Chalcophyllite (#85)

Bright turquoise colored flat lathes to 3/16" formed in a fracture that has been broken and exposed.
North America Mammoth Mine, Tintic Distric, Juab County, Utah 3" x 1 ¾" x ¼"


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The chemical formula for chalcophyllite looks complicated: Cu18Al2(SO4)3(AsO4)3(OH)27 · 33H2O. It is an uncommon secondary mineral, meaning that it is an alteration product of some other minerals - in this case, of arsenic-bearing copper minerals in a sulfide ore zone. It forms in the oxidized zone of the ore deposit, which is a region above the water table where surface and/or ground water and dissolved chemicals percolate through the rocks, resulting in chemical reactions that alter the ore minerals.

The name of this mineral, like those of many others, is of Greek derivation: "chalco" comes from the Greek word for copper, and "phyll" from the Greek for leaf, an allusion to the platy "leaf-like" crystal habit. Perhaps if you look closely you can make out the leaf-like crystals in this specimen.