall |
| Chalcosiderite | CuFe3+6(PO4)4(OH)8(H2O)4 | 1814 | United Kingdom; area not securely recorded |
| Vivianite | Fe2+3(PO4)2(H2O)8 | 1817 | Cornwall |
| Olivenite | Cu2(AsO4)(OH) | 1820 | Cornwall |
| Clinoclase | Cu3(AsO4)(OH)3 | 1830 | Cornwall |
| Cassiterite | SnO2 | 1832 | United Kingdom; area not securely recorded |
| Smithsonite | Zn(CO3) | 1832 | England; Somerset usually cited |
| Mendipite | Pb3O2Cl2 | 1839 | The Mendip Hills, Somerset |
| Langite | Cu4(SO4)(OH)6(H2O)·H2O | 1864 | Cornwall |
| Botallackite | Cu2Cl(OH)3 | 1865 | Botallack, St Just |
A count of type localities across this site's reference set
Method. On 20 September 2026 every species with a page on this site — 65 of them, including the eight published today — was matched against the IMA-CNMNC Master List for September 2026, and the Country column read off. Three could not be matched because the site's page covers a group rather than a single species: apophyllite, heulandite and stilbite. The remaining 62 were counted.
Result: unknown 17, United Kingdom 11, Germany 6, Italy 5, Romania 3, and then a long tail of ones and twos — Chile, Spain, Russia and Austria with two each; France, the USA, Argentina, Kazakhstan, Slovakia, South Africa, Japan, the Czech Republic, Mexico and Belgium with one each.
Two things about that distribution are worth saying plainly. The first is the obvious one: this site is British in emphasis, and the type-locality count confirms it rather than merely asserting it. The second is less obvious. The seventeen entries recorded as unknown are nearly all grandfathered species — minerals known and named before anyone thought to record where the first specimen came from. Talc's original reference is a 1546 reprint; pyrite's is given as original paper?. The commonest type locality among the species a collector actually handles is no type locality at all.
This is the same class of negative result as the pseudomorph count on what a pseudomorph is, and it has a similar consequence: a property that sounds like a fixed fact about a species often turns out not to be recorded at all. What a type locality is, and why it matters to a label, is set out on what is a type locality.
Why the window is 1805 to 1865, and what closed it
All eleven British-described species on this site fall in a 60-year window. That is not a coincidence and it is not selection bias in the sample; it is the history of Cornish mining written into the mineral list.
Most of the eleven are Cornish. The period runs from the peak of Cornish copper through the development of deep tin, and it coincides with the arrival of systematic analytical chemistry. Mines going deeper into oxidised zones produced unfamiliar material at exactly the moment chemists acquired the means to characterise it. Pharmacosiderite in 1813, chalcosiderite in 1814, olivenite in 1820, clinoclase in 1830 — all from the copper-arsenic ground around Gwennap and St Day.
What closed the window was the collapse of Cornish metal mining in the later nineteenth century, not the exhaustion of Cornish mineralogy. The ground is still there; the access is not. Later British species exist, but on this site's list the sequence stops at botallackite in 1865.
For the collector the consequence is that Cornish type material is nearly all old material, held in cabinets and museums rather than coming out of the ground, and that provenance chains matter more here than almost anywhere. The argument is made at length on buying old collection material.
Identifying pharmacosiderite, and what else is cubic and green
The Handbook of Mineralogy gives pharmacosiderite as cubic, point group -43m, commonly in good crystals, dominantly cubic, striated diagonally, with minor {111}, {011} and {122}, reaching 2 cm. Colour runs olive-green, grass-green, emerald-green, honey-yellow, yellowish brown and dark brown, and less commonly hyacinth-red.
Hardness is 2.5 and density 2.797 measured against 2.90 calculated — the gap between the two is the variable water. Lustre is adamantine to greasy. It is weakly piezoelectric and pyroelectric, and it is optically odd: isotropic in principle but commonly anomalously biaxial, sometimes showing six biaxial sectors in a single crystal.
The combination that identifies it is a diagonally striated green cube that a fingernail will nearly mark. Fluorite is cubic and green but is hardness 4 with perfect octahedral cleavage. Pyrite is cubic and striated but is hardness 6 to 6.5 and metallic. Nothing else green, cubic and soft occurs in an arsenic-bearing oxidised zone.
The associates the Handbook lists are the confirmation: scorodite, beudantite, carminite, arseniosiderite, symplesite, jarosite and limonite. That is the standard oxidised-arsenopyrite assemblage, and it is the same suite described on the scorodite page.
Arsenic, water, and how to keep one
Pharmacosiderite is an arsenate and it was named for the fact. That does not make it a hazard on a shelf: it is a stable crystalline solid, and the ordinary precautions for arsenic-bearing species apply — do not abrade it, do not breathe dust from it, keep it away from food preparation and wash your hands after handling. Preparation, not display, is the exposure route, which is the argument set out on is arsenopyrite dangerous. Nothing on this page is health, safety or legal advice.
The storage question that is specific to this species is water. The formula carries 6 to 7 H2O, and a species with a variable hydration state has a hydration state that can change. The measured density of 2.797 against a calculated 2.90 is that variation showing up in the numbers.
The practical answer is a stable, middling humidity rather than a dry one. A cabinet kept aggressively dry with silica gel is the wrong environment for a variably hydrated arsenate, and it is also the wrong environment for chalcanthite — while being the right environment for pyrite. That conflict, and the narrow band that resolves it, is worked out on how to store soluble mineral specimens.
The Handbook lists it from the Carharrack and Tincroft mines and Wheals Gorland and Unity at Gwennap in Cornwall, at many mines in the Caldbeck Fells, from Vaulry and the Cap Garonne mine in France, from Dernbach, Horhausen, the Clara mine and Schneeberg in Germany, from Nová Baña in Slovakia, from the Mammoth mine in Utah and the Majuba Hill and Northumberland mines in Nevada, from Tsumeb, and from Broken Hill. Nothing here is for sale and there is no catalogue; the route is the wanted list, and the section index is at mineral species.