Cobaltite
Cobaltite is hardness 5.5, density 6.33, in pseudocubic crystals to 8 cm. It is one of eight silver-white species a density ladder separates without ambiguity.
Cobaltite is a cobalt arsenic sulphide, hardness 5.5 and measured density 6.33, in silver-white pseudocubic or pseudopyritohedral crystals to 8 cm with striated faces like pyrite, perfect {001} cleavage and a greyish-black streak. Its Cornish locality is the Botallack mine at St Just, and its finest modern crystals come from Bou Azzer in Morocco.
In short
- Density 6.33 against pyrite's 5.018. Cobaltite is 26% heavier than the pyrite it can be taken for, and heft is the first thing that gives it away in the hand.
- It is silver-white, not brass-yellow. Pyrite is pale brass; cobaltite has no yellow in it at all. The streak is greyish black where pyrite's is greenish to brownish black.
- Crystals reach 8 cm, pseudocubic or pseudopyritohedral with striated faces, so closely mimicking pyrite that the Handbook of Mineralogy describes the striations by reference to it.
- It is orthorhombic, not cubic, despite the habit. Twinning about [111] as a pseudocubic three-fold axis is recorded as rare, and twin lamellae give a flame-like texture in polished section.
- Cornwall, Bou Azzer, Broken Hill and Skutterud are all on the Handbook's locality list, which is unusual: cobaltite links four separate parts of this site's coverage through a single species.
| Species | Formula | Hardness | Density | Colour and streak |
|---|---|---|---|---|
| Marcasite | FeS2 | 6 to 6.5 | 4.887 | Tin-white fresh, tarnishing pale bronze; greyish-black streak |
| Pyrite | FeS2 | 6 to 6.5 | 5.018 | Pale brass-yellow; greenish to brownish black streak |
| Gersdorffite | NiAsS | 5.5 | 5.9 | Silver-white to steel-grey; greyish-black streak |
| Arsenopyrite | FeAsS | 5.5 to 6 | 6.07 | Silver-white to steel-grey; garlic odour when struck |
| Cobaltite | CoAsS | 5.5 | 6.33 | Silver-white; greyish-black streak |
| Skutterudite | CoAs3 | 5.5 to 6 | 6.5 | Tin-white, tarnishing grey or iridescent; black streak |
| Safflorite | CoAs2 | 4.5 to 5 | 7.2 | Tin-white, tarnishing readily to dark grey |
| Galena | PbS | 2.5 to 2.75 | 7.58 | Lead-grey; perfect cubic cleavage settles it at once |
A density ladder that separates eight species nothing else does
The silver-white and grey metallic minerals are the hardest group in a collection to tell apart by eye. Colour ranges only from tin-white to steel-grey across all of them, several tarnish, and habit repeats: pyrite, cobaltite, gersdorffite and skutterudite all form striated pseudocubes.
Density separates every one of them, and the steps are large. Read from the Handbook of Mineralogy sheets, the ladder runs marcasite 4.887, pyrite 5.018, gersdorffite 5.9, arsenopyrite 6.07, cobaltite 6.33, skutterudite 6.5, safflorite 7.2, galena 7.58. The smallest gap in that list is 0.13, between marcasite and pyrite; every other step is 0.17 or more.
That is within reach of an ordinary hydrostatic weighing. Our note on measuring specific gravity at home derives the minimum specimen size for a given accuracy; for a species this dense you need about 7.5 g on a scale reading to a hundredth of a gram to resolve 0.05, which is a small thumbnail.
The one caution worth stating: these species intergrow, and a mixed specimen returns a density that belongs to nothing. Skutterudite, safflorite and cobaltite occur together at Bou Azzer and at the Cobalt district in Ontario, so a reading of 6.4 on a crusty mass means less than the same reading on a clean crystal. Our note on silver-coloured pyrite works through the diagnosis in the order that actually saves time.
Pseudocubic, not cubic, and what the striations are telling you
Cobaltite is orthorhombic, pseudocubic. The crystals look cubic, they look pyritohedral, they carry the striated faces pyrite is famous for — and the underlying symmetry is lower than any of that suggests.
The Handbook records the striations explicitly by comparison: combinations having striated faces as with pyrite. That is the trap. On pyrite, striations on adjacent cube faces run at right angles to one another, a consequence of the alternation between cube and pyritohedron during growth. Cobaltite reproduces the appearance without the cubic symmetry behind it, so the striation pattern is a poor guide here and colour and density are better ones.
Twinning is recorded as rare — about [111] as a pseudocubic three-fold axis, with {011} and {111} of the pseudocubic habit as twin planes. In polished section, twin lamellae are common and produce a flame-like texture, which is a laboratory observation rather than something visible on a specimen. Our note on mineral twinning covers why rare twinning is the usual case: of 108 Handbook sheets covering this site's species, 69 record twinning and only 27 describe it as common or better.
Four localities, and why this species ties the site together
Most species pages link outward to one or two localities. Cobaltite's Handbook distribution paragraph names four this site already covers, which is worth setting out.
Bou Azzer, Morocco — the Aït Ahmane mine, the district that produces the world's best modern cobalt-arsenide specimens and which we cover in our Bou Azzer note. Cornwall — Crown's Engine House, Wheal Cock and the Botallack mine at St Just, the same mine that gives its name to botallackite; see our Cornwall note for the district. Broken Hill and Torrington in New South Wales, covered in our Broken Hill note. And Skutterud, Modum, Norway, which is not one of ours but is the type locality of skutterudite and the reason that species has its name.
The Swedish occurrences are where the fine classic crystals come from: Tunaberg in Södermanland, Riddarhyttan and Håkansboda in Västmanland, and the Vena mines near Askersund. Tunaberg material is the reference for what a sharp cobaltite looks like, and the Handbook lists its associated minerals — magnetite, sphalerite, chalcopyrite, skutterudite, allanite, zoisite, scapolite, titanite and calcite — in unusual detail for that locality specifically.
We hold no catalogue and nothing here is offered for sale. The wanted list sets out what we look for; for cobaltite that means Cornish material with an old label above almost anything else, because it is scarce and the Botallack association is historically significant. Our species notes cover the other cobalt and arsenic species of the same deposits.
Arsenic, and how much care this actually needs
Cobaltite is CoAsS — a third of the atoms are arsenic, and cobalt itself is not benign in dust form. That places it firmly in the group of specimens that need handling rules rather than alarm.
The rule that covers it is the general one and it is enough: do not create dust, do not eat, drink or smoke while handling specimens, and wash your hands afterwards. Cobaltite is a stable sulphosalt, not a soluble salt; it is not absorbed through intact skin and it does not shed anything into the air of a closed cabinet. The risk is entirely in what comes off a broken or abraded surface.
What follows from that is practical. Do not saw, grind, sand or ultrasonically clean cobaltite, and do not trim matrix with a hammer without wet methods and eye protection — the arsenides are a group where specimen preparation, not display, is the exposure. Our note on toxic minerals in a collection sets out the group rules, and our note on cleaning specimens covers why abrasive methods are wrong here for conservation reasons as well as health ones.
This is not health or safety advice and we are not qualified to give it. If you handle arsenide specimens in quantity, or prepare them, take the question to an occupational-health professional rather than to a mineral site.