Guanajuatite

Chemical formula: Bi<sup>3+</sup><sub>2</sub>Se<sup>2-</sup><sub>3</sub>

A rare bismuth selenide with a metallic luster, forming acicular or fibrous aggregates in hydrothermal veins.

## Characteristics Guanajuatite is a bismuth selenide belonging to the bismuthinite group. It typically forms compact, granular, fibrous, or radial aggregates. It rarely occurs as elongated, acicular crystals with a square cross-section, often longitudinally striated. It is steel-gray to bluish-gray in color and has a characteristic, strong metallic luster. In air, it may develop a darker, dull tarnish. ## Physical Properties It is a relatively soft mineral, with a Mohs hardness of 2.5-3.5. It has a very high density, ranging from 6.25 to 6.98 g/cm³, which is noticeable even in small specimens. It possesses perfect cleavage in one direction. Its luster is strongly metallic, and it is completely opaque. ## Colors and Varieties Typically steel-gray, lead-gray, or bluish-gray. Some specimens may exhibit a yellowish or brownish weathering tarnish. No color or commercial varieties are distinguished. ## History and Name The mineral's name comes from its discovery and description locality – the state of Guanajuato in Mexico. It was first described in 1873 by Vicente Fernández. ## Uses Due to its rarity, guanajuatite has no industrial significance. It is solely an object of interest for collectors of rare minerals and scientists.

Properties

Mohs hardness
2.5-3.5
Luster
Metallic
Streak
Grayish black
Density
6.25-6.98
Cleavage
Perfect on {010}
Fracture
Uneven
Transparency
Opaque
Crystal system
Orthorhombic

Diagnostic features

## Identification Key features include very high density, steel-gray color, metallic luster, low hardness, and perfect cleavage in one direction. It often occurs as fibrous or acicular aggregates. ## Distinguishing from Similar Minerals It can be confused with bismuthinite (Bi₂S₃), which has a very similar appearance and physical properties, but is a bismuth sulfide, not a selenide. Definitive differentiation requires chemical analysis. It is also similar to stibnite (Sb₂S₃), which, however, has a lower density and often forms better-developed, larger crystals. ## Crystal Forms Crystals are rare, acicular, elongated, and longitudinally striated. It typically forms fibrous, felted, radial, or massive, granular aggregates.

Geological environment

## Genesis It forms under hydrothermal conditions, in medium- to high-temperature ore veins. ## Mineral Associations It often co-occurs with other selenides and sulfides, such as clausthalite, naumannite, tiemannite, klockmannite, as well as with bismuthinite, native bismuth, galena, pyrite, chalcopyrite, and calcite. ## Localities The most important localities worldwide are mines in the Guanajuato region, Mexico (type locality). It is also known from St. Andreasberg in the Harz Mountains (Germany), Falun (Sweden), Tasna mine (Bolivia), as well as from the Red Cloud mine in Nevada (USA) and Kidd Creek in Ontario (Canada).

Rarity

Very rare

For collectors

## Quality Criteria The most valued specimens are those with well-formed, visible acicular crystals or distinct fibrous aggregates. Association with other rare minerals and provenance from a classic, historical locality are also important. ## Popular Localities Specimens from historical mines in Guanajuato (Mexico) and St. Andreasberg (Germany) are most sought after by collectors specializing in rare minerals.

Care and storage

## Cleaning Clean only dry, using a soft brush to remove dust. Avoid water and any chemical agents that may cause oxidation or damage. ## What to Avoid The mineral is sensitive to moisture and chemical reagents. It should be protected from acids and other aggressive substances. Due to its softness and perfect cleavage, it is very susceptible to mechanical damage (scratches, cracks). ## Storage Specimens should be stored in stable conditions, in a dry place, preferably in a closed display box, to limit air and moisture access and protect against physical damage.

Sources

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