MALACHITE AFTER AZURITE

ABOUT THE SPECIMEN Nice group of former azurite crystals that have been replaced by malachite. There are a number of well-defined points, some of them bunched together in a cute little cluster. The piece still has white matrix attached to it, and there’s some azurite that hasn’t been replaced, still clearly visible. One of the crystals has a break on the side that reveals a fantastic cross section of radiating malachite!   VIDEO   MORE INFO Milpillas is a porphyry copper deposit within the Cananea district of Sonora, discovered in 1978 but not brought into production until 2006 due to low copper prices and difficult terrain - nobody anticipated that its oxidized carbonate zones would yield what became the most significant azurite find in history. Seven such zones were intersected during development, and from 2007 onward one major pocket followed another through 2015, when the mine transitioned into sulfide-dominant ore and specimen production ended abruptly. Pseudomorphic replacement of azurite by malachite occurs because malachite is the thermodynamically stable copper carbonate under most near-surface conditions; rising pH and shifting carbonate equilibria drive the conversion while the original azurite crystal form is preserved as a template. Milpillas pseudomorphs retain sharp prismatic to tabular azurite habits in dark forest-green velvety malachite, sometimes with remnant azurite cores visible in cross-section, and plates reaching 25 cm across were documented. A particularly coveted subset shows a secondary generation of thin azurite overgrowing the malachite surface - the translucent azurite veneer acting as a partial mirror that gives a distinctive electric-blue chatoyancy not produced by azurite grown directly on matrix. Associates including plancheite, brochantite, and volborthite appear on individual specimens. With the oxidized zones now exhausted and no comparable replacement find documented in the district, all pseudomorph material is fixed to what was recovered during the decade-long bonanza.