Malachite after Azurite

specimen number:
8282077

location:
New Cornelia mine, Ajo, Pima Co., Arizona, USA

description:

A superb example for this Arizona classic, large blocky crystals of what were once azurite measuring to 1.6 cm on edge, these now replaced by green malachite, and scattered thickly across the top of a botryoidal velvety malachite matrix. In excellent condition. The specimen comes with a whole suite of previous collection labels as shown at the end of the video. The specimen comes with a custom acrylic display base.

The New Cornelia mine, located at Ajo in Pima County, Arizona, is a historic open-pit copper operation that holds distinction as the first large-scale open-pit mine in the state. Geologically, the deposit is a classic porphyry copper system hosted within the Early Eocene Cornelia quartz monzonite stock, which underwent extensive supergene enrichment and surface weathering to form a thick, carbonate-rich oxidized zone overlying lower-grade primary copper sulfides. While small-scale surface mining by native peoples and early Spanish prospectors dates back to the 1750s, systematic industrial development began around 1911 under the Calumet and Arizona Mining Company. Full-scale open-pit mining using steam shovels officially commenced in 1916 and 1917, and operations continued under the ownership of the Phelps Dodge Corporation from 1931 until the mine was ultimately shut down in 1983 due to depressed copper prices and labor disputes.

A pseudomorph is a mineral specimen that has chemically replaced another mineral atom-for-atom, completely altering its internal composition while perfectly preserving the external crystal habit and shape of the precursor species. In the case of malachite after azurite, changes in local groundwater chemistry, acidity, or carbon dioxide levels cause the chemically unstable blue azurite to alter into the more stable green copper carbonate, malachite. This gradual chemical substitution leaves a specimen that retains the distinct, sharp, bladed or prismatic crystal forms typical of azurite, but is composed entirely of a rich green malachite. The New Cornelia mine is highly regarded by collectors for producing excellent examples of these green pseudomorphs, which were recovered in abundance during expansions into the upper oxidized benches from the 1950s through the 1970s.

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