DIOPTASE WITH MIMETITE & DUFTITE
ABOUT THE SPECIMEN
This large specimen comes out of one of the world's most renowned dioptase producing locales, the Mindouli Region, from the now closed N'tola Mine. There are at least five distinct minerals on this one piece: dioptase, mimetite, duftite, calcite, and dolomite. There appears to be some wulfenite crystals in the assemblage, though we never got the piece analyzed. The mimetite is crystallized in hexagonal bipyramids and dotted along both sides; mimetite and dioptase rarely grow alongside one another. One side shows a large group of small dioptase and mimetite crystals interspersed above patches of botryoidal duftite, complete with a unique bridged arch of dioptase. On the other side, larger dioptase crystals contrast gorgeously against a backdrop of light dolomite. The dioptase ranges in color from the typical emerald green to a bright turquoise color. Throughout the piece one can see cavities in the rock with crystals growing inside and the whole specimen is like its own world. One can easily get lost for hours looking at this specimen through a loupe!
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Mindouli sits along Route Nationale 1 in western Congo-Brazzaville, where Pb-Zn-Cu mineralization is hosted in silicified Neoproterozoic limestones of the West Congolian Supergroup cropping out over roughly 3 km along a limestone escarpment - deposits worked by French colonial operators from 1948 to 1960 and documented firsthand by Demetrius Pohl on Mindat. Dioptase crystallized secondarily in the oxidized copper zone as copper silicate hydroxide, with divalent copper in the cyclosilicate structure producing the saturated emerald-green color; mimetite formed concurrently from lead-arsenate fluids in the same cavities, precipitating as short yellow to orange hexagonal prisms. Both species grew on a matrix of glistening drusy dolomite over a silicified brick-red host rock, and the combination of green dioptase clusters against orange mimetite on white dolomite - with duftite occasionally present as a third phase - produces a trichromatic specimen with no close parallel at other dioptase localities. The N'tola Mine closed around 2019, fixing all available material to existing stock. Crystals are typically small, rarely exceeding 1 cm for dioptase, but the intensity of color and three-species associations on a single matrix piece elevate the material well beyond what size alone would suggest.