Clinoclase
Clinoclase is Cu3(AsO4)(OH)3, hardness 2.5 to 3, density 4.38. Crystals reach only 9 mm, so nearly every specimen is judged as a rosette on Cornish matrix.
Clinoclase is a copper arsenate, Cu3(AsO4)(OH)3, monoclinic, hardness 2.5 to 3 and density 4.38. Crystals are needle-like and reach only 9 mm, but radiating rosettes on matrix reach 5 cm. The colour is dark greenish blue to greenish black, and the streak bluish green.
In short
- The maximum crystal size recorded for the species is 9 mm. That figure decides how clinoclase is collected: the individual crystal is a microscope object, and what you are buying is the rosette, the coverage and the matrix. Judging a clinoclase specimen by crystal size is judging it against a ceiling of 9 mm.
- Cornwall is the type country. The IMA list records the United Kingdom as the country of first description, in 1830, and the Handbook of Mineralogy's first published analysis is Cornish. Wheal Gorland and Wheal Unity, the Ting Tang and St Day United mines in Gwennap, and Wheal Hazard at St Just are the classic sources.
- It is an arsenate, and it is toxic. Handle with gloves, do not create dust, do not eat near an open drawer, and wash your hands afterwards. The Natural Sciences Collections Association names arsenic minerals as a group needing specific handling in any collection.
- Hardness 2.5 to 3 with a perfect {001} cleavage makes it fragile out of proportion to its look. The name itself records this: from the Greek for to incline and to break, for the oblique basal cleavage.
- It travels with company that looks like it. Olivenite, cornwallite, cornubite and conichalcite are the Handbook's association list, and cornubite and cornwallite in particular are green copper arsenates from the same Cornish mines. A determination by colour alone is a guess.
| Species | Formula | Colour | Hardness | Habit |
|---|---|---|---|---|
| Clinoclase | Cu3(AsO4)(OH)3 | Dark greenish blue to greenish black | 2.5–3 | Needles to 9 mm; rosettes and spheres to 5 cm |
| Cornwallite | Cu5(AsO4)2(OH)4 | Green to blackish green | 4–5 | Botryoidal crusts, fibrous within |
| Cornubite | Cu5(AsO4)2(OH)4 | Green | 4–5 | Crusts and spherules; dimorph of cornwallite |
| Olivenite | Cu2(AsO4)(OH) | Olive-green to brown | 3 | Prismatic to acicular crystals, fibrous masses |
| Conichalcite | CaCu(AsO4)(OH) | Grass-green to yellowish green | 4.5 | Botryoidal crusts, drusy coatings |
| Liroconite | Cu2Al(AsO4)(OH)4·4H2O | Sky-blue to blue-green | 2–2.5 | Flattened lenticular crystals; the Wheal Gorland species |
Why every clinoclase specimen is really a matrix specimen
The Handbook of Mineralogy describes clinoclase as rare in crystals, typically needlelike, to 9 mm, elongated along [010], and separately as occurring as rosettes and radial fibrous spherical aggregates, to 5 cm; in crusts and coatings. Those two figures are the whole collecting logic of the species.
A 9 mm ceiling means the crystal is never the point. On a well-known species with centimetre crystals you can compare specimens on crystal size and quality and largely be finished. On clinoclase the comparison is between rosettes: how many, how complete, how well separated, how good the colour, how sound the matrix, and whether the piece has been trimmed into a shape that shows them.
This also means a great deal of good clinoclase is mounted as a micromount, and that a micromounted clinoclase is not a lesser specimen than a cabinet piece — it is the format the species asks for. The individual needles are genuinely better seen at 20 times magnification than in the hand.
The quotable version: on clinoclase you are not buying crystals, you are buying coverage.
The Cornish mines, and why the label matters more than usual
The IMA-CNMNC list records clinoclase as a grandfathered species with 1830 as the year of original description and the United Kingdom as the country. The Handbook's analysis (1) is Cornish, and its X-ray powder pattern is taken from Cornwall, England and the Tintic district, Utah.
The Cornish localities it names are the roll-call of the Gwennap and St Just copper ground: Wheal Gorland and Wheal Unity, the Ting Tang and St Day United mines at Gwennap, the Providence mine at Lelant, and Wheal Hazard at St Just, with the Bedford United mines near Tavistock in Devon. Outside Britain, the Handbook singles out the Majuba Hill mine in Pershing County, Nevada for fine examples, and lists Cap Garonne in France, the Clara mine in the Black Forest, Tintic in Utah and the Dome Rock copper mine in South Australia.
Almost all Cornish clinoclase is old material. Wheal Gorland's great period was the first half of the nineteenth century, and specimens on the market today have passed through several collections. That makes the label the evidence, and an unlabelled green Cornish arsenate is a species determination without a locality — which is a much weaker object. We set out what to do about that in is a specimen without a locality worth buying, and what old labels prove in what ex coll. means.
Telling it from the other green Cornish arsenates
The single most common error is calling any dark blue-green Cornish arsenate clinoclase. The Handbook's own association list — olivenite, cornwallite, cornubite, conichalcite — is a list of minerals that occur with it, on the same matrix, in the same mines, in overlapping colours.
Three things help. Habit first: clinoclase forms discrete needles and open radiating rosettes; cornwallite and cornubite form botryoidal crusts and spherules that are fibrous inside but rarely show free crystal terminations. Hardness second: clinoclase is 2.5 to 3 and cornwallite and cornubite are 4 to 5, which is a difference you can feel with a needle probe under the microscope on an inconspicuous edge. Colour last, and only as a tiebreak: clinoclase runs to greenish black and shows a distinctly blue cast in transmitted light, where cornubite and conichalcite stay green.
Cornwallite and cornubite are dimorphs of each other — same formula, different structure — and cannot be separated without X-ray or Raman analysis. If a specimen is labelled as one of those two without a stated method, the honest catalogue entry names the pair. Clinoclase itself is dimorphous with gilmarite, which is rare enough not to be a practical worry on Cornish material.
Handling an arsenate you can damage with a fingernail
Hardness 2.5 to 3 is softer than a copper coin, and the {001} cleavage is perfect. A clinoclase rosette will take damage from being slid across a drawer, from a cotton wool fibre catching a needle, and from any attempt to clean it.
Do not wash it. Do not brush it. Do not put it in an ultrasonic cleaner — see how to clean mineral specimens for why the risk is asymmetric on soft secondary species. If there is loose dust, a gentle puff of air is the limit.
The toxicity is real and manageable. Copper arsenates are not dangerous to own; they are dangerous to abrade, inhale or ingest. Gloves when handling, a closed box rather than an open shelf, no dry brushing, no cutting or trimming without dust control, and hands washed afterwards. The Natural Sciences Collections Association's guidance for museum collections puts arsenic and antimony minerals in the group to store in lidded clear boxes and handle with gloves. Our page on toxic minerals in a collection covers the practical routine, and Cornish arsenate material we are actively looking for is listed on the wanted list.