Malachite after Azurite

specimen number:
2242089

location:
Sacramento Mine, Bisbee, Cochise Co., Arizona, USA

description:
Sharp crystals of what were once azurite to 1.0 cm in size, these now replaced by green crystalline malachite, very nice across the top of matrix. In super condition. Classic material for this locality. This was likely collected in the 1920s are slightly earlier.

The Sacramento Mine, located in the Warren Mining District of Bisbee, Arizona, was a cornerstone of the Copper Queen Consolidated Mining Company's operations. Geologically, the mine tapped into massive sulfide and oxide ore bodies formed by the replacement of Paleozoic limestones surrounding the Sacramento Hill porphyry stock. The site is famous for producing high-grade copper minerals, including brilliant blue azurite and rich green malachite, which often occurred in large, vuggy pockets within the oxidized zones. Historically, the Sacramento shaft was sunk in the early 1900s to serve as the primary hoisting and central terminal for the district's underground rail system. Mining and quarrying reached a turning point in 1917 when the top of Sacramento Hill began to be removed for open-pit mining, eventually creating a precursor to the massive Lavender Pit. The underground shaft continued to operate as a critical infrastructure point for ventilation and personnel until the mid-20th century.

A pseudomorph is a mineralogical phenomenon where one mineral replaces another while perfectly preserving the external shape or crystal form of the original species. In the case of malachite after azurite, this process occurs through chemical alteration where the blue azurite (a copper carbonate) becomes unstable due to changes in groundwater chemistry or environmental conditions, gradually transforming into the more stable green malachite. Because both minerals share a similar chemical composition, the green malachite can slowly replace the azurite molecule by molecule, resulting in a specimen that possesses the deep green color and velvety texture of malachite but retains the distinct, sharp monoclinic crystal habits originally formed by the azurite. Historically, these specimens have been highly sought after at classic localities such as Bisbee, Arizona, and Tsumeb, Namibia, where the oxidation zones of copper deposits provided the perfect conditions for this transition.

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