Cuproadamite on Limonite (#82)

Botryoidal glossy black limonite stalactites in an aesthetic grouping with scattered lustrous translucent to opaque lime green ballsand fans of cuproadamite to 3/8" maximum dimension.
North America Gold Hill, Tooele County, Utah 3 ¼" x 2 ¼" x 1 ½"

 

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Limonite is an amorphous mineral, meaning that it has no crystal structure - that is, the atoms are not arranged in any regular, repeating way. Its formula is generally written FeO(OH) · nH2O (where n represents any number), but in fact, the O:OH ratio and degree of hydration are variable. All this sounds contrary to the definition of a mineral, and it is. Limonite has been known and used as a pigment and ore of iron for so long (mined since the beginning of the Iron Age), however, that it is accepted as a mineral, despite its variable composition and lack of crystalline structure. Most limonite consists, at least in part, of a mixture of very fine iron oxides that, themselves, are other known minerals. Specimen #33, chrysocolla, is another amorphous mineral.

If you guessed that "cupro" indicates copper, you are correct. Adamite is Zn2AsO.(OH). Cuproadamite, or copper-bearing adamite, is Zn2AsO4(OH) with some Cu substituting for Zn. Another mineral, olivenite, is Cu2AsO4(OH), and ZnCuAsO4(OH) is zincolivenite. So whereas there is no specific amount of copper that defines cuprian adamite, the Zn:Cu ratio must be substantially greater than 1 :1 - that is, Cu must be at the level of a minor element or impurity. It does not take a large concentration of a chromophore to cause color, though. Pure Zn2AsO4(OH) would be colorless, but a small amount of Cu causes the pale green to blue-green color of this specimen. The "ball" is formed by the ends of closely spaced radiating crystals.