Chalcoalumite - Encyclopedia
Class : Sulfates, chromates, molybdates
Subclass : Hydrated sulfates
Crystal system : Monoclinic
Chemistry : CuAl4SO4(OH)12 3H2O
Rarity : Rare to uncommon
Chalcoalumite is a hydrated sulfate of copper and aluminum. It is a secondary mineral of the oxidation zones of copper deposits. It owes its name to its chemical composition : from the Greek khalcos (copper) and the Latin alumen (alum). It essentially forms fibrous botryoidal encrustations, sometimes small triangular, finely lamellar crystals grouped into small spherules, of a pale bluish green to blue or blue-gray color. It transforms into copper-bearing gibbsite by dehydration.
Main photo : Chalcoalumite from Holbrook Mine, Arizona, USA © Rob Lavinsky
Chalcoaluminite in the World
Twinning and special crystallizations
A twin is known to have a re-entrant angle, but is difficult to identify.
Fakes and treatments
No fakes identified for this mineral species, confusion with chrysocolla or gibbsite may occur.
Hardness : 2.5
Density : 2.29
Fracture : Irregular
Streak : White
TP : Translucent to transparent
RI : 1.520 to 1.530
Birefringence : 0.010
Optical character : Biaxial +
Pleochroism : Low
Fluorescence : None
Solubility : Nitric acid and hydrochloric acid
Magnetism : None
Radioactivity : None