Amarantite

specimen number:
2133332

location:
La Compania Mine, Sierra Gorda, Antofagasta, Chile

description:
Red masses throughout matrix. Good example for the species.

The La Compania mine, situated just east of the village of Sierra Gorda in the Antofagasta Region of northern Chile, is a classic historic underground operation located within one of the most hyper-arid zones of the Atacama Desert. Geologically, the deposit sits inside a prolific Paleocene-aged metallogenic belt, where mineralization is characterized by heavily oxidized porphyry copper and silver vein systems hosted within Mesozoic volcanic and sedimentary sequences. The extreme aridity of the environment permitted intense supergene alteration, transforming primary iron and copper sulfides into a remarkable suite of rare, water-soluble sulfate minerals. Historically, the mine was worked primarily during the late 19th and early 20th centuries, with its peak operational era spanning from the late 1880s through the 1920s as miners extracted high-grade silver and copper ores. While large-scale industrial focus in the district has since shifted to massive modern open-pit copper-molybdenum operations nearby, the abandoned underground workings of La Compania remain highly significant to mineralogists as an important locality for exotic secondary species.

Amarantite is a rare secondary iron sulfate mineral that forms almost exclusively in extremely arid, desert environments through the oxidation of primary iron-bearing sulfide minerals like pyrite. Crystallizing in the triclinic system, it typically occurs as distinct, radiating or matted aggregates of tiny, bladed crystals and needles that exhibit a highly characteristic amaranth-red, brownish-red, or deep orange-red coloration. The mineral features perfect cleavage along two planes, a relatively low Mohs hardness of 2.5, and a distinct lemon-yellow streak when scratched. Because amarantite is highly water-soluble, it can decompose easily when exposed to moisture or liquid water, making its long-term preservation dependent on the absolute dryness of its environment. It is frequently found in close geochemical association with other rare secondary desert minerals such as hohmannite, fibroferrite, copiapite, coquimbite, and chalcanthite.

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