Overview
Aguilarite is a rare silver selenide-sulfide, a genuine collector’s curiosity from the great silver district of Guanajuato, Mexico. A soft, heavy, metallic mineral, it forms lead-grey to iron-black masses and the occasional crude crystal, and is valued less for beauty than for its rarity, its silver content and its place in mineralogical history. It was named in honour of Ponciano Aguilar, the superintendent of the San Carlos Mine who recognised the first specimens in the late 19th century, making the species a small monument to Mexican mining heritage.
Composition & structure
Aguilarite has the ideal formula Ag4SeS, a silver selenide in which selenium and sulfur share the anion sites. It sits within a solid-solution relationship between acanthite (silver sulfide) and naumannite (silver selenide), so its real composition shifts along that selenium-sulfur range. The mineral is monoclinic. Because silver, selenium and sulfur substitute and reorganise readily, aguilarite is part of a chemically subtle family in which careful analysis is needed to distinguish true aguilarite from its acanthite- and naumannite-like neighbours.
| Formula | Ag4SeS |
| Crystal system | Monoclinic |
| Mohs hardness | 2.5 |
| Lustre | Metallic |
| Colour | Bright lead-grey on fresh surfaces, dull iron-black on exposure |
| Type locality | San Carlos Mine, La Luz, Guanajuato, Mexico |
Formation & occurrence
Aguilarite is an uncommon, relatively low-temperature mineral of hydrothermal vein deposits that are rich in silver and selenium yet notably poor in sulfur. It is part of the bonanza silver ore assemblage, crystallising from cooling mineralising fluids alongside other silver species. Typical associates include acanthite, naumannite, pearceite, proustite, stephanite, native silver, calcite and quartz. The combination of abundant silver and selenium with a deficiency of sulfur is the key chemical signature that allows aguilarite to form rather than the more common silver sulfides.
Identification & similar species
Aguilarite is soft (hardness about 2.5), sectile and very dense, and tarnishes from bright lead-grey to dull black on exposure to air, all useful field clues. However, it closely resembles acanthite and naumannite, and the three cannot be reliably separated by eye; confident identification depends on chemical analysis to pin down the selenium-to-sulfur ratio. Its grey-black streak and metallic lustre are shared with several silver minerals, so locality data and analytical confirmation carry real weight when labelling specimens.
Notable localities & collecting
The San Carlos Mine at La Luz, near the city of Guanajuato, is the type locality and the classic source, and the broader Guanajuato silver district, including the La Sirena Mine, remains the area most associated with the species. Scattered occurrences have been reported from silver-selenium deposits elsewhere in the Americas, Europe and beyond, but fine, well-documented aguilarite is genuinely scarce. For collectors the appeal is its rarity, its silver chemistry and its historic Guanajuato pedigree; choice specimens are usually small and best sourced with reliable locality and analytical provenance.