Goethite after Gypsum

specimen number:
2133429

location:
Naica, Chihuahua, Mexico

description:
A crystal grouping of what was once gypsum, this now replaced by goethite. Sharply formed and in great condition.

The Naica mining district, located in Saucillo Municipality, Chihuahua, Mexico, is a world-renowned lead, zinc, and silver deposit famous for its giant selenite gypsum crystals. Geologically, the deposit is a carbonate replacement and skarn-type system hosted within thick sequences of Cretaceous Aurora Limestone. Mineralization was driven by Tertiary-aged dacitic intrusions that channeled high-temperature, metal-rich hydrothermal fluids through localized fault systems and fractures, dissolving the limestone and replacing it with massive base metal sulfides, while cooler, calcium-sulfate-rich waters later filled deep structural cavities to form enormous gypsum crystals. Commercial underground mining operations began around 1794 following the discovery of surface silver veins, expanding significantly in 1900 with the establishment of the Compania Minera de Naica. Large-scale corporate operations, managed for decades by Grupo Penoles, processed thousands of tons of ore daily until 2015, when a catastrophic underground flooding event breached a major aquifer, forcing an indefinite suspension of primary ore extraction and mine closure.

In addition to its well-known cave crystals, Naica has produced notable pseudomorphs, particularly goethite after gypsum. These specimens formed when late-stage, iron-rich fluids completely replaced the original prismatic gypsum crystals, leaving behind dark brown to black iron oxide structures or hollow epimorph casts that perfectly preserve the distinct monoclinic shape and habit of the original selenite.

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