Azurite with Arsentsumebite after Mimetite

specimen number:
2262237

location:
Tsumeb Mine, Otjikoto, Namibia

description:

Lustrous blue bladed crystals of azurite 1.0 cm in size on a matrix of what were once prismatic crystals of mimetite, these now replaced by arsentsumebite. Colorful combination piece good condition with just very minimal micro-damage to some of the pseudomorph crystals on the matrix.

The Tsumeb mine, situated in the Oshikoto Region of northern Namibia, is universally regarded by mineralogists as the most celebrated and mineralogically diverse polymetallic deposit on Earth, famous for producing over three hundred distinct mineral species. Geologically, this world-class deposit is hosted within a massive, steeply dipping pipe-like structure that cuts vertically through a thick sequence of Neoproterozoic-age dolostones and limestones belonging to the Otavi Group. This unique pipe structure, filled with complexly fractured feldspathic sandstone, acted as a primary conduit for deeply circulating hydrothermal fluids that deposited immense volumes of copper, lead, zinc, silver, germanium, and arsenic sulfides. Over hundreds of millions of years, deep groundwater circulation subjected these primary sulfides to intense, multi-stage supergene oxidization, creating three distinct, deeply buried oxidation zones that produced unparalleled quantities of spectacular, highly colorful secondary arsenate, carbonate, and silicate minerals. Historically, local tribes mined surface outcroppings for copper long before European arrival, but formal corporate underground mining began in 1897 under the South West Africa Company and later the Otavi Minen- und Eisenbahn-Gesellschaft (OMEG). Large-scale corporate operations ran continuously under various owners, most notably the Tsumeb Corporation Limited (TCL), through the 20th century until declining ore grades, labor strikes, and deep flooding forced the permanent closure of all underground mining operations in 1996.

Among the many extraordinary mineralogical phenomena discovered during these decades of deep underground extraction, the Tsumeb mine produced some of the world's most sought-after pseudomorphs, including the exceedingly rare replacement of mimetite by arsentsumebite. Geologically, this specific type of pseudomorphism occurred when primary lead-arsenic fluids first crystallized into sharp, elongated hexagonal prisms or barrel-shaped crystals of mimetite within the deeply oxidized zones of the dolostone pipe. At a later stage, changing geochemical conditions introduced highly specialized copper- and sulfate-rich secondary solutions that reacted directly with the pre-existing mimetite crystals. Over time, these solutions slowly dissolved the lead arsenate molecule by molecule, simultaneously replacing it with the rare, hydrated lead copper arsenate sulfate mineral, arsentsumebite, while perfectly preserving the original hexagonal morphology and crystalline outline of the initial mimetite prism. Historically, these striking specimens were found in very limited pockets during the mid-20th century mining of the deeper second oxidation zone, appearing as vibrant, velvety, apple-green to dark emerald-green crusts and pseudomorphic crystals that stood in stark contrast to the surrounding dark gossan matrix.

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