Goethite after Gypsum

specimen number:
8701003

location:
Santa Eulalia, Chihuahua, Mexico

description:
Classic Mexican specimen with lustrous black goethite, this forming a replacement after a previous a now gone crystal of gypsum. In great condition. The hollow nature of the cast is easily seen in the video. There are no points of attachment.

The Santa Eulalia mining district in Chihuahua, Mexico, is famous for its massive silver-lead-zinc carbonate replacement deposits hosted within Cretaceous limestones. Geologically, the district is comprised of vast chimneys and mantos where primary sulfides were subjected to extensive oxidation, creating a diverse suite of secondary minerals. A significant mineralogical feature of the site is the occurrence of goethite casts after gypsum, specifically replacing large, individual selenite crystals or complex clusters. These pseudomorphs form when iron-rich solutions deposit goethite as a crust or total replacement over the gypsum, preserving the original monoclinic crystal forms, such as long blades or swallowtail twins, in a dark brown or metallic-looking iron oxide. Historically, mining and quarrying began in the early 1700s and was so productive that it led to the founding of Chihuahua City. Major operations like the Potosi and San Antonio mines flourished throughout the 20th century, producing immense quantities of ore. While many mines have transitioned to intermittent production, the district remains a premier locality for studying oxidation zones and collecting world-class pseudomorphs.

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