Malachite after Azurite

specimen number:
8311259

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

description:

A grouping of prismatic crystals of what were once azurite, these now replaced by green malachite. Little matrix attached, in very good condition. A very choice thumbnail from this classic locality.

The Sacramento Mine, located in Bisbee, Cochise County, Arizona, was a cornerstone of the Warren Mining District and played a pivotal role in the transition to large-scale open-pit mining. Geologically, the deposit was part of a massive porphyry copper system where mineral-rich fluids associated with the Jurassic-aged Sacramento Stock intruded and replaced Paleozoic limestones, specifically the Escabrosa and Martin formations. This created vast bodies of high-grade copper sulfides and spectacular secondary carbonates in the oxidation zone. Historically, the Sacramento Hill was initially worked through underground methods starting in the late 1800s, but major open-pit operations commenced in 1917 under the Phelps Dodge Corporation. The pit was a primary producer for decades until its ores were largely exhausted, leading to its eventual incorporation into the larger Lavender Pit operation which continued until 1974. Today, the site remains a classic locality for its complex mineralogy and its historical significance in the American West copper boom.

In mineralogy, a pseudomorph occurs when one mineral replaces another while preserving the original crystal shape (the name literally translates to "false form"). At Bisbee, this process is famously illustrated by malachite after azurite, where changing chemical conditions in the oxidation zone caused unstable blue azurite crystals to be replaced by stable green malachite. Because the chemical transition happens slowly, the resulting specimen retains the sharp, monoclinic crystal habit of the original azurite but is composed entirely of fibrous or velvety green malachite. These "after" specimens are highly prized by collectors for their sharp geometry and the vivid contrast they provide between the form of one mineral and the chemistry of another, with the Sacramento Mine being a prolific source of these geological transformations.

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