Azurite altering to Malachite (repaired)

specimen number:
8282055

location:
Chessy-les-Mines, Rhone, Rhone-Alpes, France

description:

Incredibly good for this old locality, large terminated sharp crystals of azurite measuring to 3.5 cm in height or coming off of a thin matrix. These crystals are in the process of being replaced by green malachite. The large crystal has been repaired as shown in the video. In super condition.

The Chessy-les-Mines deposit, located northwest of Lyon in the Rhone department of France, is a classic European mining locality famous for its rich copper minerals. Geologically, it is a volcanogenic massive sulfide (VMS) deposit hosted within Lower Devonian volcanic and sedimentary sequences, where primary copper-iron sulfides underwent intense secondary weathering and oxidation. This chemical alteration created a world-renowned oxidized zone rich in cuprite, malachite, and the deep blue copper carbonate azurite, which typically formed sharp, distinct crystals within a soft, yellowish clay matrix. Organized mining and underground quarrying for copper at Chessy dates back to medieval times, with major industrial operations expanding in the mid-1700s and reaching peak production during the early 1800s. Commercial extraction of copper ore and the associated by-product of sulfuric acid ceased around 1877, leaving the locality closed for over a century, though the site remains celebrated in historic collections for producing some of the finest azurite crystals ever discovered.

Malachite after azurite pseudomorphs represent a fascinating mineralogical process where one mineral completely replaces another while perfectly preserving the original crystal habit. Geologically, this chemical transformation occurs within the upper oxidized zones of copper deposits, where changing environmental conditions, such as increased moisture or shifting acidity levels, alter the stability of the primary mineral. Because azurite and malachite are closely related copper carbonates, a subtle chemical change causes the azurite crystal lattice to alter into the more stable molecular structure of malachite. This slow, atom-by-atom replacement typically progresses from the exterior surface inward, resulting in a solid specimen that possesses the exact, sharp tabular or prismatic crystal shape of the original azurite, but consists entirely of fibrous, vibrant green malachite.

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