Cumengeite - Encyclopedia

Class : Halides
Subclass : Oxychlorides
Crystal system : Tetragonal
Chemistry : Pb₂₁Cu₂₀Cl₄₂(OH)₄₀
Rarity : Very rare

Cumengeite is a very rare secondary mineral in copper deposits, typical of arid climates in a chlorinated environment (evaporite lagoons and marine coastlines); it therefore shares the deposits of boleite and pseudoboleite. It is also found in the slag of foundries submerged at sea. It was named in honor of Edouard Cumenge, a French mining engineer at Boléo where the first specimens were discovered. It appears in octahedral or cubo-octahedral crystals, of a magnificent indigo blue hue, often forming epitaxial aggregates with boleite : each face of the boleite cube supports a tetragonal pyramid of cumengeite giving very original complex buildings which evoke twins with six crystals. It is a mineral highly sought after by collectors.

Main photo : Cumengeite from Amelia Mine, Boleo, Mexico © Kiyoshi Kiikuni

Cumengeite from Cap Garonne, Var, France © Jean-Marie Claude

Cumengeite epitaxy on boleite from Amelia Mine, Boleo, Mexico © John Rakovan

Cumengeite from Lavrion, Greece © Pascale et Daniel Journet

Cumengeite from Penpol, Cornwall, England © Pascal Chollet

Cumengeite in the World

The most spectacular cumengeite epitaxies reach 3.5 cm (8 cm according to some sources). They were discovered at the Amelia mine, El Boleo district (Mexico). Isolated 4 mm crystals come from the copper deposits of English Cornwall. Cumengeite is also known in the Laurion slag (Greece) and in several other copper deposits.

Cumengeite in France

In France, cumengeite is reported at Cap Garonne (Var).

Twinning

No known twin but a very unique epitaxy on boleite.

Fakes and treatments

No fakes listed for this mineral species.

Hardness : 2.5
Density : 4.65
Fracture : Irregular
Streak : Blue

TP : Translucent
RI : 1.926 to 2.041
Birefringence : 0.100
Optical character : Uniaxial -
Pleochroism : Visible
Fluorescence : None

Solubility : Nitric acid

Magnetism : None
Radioactivity : None