Aguilarite

Chemical formula: Ag<sup>1+</sup><sub>4</sub>Se<sup>2-</sup>S<sup>2-</sup>

Aguilarite is a rare silver selenosulfide with a metallic luster, occurring mainly in massive aggregates.

## Characteristics Aguilarite is a mineral from the sulfide group, chemically classified as silver selenosulfide. It represents an intermediate member in the isomorphic series between acanthite (Ag₂S) and naumannite (Ag₂Se). It rarely forms well-developed crystals, which take on skeletal, acicular, or filiform habits. It usually occurs as massive, granular, or anhedral aggregates, often intergrown with other silver minerals. Its surface is typically dull or tarnished, but a fresh fracture reveals a strong, metallic luster. ## Physical Properties This mineral is relatively soft, with a Mohs hardness of 2.5, meaning it can be scratched with a copper wire. It is opaque and has a high density, ranging from 7.4 to 7.58 g/cm³, which makes specimens noticeably heavy for their size. It possesses indistinct cleavage and an uneven or subconchoidal fracture. ## Colors and Varieties Aguilarite is iron-black to grayish-black in color. Due to its position in the solid solution series, no distinct colored varieties are recognized. The chemical composition transitions smoothly from sulfur-rich acanthite to selenium-rich naumannite, with aguilarite representing the intermediate member. ## History and Name The mineral's name, given in 1891 by Frederick Augustus Genth, honors Ponciano Aguilar, the superintendent of the San Carlos mine in Guanajuato, Mexico. This was the first locality where the mineral was identified. ## Uses Aguilarite has no significant industrial application. It is primarily of interest to collectors of rare minerals and scientists studying ore deposits. In places of abundant occurrence, it can be a minor source of silver.

Properties

Mohs hardness
2.5
Luster
Metallic
Streak
Greyish black
Density
7.4 - 7.58
Cleavage
Imperfect on {010}
Fracture
Subconchoidal to uneven
Transparency
Opaque
Crystal system
Monoclinic

Diagnostic features

## Identification Helpful features for identifying aguilarite include its black color, strong metallic luster on a fresh surface, high density, and softness. Its occurrence in paragenesis with other silver, selenium, and sulfur minerals in hydrothermal veins is of key importance. ## Distinguishing from Similar Minerals Aguilarite is visually indistinguishable from acanthite and naumannite, with which it forms an isomorphic series. These three minerals often co-occur and form intergrowths. Definitive differentiation is only possible using advanced analytical methods, such as X-ray microanalysis (EDS/WDS), which allows for the determination of the selenium to sulfur ratio in the mineral. ## Crystal Forms Well-formed crystals are rare. If they occur, they take the form of elongated, acicular, or hair-like habits, often with a skeletal structure. Most commonly, it is found in massive, anhedral, and granular aggregates.

Geological environment

## Genesis Aguilarite is a hydrothermal mineral. It forms in low-sulfidation epithermal veins, rich in silver and selenium. It crystallizes under relatively low-temperature conditions from solutions circulating in rock fractures. ## Mineral Associations This mineral often co-occurs with other silver ores. Its typical associated minerals include: acanthite, naumannite, native silver, stephanite, pearceite, polybasite, as well as gangue minerals such as calcite and quartz. ## Localities The most important and classic locality for aguilarite is its type locality – the San Carlos mine in Guanajuato, Mexico. It is also known from the historic Comstock Lode in Nevada (USA), as well as occurrences in Japan (Hokkaido island), Argentina (San Juan province), and Russia (Tuva Republic).

Rarity

Rare

For collectors

## Quality Criteria The most highly valued by collectors are specimens with well-developed, even if small, crystals with acicular or skeletal habits. Due to their extreme rarity, even rich, massive aggregates from an analytically confirmed type locality (Guanajuato) are considered valuable. The attractiveness of a specimen is enhanced by intergrowths with other rare silver minerals, such as naumannite or stephanite. ## Popular Localities Specimens from historical finds in Guanajuato, Mexico, are by far the most sought-after and valued in the collector's market. Material from this locality is considered classic for this species.

Care and storage

## Cleaning Aguilarite specimens should only be dry-cleaned, using a soft brush to remove dust. Contact with water, especially tap water, can accelerate the oxidation process and dull the surface. Ultrasonic cleaners and all chemical agents should be avoided. ## What to Avoid The mineral is very soft and susceptible to scratches. It should be protected from contact with harder minerals (e.g., quartz). As a sulfide, it is sensitive to moisture and acidic environments, which can cause its slow decomposition and tarnishing. It should not be exposed to prolonged sunlight or sudden temperature changes. ## Storage It is recommended to store specimens in a dry place, preferably in closed, padded collector boxes or display cases, to limit air and moisture exposure and protect against mechanical damage.

External references

Sources

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