Malachite after Azurite

specimen number:
2063080

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

description:
Classic thumbnail specimen for this locality, this a cluster of what were once large bladed or blocky crystals of azurite, these now replaced by velvety green crystallized malachite. A second-generation of blue azurite crystals is present. In good condition.

The New Cornelia Mine in Ajo, Arizona, is a massive porphyry copper deposit hosted within a quartz monzonite stock and surrounding volcanic rocks, featuring significant secondary enrichment zones. Small scale Spanish and Mexican prospecting began around 1750, but modern industrial open pit mining took shape in the early 20th century under the New Cornelia Copper Company and later Phelps Dodge, operating continuously until production ended in the 1980s. Mineral collectors historically prized the locality for its rich variety of secondary copper specimens, most notably beautiful blue ajoite crystals, shattuckite, and malachite.

A pseudomorph occurs when one mineral replaces another while preserving the original crystal shape and form of the first mineral. In the specific case of malachite after azurite, the copper carbonate mineral azurite is gradually altered and replaced by malachite, another copper carbonate, due to changing chemical conditions in the oxidation zones of copper deposits. This chemical transformation retains the distinct crystal habit and sharp geometric forms of the original azurite crystals, but completely changes their classic deep blue color into a vibrant emerald green. Famous localities for this classic specimen type include Chessy-les-Mines in France, Tsumeb in Namibia, and various copper districts in the southwestern United States.

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