Ferrofettelite
Chemical formula: Ag¹⁺₁₆Fe²⁺As³⁺₄S²⁻₁₅
Ferrofettelite is a rare silver, iron, and arsenic sulfide, forming thin, hexagonal plates with a metallic luster and cherry-red color.
Properties
- Mohs hardness
- 1.5
- Luster
- Metallic
- Streak
- Dark reddish-gray
- Density
- 5.74
- Cleavage
- Perfect on {0001}
- Fracture
- Irregular/Uneven
- Transparency
- Opaque
- Crystal system
- Monoclinic
Diagnostic features
## Identification Key diagnostic features include its form as thin, hexagonal plates, cherry-red color, metallic luster, and extreme softness. Its occurrence in paragenesis with other silver and arsenic sulfosalts in hydrothermal veins is also an important clue. ## Distinguishing from Similar Minerals It can be confused with fettelite, from which it is virtually impossible to distinguish without advanced chemical analyses (EDS/WDS). Proustite and pyrargyrite, although having similar composition and red color, typically form better-developed, prismatic trigonal crystals, rather than thin hexagonal plates. Dark red cinnabar also forms different crystal habits and has a distinct environment of occurrence. ## Crystal Forms It crystallizes in the hexagonal system, forming characteristic, very thin, tabular (platy) crystals with a hexagonal outline. These crystals often group into fan-like or rosette aggregates.
Geological environment
## Genesis Ferrofettelite is a hydrothermal mineral, forming at low to medium temperatures. It forms in polymetallic ore veins rich in silver, arsenic, and sulfur. ## Mineral Associations It most commonly co-occurs with other sulfosalts and sulfides. At the type locality (Příbram), it was found in association with fettelite, native arsenic, stephanite, pyrargyrite, proustite, acanthite, native silver, dyscrasite, kutinaite, and calcite. ## Localities This is an extremely rare mineral, known from only a few localities worldwide. Besides the type locality in Příbram, Czech Republic, its occurrence has been confirmed in the Uchucchacua mine in Peru and the Svätodušná mine in Banská Štiavnica, Slovakia.
Rarity
Extremely rare
For collectors
## Quality Criteria As a micromineral, ferrofettelite is primarily valued for the richness and size of its crystalline aggregates on the matrix. Specimens with clearly visible, well-formed rosettes or plates of intense red color, contrasting with the substrate, are most highly prized. Association with other rare minerals is also important. ## Popular Localities The most sought-after specimens, constituting the type material, come from vein No. 21 in Příbram, Czech Republic. They serve as the reference standard for this mineral.
Care and storage
## Cleaning Due to its extreme softness and delicacy, mechanical cleaning is not recommended. If absolutely necessary, compressed air can be used to remove dust. Avoid contact with water and any chemicals. ## What to Avoid Avoid touching, abrasion, and any mechanical stress. The mineral is sensitive to light, which can cause it to darken. It should be protected from moisture, high temperatures, and chemicals that could cause its decomposition. ## Storage Ferrofettelite specimens should be stored in stable, sealed containers (e.g., "perky boxes"), away from light, in a dry environment with a constant temperature. It is best to store them in their original form, on a rock matrix, without attempting to isolate the delicate crystals.