Chalcophyllite
Chalcophyllite, Cu18Al2(AsO4)4(SO4)3(OH)24·36H2O: emerald-green hexagonal plates with a micaceous cleavage, hardness 2, density 2.67 to 2.69, 27.91% water.
Chalcophyllite is a hydrated copper aluminium arsenate sulphate, Cu18Al2(AsO4)4(SO4)3(OH)24·36H2O on the IMA list (G, 1841, United Kingdom): emerald-green to blue hexagonal plates to 1 cm with a perfect micaceous cleavage, hardness 2, density 2.67 to 2.69. Worked from its formula it is 27.91% water and carries both arsenate and sulphate.
In short
- The arsenate–sulphate key. The Handbook of Mineralogy chalcophyllite sheet names 16 associated minerals. Looked up by script on the September 2026 IMA-CNMNC Master List, 7 have arsenate in their formula and 6 sulphate; only one, parnauite, Cu9(AsO4)2(SO4)(OH)10(H2O)4·3H2O, has both, as chalcophyllite does. Their arsenic-to-sulphur ratios differ: 4 to 3 in chalcophyllite, 2 to 1 in parnauite.
- Chalcophyllite against parnauite. On their sheets: hexagonal micaceous plates to 1 cm, uniaxial, against orthorhombic laths to 4 mm in bowties and fans, biaxial. Worked as 20 − 20/D, a 20.00 g piece reads 12.51 to 12.57 g as chalcophyllite and 13.53 g as parnauite.
- The lightest of its arsenate company. Of the eight arsenates (chalcophyllite and the seven on its association line), chalcophyllite reads lowest in water. The nearest is pharmacosiderite at 12.85 g, 0.28 g above, a cubic, isotropic mineral on its sheet; the heaviest, cornubite, reads 15.69 g.
- Documented failure: the 1.618 reading. The sheet gives ω 1.618 and ε 1.552 for chalcophyllite, and ω 1.680 and ε 1.618 for dehydrated material. A single refractometer reading of 1.618 therefore fits both states; only the second reading, 1.552 or 1.680, says which.
- Where. The Distribution field names large crystals from Wheals Gorland, Muttrell and Unity and the Ting-Tang mine, Gwennap, Cornwall; the Old Potts Gill mine, Caldbeck Fells, Cumbria; Saxony, Schwaz and Špania Dolina; Cap Garonne and Salsigne, France; Bisbee, Majuba Hill and Tintic, USA; fine examples from El Teniente, Chile; and Maharahara, New Zealand.
| Species | Formula (IMA) | IMA status | Arsenate | Sulphate | As by mass | S by mass | System (sheet) | Hardness | Density | 20.00 g piece in water | Optical class (sheet) |
|---|---|---|---|---|---|---|---|---|---|---|---|
| Chalcophyllite | Cu18Al2(AsO4)4(SO4)3(OH)24·36H2O | G, 1841, United Kingdom | Yes | Yes | 9.67% | 3.10% | Hexagonal (point group −3) | 2 | 2.67 to 2.69 | 12.51 to 12.57 g | Uniaxial (−) |
| Parnauite | Cu9(AsO4)2(SO4)(OH)10(H2O)4·3H2O | A, 1978-014, USA | Yes | Yes | 12.06% | 2.58% | Orthorhombic | About 2 | 3.09 | 13.53 g | Biaxial (−) |
| Pharmacosiderite | KFe3+4(AsO4)3(OH)4·6-7H2O | G, 1813, United Kingdom | Yes | No | 26.28% | 0% | Cubic | 2.5 | 2.797 | 12.85 g | Isotropic; commonly anomalously biaxial |
| Mansfieldite | Al(AsO4)(H2O)2 | G, 1948, USA | Yes | No | 37.10% | 0% | Orthorhombic | 3.5 | 3.03 | 13.40 g | Biaxial (+) |
| Lavendulan | NaCaCu5(AsO4)4Cl(H2O)5 | G, 1837, Czech Republic | Yes | No | 28.22% | 0% | Orthorhombic | 2.5 | 3.54 | 14.35 g | Biaxial (−), nearly uniaxial (−) |
| Strashimirite | Cu4(AsO4)2(OH)2·2.5H2O | A, 1967-025, Bulgaria | Yes | No | 24.52% | 0% | Monoclinic | n.d. | n.d.; calc. 3.67 | 14.55 g (calc.) | Biaxial (−) |
| Clinoclase | Cu3(AsO4)(OH)3 | G, 1830, United Kingdom | Yes | No | 19.69% | 0% | Monoclinic | 2.5 to 3 | 4.38 | 15.43 g | Biaxial (−) |
| Cornubite | Cu5(AsO4)2(OH)4 | A, 1962 s.p., United Kingdom | Yes | No | 22.58% | 0% | Triclinic | About 4 | 4.64 (fibrous) | 15.69 g | Biaxial (−) |
| Connellite (sulphate, no arsenic) | Cu36(SO4)(OH)62Cl8·6H2O | G, 1850, USA | No | Yes | 0% | 0.84% | Hexagonal | 3 | 3.36 to 3.41 | 14.05 to 14.13 g | Uniaxial (+) |
What is chalcophyllite? Copper, aluminium, arsenate, sulphate and water
Chalcophyllite is Cu18Al2(AsO4)4(SO4)3(OH)24·36H2O, on the September 2026 IMA list and on the Handbook of Mineralogy chalcophyllite sheet alike. The list gives status G (grandfathered), 1841, with the United Kingdom as the country of first description. The sheet files it as hexagonal, point group −3 and space group R−3 (overbars read from the rendered sheet), in hexagonal platy crystals to 1 cm, flattened on [0001], with a prominent {01−12} form and sometimes triangular striations on {0001}; also in rosettes, drusy and foliated massive. Twinning on {10−10} as twin plane is called probably universal.
