Argentopolybasite

Chemical formula: [Ag₁+₉Ag₁+S₂-₄][Ag₁+₆Sb₃+₂S₂-₇]

Argentopolybasite is a rare silver mineral from the sulfosalt group, forming thin, tabular crystals with a metallic luster and a cherry-red internal tint.

## Characteristics Argentopolybasite is a complex silver and antimony sulfosalt belonging to the polybasite group. It forms characteristic, thin, tabular crystals with a hexagonal outline, often arranged in fan-shaped or rosette-like aggregates. It is usually opaque and has a metallic, black color, but on thin edges or under strong transmitted light, it exhibits a deep, cherry-red color, which is its distinguishing feature. ## Physical Properties This mineral is relatively soft, with a Mohs hardness of 2.5. It has a metallic luster, and its density is significant, around 6.3 g/cm³. It is brittle and exhibits an uneven fracture. Thin lamellae of the mineral are somewhat flexible. ## Colors and Varieties The dominant color is black or iron-black. Characteristic internal reflections range from cherry-red to ruby-red, visible in transmitted light on thin fragments. No named varieties of this mineral are distinguished. ## History and Name The name "argentopolybasite" refers to its chemical composition – a high silver content (Latin: *argentum*) – and its structural and visual similarity to polybasite. The mineral was formally described and recognized by the International Mineralogical Association (IMA) in 1966. ## Uses Due to its rarity and small accumulations, argentopolybasite is not significant as a source of silver. However, it is a valued and sought-after collector's mineral.

Properties

Mohs hardness: 2.5
Luster: Metallic
Streak: Black
Density: 6.3
Cleavage: Imperfect on {001}
Fracture: Uneven
Transparency: Opaque, translucent in thin splinters
Crystal system: Monoclinic

Diagnostic features

## Identification Key diagnostic features include the hexagonal outline of thin, tabular crystals, black color with a metallic luster, and characteristic, deep, cherry-red internal reflections visible on edges or in transmitted light. Low hardness (2.5) is also helpful. ## Distinguishing from Similar Minerals Argentopolybasite can be confused with polybasite, stephanite, acanthite, or hematite. It differs from polybasite by a subtly different color of internal reflections and chemical data (higher silver content). Stephanite forms thicker, prismatic crystals. Acanthite rarely forms such well-developed tabular crystals and lacks red reflections. Hematite in the form of thin tabular crystals (so-called iron rose) is much harder (5.5-6.5). ## Crystal Forms It most often occurs as thin, flat, tabular crystals with a hexagonal outline. These crystals often form fan-shaped, rosette-like, or irregular aggregates. Prismatic forms are less common.

Geological environment

## Genesis It is a low- to medium-temperature hydrothermal mineral. It forms in ore veins rich in silver, lead, and zinc, where it crystallizes in the late stages of deposit formation. ## Mineral Associations It often co-occurs with other silver minerals such as stephanite, acanthite, proustite, pyrargyrite, as well as with galena, sphalerite, chalcopyrite, quartz, calcite, and dolomite. ## Localities Important worldwide localities include: Fresnillo mine in Mexico (type locality); Uchucchacua mine in Peru; Příbram in the Czech Republic; Freiberg in Saxony (Germany); Comstock Lode mine in Nevada and various localities in Colorado (USA).

Rarity

Rare

For collectors

## Quality Criteria The most valued specimens are those with sharp, well-formed, undamaged crystals with a distinct hexagonal outline. Aesthetically pleasing groups of crystals forming rosettes or fans, set on a contrasting matrix (e.g., white quartz), are highly prized. The large size of individual crystals and the intensity of red internal reflections significantly increase the specimen's value. ## Popular Localities Specimens from classic localities such as Fresnillo in Mexico, Freiberg in Germany, or Příbram in the Czech Republic are particularly sought after by collectors due to their historical significance and high quality.

Care and storage

## Cleaning Specimens should only be cleaned very carefully, using a soft brush to remove dust. Compressed air can be used from a safe distance. Avoid water and any chemicals that may cause oxidation or damage to delicate crystals. ## What to Avoid Avoid contact with chemicals, especially acids and cleaning agents. The mineral is sensitive to temperature changes and prolonged exposure to strong light, which can dull its luster. Protect from moisture to prevent oxidation processes. ## Storage Argentopolybasite is brittle and soft, so it should be stored in a separate, padded box, away from harder minerals that could scratch it. It is best displayed in a closed cabinet, protecting it from dust and humidity fluctuations.

Sources

IMA Mineral List (CNMNC)