Malachite after Azurite

specimen number:
8311385

location:
Milpillas Mine, Sonora, Mexico

description:
Bladed crystals of what were once azurite to 3.0 cm in size across the top of matrix, these now replaced by attractive chatoyant green crystallized malachite. This is in terrific condition.

The Milpillas Mine, situated in the Cuitaca District of Sonora, Mexico, is a modern copper operation that has become legendary for its world-class secondary mineral specimens. Geologically, it is a high-grade porphyry copper deposit where intense supergene enrichment occurred, as surface waters leached copper from the upper layers and concentrated it in a rich oxidation zone below. This process resulted in spectacular pockets of carbonates and silicates hosted within a weathered volcanic porphyry. Historically, the mine was developed by Industrias Penoles and began commercial production in 2006. While it remains a significant industrial site for copper cathode production, its legacy in the mineralogical community was cemented by the discovery of extraordinary azurite, malachite, and brochantite between 2006 and the mid-2010s. The mine is still active today, though the most prolific period for collectible mineral pockets has largely passed, leaving behind specimens that are considered benchmarks for their respective species.

A pseudomorph is a mineral that appears in the external crystal form of another mineral species, essentially acting as a "false form." This geological phenomenon occurs when one mineral replaces another through a chemical process while preserving the original crystal shape. One of the most iconic examples of this is malachite after azurite, which occurs frequently in the oxidation zones of copper deposits like Milpillas or Bisbee. As environmental conditions change, such as an increase in moisture or a decrease in carbon dioxide, the chemically unstable blue azurite (a copper carbonate) is replaced by the more stable green malachite. The result is a specimen that possesses the deep green color and silky luster of malachite but retains the sharp, monoclinic blade or tablet shapes characteristic of the original azurite. These specimens are highly valued by collectors because they provide a frozen snapshot of the chemical evolution occurring within an ore body.

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