Pyrostilpnite
Chemical formula: Ag¹⁺₃Sb³⁺S²⁻₃
Pyrargyrite is a rare silver antimony sulfide, prized by collectors for its fiery-red, tabular crystals with an adamantine luster.
Properties
- Mohs hardness
- 2
- Luster
- Adamantine
- Streak
- Yellow-orange
- Density
- 5.94
- Cleavage
- Perfect on {001}.
- Fracture
- Conchoidal
- Transparency
- Transparent,Translucent
- Crystal system
- Monoclinic
Diagnostic features
## Identification Key diagnostic features of pyrargyrite include its fiery-red color, characteristic tabular or bladed crystal habit, often forming fan-shaped aggregates, and a very strong, almost adamantine luster. An orange-yellow streak and extreme softness (Mohs hardness of 2) are also helpful. ## Distinguishing from Similar Minerals Pyrargyrite is sometimes confused with its dimorphic counterpart, proustite. However, proustite typically forms better-developed, prismatic or scalenohedral crystals and has a slightly lighter, red streak. It differs from cinnabar by its crystal form and stronger luster. It differs from realgar by its higher density and stability (realgar is unstable in light). ## Crystal Forms Pyrargyrite crystals are usually thin, flattened, with a tabular or bladed appearance. They often exhibit striations on their faces. They coalesce into characteristic fan-shaped, sheaf-like, or radial aggregates. Prismatic forms are less common.
Geological environment
## Genesis Pyrargyrite is a hydrothermal mineral, crystallizing at low temperatures. It forms in the final stages of the formation of ore veins rich in silver and antimony. It is one of the last minerals to crystallize in the mineralization sequence. ## Mineral Associations It most commonly co-occurs with other silver sulfosalts, such as proustite, stephanite, and polybasite. It is also accompanied by acanthite, native silver, galena, sphalerite, as well as gangue minerals like calcite, dolomite, and quartz. ## Localities The most important historical and contemporary localities for pyrargyrite are in Germany (Freiberg and St. Andreasberg in the Harz Mountains), Czech Republic (Jáchymov, Příbram), Chile (Chañarcillo), Bolivia (Colquechaca), and the United States (Idaho, Nevada).
Rarity
Rare
For collectors
## Quality Criteria The most highly valued pyrargyrite specimens are characterized by well-formed, sharp crystals with an intense, fiery-red color and strong luster. Rich groups of crystals forming aesthetic, fan-shaped or radial aggregates on a contrasting matrix (e.g., on white calcite) are particularly sought after. Crystal size is also important, although even small but well-formed groups achieve high value. Damage-free specimens are rare and represent significant value for collectors. ## Popular Localities Classic, historical specimens come from mines around Freiberg in Saxony (Germany) and from St. Andreasberg in the Harz Mountains. In recent decades, beautiful specimens from the Uchucchacua mine in Peru have also entered the market, often distinguished by excellent crystal development and luster.
Care and storage
## Cleaning Pyrargyrite specimens should be cleaned with the utmost care. Only gentle brushing with a soft brush is permissible to remove dust. The use of water, and especially ultrasonics, is absolutely inadvisable and can destroy delicate crystals. ## What to Avoid The mineral is sensitive to light – prolonged exposure to strong illumination, especially sunlight, can cause it to darken and lose its luster. All chemicals, acids, and detergents should be avoided. Due to its low hardness, it is extremely susceptible to scratches and mechanical damage. Sudden temperature changes should also be avoided. ## Storage Pyrargyrite specimens should be stored in separate, padded boxes, away from harder minerals. It is best to keep them in a dark and dry place to protect them from light and moisture. Display should be in enclosed showcases with limited lighting.