On the sheet. Cleavage {0001} perfect and micaceous, {10−11} in traces; hardness 2; measured density 2.67 to 2.69, calculated 2.684. Emerald-green, grass-green, bluish green or blue, with a pale green to bluish green streak and a vitreous to subadamantine lustre, pearly on the cleavage. Uniaxial (−), with pleochroism O bluish green and E almost colourless. The name is from the Greek for copper and a leaf, for its micaceous look; type material is at the Mining Academy, Freiberg. The sheet prints no type locality.
Its chemistry, worked. On standard atomic weights the formula mass is 3098.34, giving CuO 46.21%, H2O 27.91%, As2O5 14.84%, SO3 7.75% and Al2O3 3.29%, the ideal column on the sheet. The 36 water molecules alone are 20.93% of the mass; the 24 hydroxyls make up the rest of the water figure. The Cornish analysis on the sheet reads As2O5 13.26 and SO3 7.98, close to ideal. Arsenic is 9.67% by mass; arsenic minerals are covered on toxic minerals in a collection, and nothing here is health or safety advice.
Which minerals is chalcophyllite found with? The arsenate–sulphate key
Chalcophyllite sits between two families on its own association line: arsenates and sulphates. Of its 16 listed companions, 7 are arsenates, 6 are sulphates and only parnauite is both.
Method. The 16 names were read from the sheet’s Association field: azurite, malachite, brochantite, chrysocolla, spangolite, connellite, cuprite, cyanotrichite, strashimirite, parnauite, lavendulan, cornubite, langite, clinoclase, pharmacosiderite and mansfieldite. A script looked each up on the September 2026 IMA list and tested its formula for (AsO4) and (SO4). Arsenates: strashimirite, parnauite, lavendulan, cornubite, clinoclase, pharmacosiderite, mansfieldite. Sulphates: brochantite, spangolite, connellite, cyanotrichite, parnauite, langite. The rest are carbonates (azurite, malachite), a silicate (chrysocolla) and an oxide (cuprite).
What the key separates. Chalcophyllite has four arsenate groups to three sulphates; parnauite two to one. On their sheets chalcophyllite forms hexagonal micaceous plates and is uniaxial, parnauite lathlike crystals to 4 mm in bowties, rosettes and fans, orthorhombic and biaxial. Worked as 20 − 20/D, chalcophyllite reads 12.51 to 12.57 g in water for 20.00 g and parnauite 13.53 g. Connellite, a sulphate without arsenic, is azure-blue, acicular and reads 14.05 to 14.13 g. The method is on how to measure specific gravity at home.
Does chalcophyllite lose water? The 1.618 trap
The sheet records refractive indices for dehydrated chalcophyllite as well as for fresh material: ω 1.618 and ε 1.552 fresh, ω 1.680 and ε 1.618 dehydrated. It gives no rate, temperature or humidity for the change, and this page adds none. The birefringence barely moves, 0.066 fresh against 0.062 dehydrated; both readings shift up by 0.062 to 0.066.
Documented failure. The fresh ω and the dehydrated ε are the same number, 1.618. A refractometer showing one shadow edge at 1.618 cannot say which state the plate is in. The second edge decides it: 1.552 for fresh material, 1.680 for dehydrated. Questions about keeping hydrated arsenates belong on toxic minerals in a collection for the hazard side; this page gives no storage, cleaning or drying advice.
How do you tell chalcophyllite from liroconite?
The leaf is the test: chalcophyllite cleaves into thin pearly plates on {0001}; liroconite forms lenticular crystals with indistinct cleavage. Both are copper aluminium arsenates from Wheals Gorland, Muttrell and Unity and the Ting Tang mine at Gwennap on their sheets, and both list bluish green and emerald-green. Chalcophyllite is uniaxial and carries sulphate; liroconite is biaxial and carries none. The weight comparison between the two, 12.51 to 12.57 g against 13.20 to 13.36 g, sits within the liroconite page’s own line and is not repeated as an asset here.
Where chalcophyllite is found
The Distribution field names, in Germany, the Altväter samt Eschig mine near Sayda and Schmiedeberg, Saxony; the Virneberg mine near Rheinbreitbach; and the Clara mine near Oberwolfach, Black Forest. Schwaz, Tirol, Austria. Špania Dolina (Herrengrund), Slovakia. Moldava, Czech Republic. Nizhni Tagil, Urals, Russia. In England, large crystals from Wheals Gorland, Muttrell and Unity and the Ting-Tang mine, Gwennap, and a number of other places in Cornwall, and the Old Potts Gill mine, Caldbeck Fells, Cumbria. The Cap Garonne mine, Var, and the Salsigne mine, Aude, France. In the USA, the Calumet and Arizona mine, Bisbee; the Majuba Hill mine, Nevada; and the Mammoth and Ajax mines, Tintic district, Utah. Fine examples from the El Teniente mine, O’Higgins Province, Chile, and the Maharahara copper mine, New Zealand.
Occurrence and type material. The sheet calls it a rare secondary mineral in the oxidized zone of some arsenic-bearing hydrothermal polymetallic deposits. Its X-ray pattern is from El Teniente; type material is at Freiberg. The rest of the reference section is at the species index. We hold no catalogue and nothing here is offered for sale; the wanted list is the only commercial route.