Malachite after Azurite (O-0008)

specimen number:
9273153

location:
El Cobre Mine, Zacatecas, Mexico

description:

A superb miniature for this Mexican classic with bladed crystals of what were once azurite to 1.0 cm in size across the top of the malachite matrix, these crystals then replaced by velvety green crystallized malachite. In super condition.

The El Cobre mine, located in the Concepcion del Oro Municipality of Zacatecas, Mexico, is a notable underground copper producer characterized by polymetallic mineralization. Geologically, the deposit consists of a classic contact-metamorphic skarn system formed where a Tertiary granodiorite stock intruded into Mesozoic sedimentary formations, specifically the upper Jurassic Zuloaga Limestone and Cretaceous Cupido Limestone. This intrusion generated high-temperature hydrothermal fluids that replaced the surrounding carbonate rocks, depositing rich zones of chalcopyrite, pyrite, and magnetite, while subsequent supergene weathering formed secondary copper zones near the surface. Underground operations at the property were initiated on a commercial scale in the late nineteenth century and expanded under corporate entities like the Mazapil Copper Company, which developed extensive shafts and tramways to feed regional smelters. The mine operated through various phases of modernization until falling copper grades and economic factors led to the cessation of primary industrial extraction in the mid-twentieth century.

Pseudomorphs represent a distinct mineralogical occurrence where a mineral replaces or alters an existing crystal while completely retaining the original external crystal form. In the specific case of malachite after azurite, the transformation involves a chemical alteration between two copper carbonate minerals with different structural symmetries and water contents. The process begins with a primary crystal of azurite, a monoclinic copper carbonate that typically forms sharp, tabular, or prismatic crystals. When the chemical composition of the surrounding groundwater changes, typically through a decrease in carbon dioxide or an increase in moisture, the azurite becomes chemically unstable. The fluid gradually leaches carbon dioxide from the crystal lattice while introducing water, converting the compound into malachite, a monoclinic copper carbonate with a higher hydroxyl content, molecule by molecule. The resulting specimen preserves the precise, sharp crystal angles of the original azurite, but it is composed entirely of fibrous, deep green malachite.

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