Overview
Chalcophyllite is a strikingly coloured secondary copper mineral, prized by collectors for its emerald- to blue-green platy crystals that flash with a pearly to vitreous lustre. Its name comes from the Greek words for “copper” and “leaf,” a direct nod to both its copper content and the thin, micaceous plates in which it commonly grows. A classic species of oxidised copper deposits, chalcophyllite is best known from the historic mines of Cornwall, England, where fine examples have been treasured for well over two centuries.
Composition & structure
Chalcophyllite is a hydrated copper-aluminium arsenate-sulfate hydroxide, one of the more chemically complex secondary copper minerals. It carries a large amount of water in its structure, and the exact formula varies slightly with humidity as that water is gained or lost. It is commonly written as Cu18Al2(AsO4)3(SO4)3(OH)27·36H2O, with an alternative arsenate-rich variant also recognised. The mineral is trigonal, and its layered atomic structure gives rise to the characteristic tabular, leaf-like crystals and the perfect basal cleavage that lets them split into thin flakes.
| Formula | Cu18Al2(AsO4)3(SO4)3(OH)27·36H2O |
| Crystal system | Trigonal |
| Mohs hardness | 2 |
| Lustre | Vitreous to subadamantine, pearly on cleavage faces |
| Colour | Emerald-green, grass-green to blue-green |
| Type locality | Germany (originally described from German material) |
Formation & occurrence
Chalcophyllite is an uncommon secondary mineral that forms in the oxidised zones of arsenic-bearing copper deposits. There, descending oxygen- and water-rich solutions attack primary copper-arsenic ores, liberating copper, arsenic, aluminium and sulfate that recombine to crystallise chalcophyllite on fracture surfaces and in cavities. Because its formation depends on this particular weathering chemistry, it is generally a minor associate rather than a major ore mineral, found alongside other oxidation-zone species such as malachite, azurite, brochantite, olivenite and cuprite.
Identification & similar species
The combination of vivid green colour, hexagonal tabular or foliated habit, very soft feel (hardness only about 2) and perfect micaceous cleavage is highly diagnostic. The plates are flexible and split into thin pearly leaves, a behaviour that sets chalcophyllite apart from harder green copper minerals. It can be confused with spangolite, tabular brochantite or other green secondary species, but these differ in habit, cleavage and chemistry. The pale green streak and association with arsenate-bearing oxidation zones help confirm an identification.
Notable localities & collecting
Cornwall, England is the most celebrated source, with Wheal Gorland and other mines of the St Day and Gwennap districts producing the classic emerald-green plates that define the species for most collectors. Excellent material has also come from the Majuba Hill mine in Nevada and from localities in Idaho in the United States, as well as from copper deposits in Germany, where the mineral was first described. Fine chalcophyllite is genuinely scarce: its softness and water content make it fragile and sensitive to drying, so well-preserved crystallised specimens command real attention. Collectors should store it away from heat and very dry air to keep the lustrous green plates intact.