Calcite with Duftite (#57)
|Africa||Tsumeb, Namibia||10 ½" x 8" x 2 ½"|
Calcite, CaCO3, is featured in five specimens in this display. It is often colorless, but also occurs in yellow, brown, green, blue, red, and some other colors. Calcite is allochromatic, meaning that its color depends on and is caused by something other than its essential chemical elements. Minerals that owe their color to particular, specific chemical elements that are always present are called idiochromatic minerals. All specimens of an idiochromatic mineral have essentially the same color, although there can be differences in shade, depending on how much chemical variation occurs in that mineral. Duflite is an idiochromatic mineral, green or olive green to grey green.
The chemical formula of duftite is PbCu(AsO4)(OH), where Pb is lead, Cu is copper, and As is arsenic. The green color is due to the presence of copper. At Tsumeb, duftite in association with calcite either coats the outer surfaces of the calcite crystals, or else it coated them during stages in the growth of the calcite and now appear as green "phantoms" or "ghosts" beneath the final crystal surfaces.
The Tsumeb Mine was remarkable. In nearly a century, it produced over $5 billion in lead, zinc, copper, germanium, and other economically important metals. In addition, over 250 different mineral species were identified in that mine, 40 of which occur in no other known locality. Some of the finest mineral specimens in the world have come from Tsumeb. After depletion of the economic ore deposit and closure of the mine in 1996, it came under new ownership and was renamed the Ongopolo Mine. It now produces collectible mineral specimens from the upper levels of the old mine.