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Chalcophyllite

Cu18Al2(AsO4)4(SO4)3(OH)24·36H2O

Locality:

Madonna di Fucinaia (Madonna della Fucinaia) slag heaps, Campiglia Marittima, Campigliese, Livorno Province, Tuscany, Italy

1.31 mm group of Chalcophyllite crystals.

Photo Matteo Chinellato

Chalcophyllite, which is named from the Greek words for copper and leaf, is a rare secondary mineral that forms in the oxidation zone of copper ore deposits. It is a great collection mineral because it has a high luster, an attractive blue-green to green color and it forms nice tabular six-sided crystals that can be arranged into rosettes. 

Chalcophyllite is unusual in that it has three different anion groups in its chemistry. Most minerals have just one principle anion group with possibly some hydroxides or halides along for the ride. Classifying those minerals is relatively easy as they are generally classed by their most complex or highest electronegative anion group. In the case of chalcophyllite, it has the same number of arsenate anion groups as sulfate anion groups.  Another unusual aspect of chalcophyllite’s chemistry is its large amount of hydroxides and water molecules. Half of this mineral is either water or hydroxide.