Paraguanajuatite

Chemical formula: Bi³⁺₂Se²⁻₃

Paraguanajuatite is a rare bismuth selenide with a metallic luster, occurring as platy or fibrous aggregates.

## Characteristics Paraguanajuatite is a rare mineral from the tetradymite group, chemically classified as bismuth selenide. It forms characteristic platy or lamellar crystals, which often arrange into radial, fibrous, or foliated aggregates. Its appearance is typically metallic, with a lead-gray color and a distinct luster on fresh surfaces. ## Physical Properties This mineral is relatively soft, with a Mohs hardness of 2.5-3, meaning it can be scratched by a copper wire. It is opaque and quite dense – its specific gravity is approximately 6.2 g/cm³. It possesses perfect cleavage in one plane, which explains its tendency to form platy and foliated shapes. ## Colors and Varieties Paraguanajuatite occurs in a uniform, lead-gray color. No colored varieties or trade names are known. ## History and Name The mineral's name refers to its discovery location. It was first identified in 1948 at the Santa Catarina mine in Guanajuato, Mexico. The name is a combination of "para" (indicating similarity to guanajuatite) and "Guanajuato," the type locality. ## Uses Paraguanajuatite has no industrial significance. Its occurrence is too rare to be a source of bismuth or selenium. It is solely an object of interest for collectors specializing in rare minerals or systematics.

Properties

Mohs hardness
2.5-3
Luster
Metallic
Density
6.2
Cleavage
{0001}
Transparency
Opaque
Crystal system
Trigonal

Diagnostic features

## Identification Characteristic features of paraguanajuatite include its lead-gray color, metallic luster, low hardness (2.5-3), and typical forms of occurrence – platy, lamellar, or fibrous aggregates. Perfect cleavage in one direction is also a key indicator. ## Distinguishing from Similar Minerals Paraguanajuatite can be confused with other bismuth selenides and sulfides, such as guanajuatite, tetradymite, or bismuthinite. Guanajuatite has a very similar composition and appearance but crystallizes in the orthorhombic system. Tetradymite contains tellurium instead of selenium. Bismuthinite is a sulfide and has a slightly different crystal structure. Definitive differentiation of these minerals often requires advanced analytical methods, such as X-ray diffraction (XRD). ## Crystal Forms Crystals are usually poorly formed, appearing as thin tablets, lamellae, or needles. Most often, it forms platy, fibrous, radial, or massive, granular aggregates.

Geological environment

## Genesis Paraguanajuatite is a hydrothermal mineral. It forms in ore veins with low to medium formation temperatures. It is typically associated with deposits rich in bismuth and selenium. ## Mineral Associations This mineral co-occurs with other selenides and sulfides. In its type locality (Guanajuato, Mexico), it was found in association with guanajuatite, clausthalite, naumannite, native bismuth, quartz, and calcite. ## Localities The most important and historical locality is the Santa Catarina mine in Guanajuato, Mexico. Other confirmed localities include the Tsumeb mine in Namibia, the Zalesi vein in the Czech Republic, and some deposits in Argentina (Sierra de Cacheuta) and Bolivia (Pacajake mine).

Rarity

Very rare

For collectors

## Quality Criteria Specimens with well-formed, even if small, platy crystals or distinct radial aggregates are most valued by collectors. Association with other rare minerals is also important, increasing the scientific and aesthetic value of the specimen. Purity and lack of mechanical damage are crucial, given the mineral's great delicacy. ## Popular Localities Specimens from the type locality, the Santa Catarina mine in Mexico, have the greatest historical value and are most sought after by specialized collectors.

Care and storage

## Cleaning Paraguanajuatite specimens are very soft and delicate. They should only be cleaned with compressed air to remove dust. Any contact with water, and especially with chemicals, is inadvisable. Avoid ultrasonic cleaners and mechanical cleaning. ## What to Avoid The mineral is susceptible to scratching due to its low hardness. Contact with harder minerals should be avoided. As a selenide, it may react with acids. It should be protected from moisture and sudden temperature changes. ## Storage It is recommended to store specimens in separate, padded collector boxes to prevent abrasion and mechanical damage. They should be kept in a dry place, away from direct sunlight and heat sources.

External references

Sources

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