Botallackite
Botallackite is Cu2Cl(OH)3, type locality the Botallack mine in Cornwall, 1865. It is one of three polymorphs no bench test can separate from each other.
Botallackite is a copper chloride hydroxide, Cu2Cl(OH)3, monoclinic, named in 1865 for the Botallack mine at St Just in Cornwall, which is its type locality. It occurs as crusts of minute platy interlaced crystals, mountain-green to bluish green, with a measured density of about 3.6. The Handbook of Mineralogy gives no hardness figure.
In short
- The type locality is the Botallack mine, St Just, Cornwall, and the IMA master list gives the United Kingdom as the country of first description, 1865. Type material is held at the Natural History Museum in London and at Harvard.
- Cu2Cl(OH)3 has three polymorphs, not four. Atacamite, botallackite and clinoatacamite share the formula. Paratacamite was long counted with them and no longer belongs: on the September 2026 IMA list it is Cu3Cu(OH)6Cl2, a different composition.
- No bench test separates them. Hardness runs 3 to 3.5 across the set, densities run 3.6 to 3.78, and all are green. The determination is X-ray or Raman, and a specimen labelled by eye is a guess.
- Habit is the one hint. Atacamite forms slender crystals to 10 cm; botallackite is crusts of minute plates; clinoatacamite reaches 1 mm. A large green chloride crystal is atacamite by default.
- Seawater is the common thread. The Handbook records botallackite from copper deposits weathered in contact with seawater, from slag immersed in seawater, and from sulphides in subsea black smoker deposits — including along the Mid-Atlantic Ridge.
| Species | Formula (IMA, Sept 2026) | System | Hardness | Density | Crystals to |
|---|---|---|---|---|---|
| Atacamite | Cu2Cl(OH)3 | Orthorhombic | 3 to 3.5 | 3.745 to 3.776 | 10 cm |
| Botallackite | Cu2Cl(OH)3 | Monoclinic | Not stated; soft | about 3.6 | Crusts of minute plates |
| Clinoatacamite | Cu2Cl(OH)3 | Monoclinic | 3 | 3.77 calculated | 1 mm |
| Paratacamite | Cu3Cu(OH)6Cl2 — no longer the same formula | Hexagonal | 3 | 3.72 to 3.74 | 1.5 cm |
| Langite | Cu4(SO4)(OH)6 · 2H2O | Monoclinic | 2.5 to 3 | 3.28 to 3.34 | 7 mm |
Paratacamite is no longer a polymorph, and most sources have not caught up
For most of the twentieth century the standard account was that Cu2Cl(OH)3 had three polymorphs — atacamite, botallackite and paratacamite — and clinoatacamite was added as a fourth when it was described in 1996. The 2001 Handbook of Mineralogy still says exactly that on the botallackite sheet: polymorphous with atacamite, clinoatacamite, and paratacamite.
The IMA-CNMNC master list for September 2026 does not. It gives paratacamite as Cu3Cu(OH)6Cl2, alongside paratacamite-(Mg) and paratacamite-(Ni), both approved in 2013. That is a different formula from Cu2Cl(OH)3, and a mineral with a different composition is not a polymorph of anything.
So the true polymorph set today is three: atacamite, botallackite and clinoatacamite. This matters to a collector for a narrow but real reason. A lot of old Cornish and Chilean material is labelled paratacamite, and much of that material was determined before clinoatacamite existed as a species and before paratacamite was redefined. Those labels are historically informative and analytically obsolete at the same time, which is a common combination and is covered in our note on what an old label is worth.
Why you cannot identify these by eye, and what to write on the label instead
Read the comparison table as a negative result, because that is what it is. Hardness: 3 to 3.5, not stated, 3, 3. Density: 3.6 to 3.78 across the whole set. Colour: green in every row. There is no combination of scratch, streak, weight and colour that will tell you which of the three you are holding.
Habit narrows it. Atacamite forms slender crystals prismatic along [001], striated, reaching 10 cm, and it has a distinctive apple-green streak. Botallackite occurs as crusts of minute platy interlaced crystals — so small that the individual plates are a microscope object. Clinoatacamite occurs as complex intergrowths and twinned grains to 1 mm, and as powdery aggregates. A green copper chloride crystal you can see with the naked eye is overwhelmingly likely to be atacamite; a green chloride crust is the one that needs analysis.
The honest label for undetermined material is the one that says so. Copper chloride hydroxide, Cu2Cl(OH)3 group, undetermined is a better record than a confident species name that a Raman spectrum would overturn. Our page on labelling and cataloguing sets out the three certainty states that make this workable.
Botallack, St Just, and the seawater mechanism
The Botallack mine at St Just is one of the most photographed industrial sites in Britain — the Crowns engine houses stand on the cliff face with workings running out under the Atlantic — and it is the type locality for botallackite. The Handbook names Wheal Cock, West Wheal Owles, and the Botallack and Levant mines at St Just, along with a few other Cornish places.
The occurrence description explains why this corner of Cornwall. Botallackite forms in copper-bearing deposits in areas with high chlorine concentrations, or weathered while exposed to seawater; as a reaction product of slag immersed in seawater; and as a weathering product of sulphides in subsea black smoker deposits. St Just's mines ran out under the sea. The chlorine came with the ground.
The other occurrences follow the same rule. Southwick Cliffs near Dalbeattie in Kirkcudbrightshire, where it is associated with gypsum, brochantite and connellite; the Castletown mine at Lochgilphead; the Dooneen mine at Allihies in County Cork. Then a run of slag localities — Juliushütte at Astfeld in the Harz, Laurium in Greece, the Argent lead-zinc mines east of Johannesburg — and finally the TAG hydrothermal field on the Mid-Atlantic Ridge. Our Cornwall locality notes cover the St Just district in more detail.
Handling: soft, in crusts, and not a specimen to clean
The Handbook declines to give botallackite a Mohs number, recording only [Soft.], with a good cleavage in one direction perpendicular to Z. Comparison with its polymorphs puts it near 3. In practice this is a crust of tiny plates on a matrix, and crusts of tiny plates on a matrix are the single most cleaning-sensitive thing a collection holds.
Do not brush it, do not put it in an ultrasonic tank, and be careful with water: these are chlorides forming in a chloride-rich environment, and the boundary between washing a specimen and redissolving the thing you are washing is not one to explore on a type-locality piece. A puffer and patience is the whole toolkit. Our page on cleaning mineral specimens covers which species tolerate what.
Botallackite is also a copper secondary in a mine environment, so the usual company applies — atacamite, brochantite, langite and connellite all turn up on the same pieces. A specimen carrying two or three of them is more informative than a single-species crust, and generally harder to find. If that is what you are after, put it on a wanted list with the species combination stated, because no catalogue is indexed that way.