Chalcosiderite
Chalcosiderite is the iron end of the turquoise series, hardness 4.5, density 3.22, crystals only 3 mm long. Cornwall is the classic locality. Habit and care.
Chalcosiderite is a copper iron phosphate of the turquoise group, triclinic, hardness 4.5, measured density 3.22. It forms a series with turquoise, iron replacing aluminium. Crystals are short prismatic, typically in sheaves, and reach only 3 mm. The classic locality is the West Phoenix mine in Cornwall.
In short
- Crystals reach 3 mm. That is the whole species. The Handbook of Mineralogy records short prismatic crystals, typically in sheaves, to 3 mm, and crusts. Anything sold as a large chalcosiderite crystal is something else.
- It is the iron end of the turquoise series. Turquoise is CuAl6(PO4)4(OH)8·4H2O; chalcosiderite substitutes ferric iron for the aluminium. Same structure, same group, and the colour swings from turquoise-blue to siskin-green as iron comes in.
- Hardness 4.5 and density 3.22, with one perfect cleavage and a second less perfect one. Harder than most collectors expect from a hydrated phosphate, and hard enough to survive normal handling — which is fortunate, given the crystals cannot be seen without magnification.
- The name is descriptive, not honorific: from the Greek for copper and iron, for the two elements in the formula. It says nothing about a person or a place, which is unusual for a Cornish classic.
- The West Phoenix mine in Cornwall is the reference locality — the Handbook's chemical analysis and its X-ray powder pattern are both taken from Cornish material.
| Species | Formula | Colour | Hardness |
|---|---|---|---|
| Turquoise | CuAl6(PO4)4(OH)8·4H2O | Sky-blue to green | 5–6 |
| Chalcosiderite | CuFe6(PO4)4(OH)8·4H2O | Pale siskin-green to dark green | 4.5 |
| Faustite | (Zn,Cu)Al6(PO4)4(OH)8·4H2O | Apple-green | 4.5–5 |
| Planerite | Al6(PO4)2(PO3OH)2(OH)8·4H2O | Pale green to white | Around 5 |
| Aheylite | (Fe,Zn)Al6(PO4)4(OH)8·4H2O | Pale blue-green | 5–5.5 |
A species you need a microscope to collect
Chalcosiderite is a micromount mineral, and stating that plainly is the single most useful thing a page about it can do. The Handbook of Mineralogy gives the crystal habit as short prismatic, with a number of forms noted, typically in sheafs, to 3 mm; in crusts. Three millimetres is the recorded maximum, not the typical size.
This changes what buying one means. There is no cabinet specimen of chalcosiderite in the sense that there is a cabinet specimen of fluorite. What is offered is a piece of matrix carrying crusts and sheaves that resolve into crystals only under magnification, and the quality of the specimen is a statement about the coverage, the sharpness of the sheaves under a stereo microscope, and the locality — not about size.
Two practical consequences follow. First, you cannot assess this species from a normal photograph; you need a photomicrograph, and a dealer who works in micromounts will have one. Second, the crystals being small does not make them robust in the way that small hard crystals usually are. They are sheaves standing proud of a crust, and a fingertip flattens them.
If you are starting on micromounts, our guide to specimen sizes and grading explains the size conventions and where the micromount category sits relative to thumbnails and miniatures.
The turquoise connection, and what the colour is telling you
Chalcosiderite forms a series with turquoise. The two share the triclinic turquoise-group structure and differ in one position: turquoise has aluminium, chalcosiderite has ferric iron. Everything else about the formula is the same — a copper atom, four phosphate groups, eight hydroxyls and four waters.
The colour follows the iron. Turquoise proper is sky-blue to green; chalcosiderite is what the Handbook calls pale siskin-green to dark green. Intermediate compositions exist along the series and take intermediate colours, which means a green “turquoise” and a pale chalcosiderite may be the same continuum sampled at different points. As with the copper sulphides, the name on the label reflects where somebody decided the boundary was.
The Handbook's analysis of West Phoenix mine material shows how untidy this is in practice: 42.81% Fe2O3 against a theoretical 48.56%, with 4.45% Al2O3 sitting in the structure where more iron should be. The Cornish type material is itself part way along the series towards turquoise.
The optical properties are more diagnostic than the colour. Chalcosiderite is biaxial negative with refractive indices of 1.775 to 1.844 — very high for a hydrated phosphate — a 2V of 22 degrees, and very strong, crossed dispersion. Under a polarising microscope it is distinctive. Under a hand lens it is a small green sheaf.
Cornwall, and the associations that identify a specimen
The West Phoenix mine in Cornwall is the locality the species is defined against: the Handbook's chemical analysis is of West Phoenix material and its X-ray powder pattern is credited simply to Cornwall, England. The association there is dufrénite and goethite — an iron-phosphate assemblage, which is exactly what the formula predicts.
Two other associations are recorded and both are useful for placing an unlabelled specimen. At the Gunheath china clay pit, also in Cornwall, chalcosiderite occurs with dufrénite, cyrilovite and leucophosphite — the china clay pits are a distinct Cornish phosphate environment quite separate from the copper-tin lodes. At the Lake Boga quarry in Australia, the associates are sampleite, libethenite, crandallite, cyrilovite, pseudomalachite, saléeite, torbernite and ulrichite.
The association is the locality clue. Goethite and dufrénite point to the lode occurrences; the full crandallite-and-uranium-mica suite points to Lake Boga; leucophosphite points to the china clay pits. On a species where the crystals are 3 mm and the label may have been rewritten twice, the matrix is often better evidence than the writing.
Beyond these, chalcosiderite is a rare mineral in the oxidised zone of some hydrothermal deposits generally. Cornish material is old-collection material, and a specimen with a nineteenth or early twentieth century label is a documented object as much as a mineralogical one — see buying old collection material. If a Cornish-labelled chalcosiderite is what you are after, it is worth putting on a wanted list rather than searching for, because material at that level surfaces when collections are dispersed and not otherwise.
Storing and handling a micromount species
Chalcosiderite is chemically among the better-behaved hydrated phosphates. It is not water-soluble, not notably light-sensitive, and at hardness 4.5 it is harder than fluorite. The risks are mechanical and environmental rather than chemical.
Dehydration is the one to watch. The formula carries four waters of hydration. Hydrated species can lose water at low relative humidity — the Natural Sciences Collections Association records blue copper sulphate crystals crumbling to a pale blue powder below 35% RH — so a very dry cabinet, or a sealed box over-loaded with silica gel intended for a pyrite problem, is the wrong environment. Keep it in the general 45 to 60% band rather than in a desiccated microclimate.
Mount it, and then handle the mount. The standard micromount practice — the specimen glued to a plastic pedestal in a hinged box, oriented so the best sheaf is presented — exists precisely so that the mineral is never touched again. It also solves the finding problem, since an unmounted 3 mm sheaf on a 4 cm piece of gossan is very easy to lose in a drawer.
Label it properly, because you cannot re-identify it later. On a species that requires a microscope to see and analysis to confirm against turquoise-series intermediates, the label carries most of the information value. Record the locality, the source, the date and whether it has been analysed. Our page on what the abbreviations on a mineral label mean covers the conventions worth following.