Ferriandrosite-(Ce)
Chemical formula: Mn²⁺Ce³⁺(Fe³⁺AlMn²⁺)(Si₂O₇)(SiO₄)O(OH)
A rare mineral from the epidote group, characterized by a complex chemical composition and occurrence as dark, almost black crystals.
Properties
- Mohs hardness
- 6.5-7
- Color
- dark brown
- Luster
- Vitreous
- Streak
- light brown
- Density
- 4.321
- Cleavage
- Good on {001}
- Fracture
- Uneven
- Transparency
- Opaque
- Crystal system
- Monoclinic
Diagnostic features
## Identification Ferriandrosite-(Ce) can be identified by its dark, almost black color, prismatic crystal habit, and vitreous luster. It is relatively hard and heavy. However, chemical analysis (e.g., EDS) is crucial for identification to confirm the complex composition with a dominance of Mn, Ce, and Fe³⁺. ## Distinguishing from Similar Minerals It can be confused with other dark minerals from the epidote group, such as allanite-(Ce) or ferriallanite-(Ce), as well as tourmaline (schorl) or pyroxenes. Distinguishing it from these minerals based on visual characteristics is extremely difficult and almost always requires advanced laboratory research. ## Crystal Forms This mineral forms well-developed, prismatic crystals, often elongated along one axis. It also occurs as granular aggregates or embedded in the host rock.
Geological environment
## Genesis Ferriandrosite-(Ce) forms as a result of metamorphic processes in manganese-rich rocks. Its occurrence is associated with metamorphosed manganese ore deposits. ## Mineral Associations It coexists with other manganese minerals and silicates, such as braunite, ganophyllite, rhodonite, tephroite, aegirine, quartz, and various varieties of garnets (spessartine). ## Localities The only known and confirmed locality for this mineral (type locality) is the Varenche mine, located in the Saint-Barthélemy valley in the Aosta Valley region of Italy.
Rarity
Very rare
For collectors
## Quality Criteria The most valuable specimens of ferriandrosite-(Ce) are those that possess well-formed, sharply terminated crystals with a distinct luster. The contrast with a light host rock (matrix) significantly enhances their visual appeal. Due to the mineral's rarity, even small but well-defined crystals are valuable to collectors specializing in systematics or rare minerals. ## Popular Localities The only source of collector specimens is the Varenche mine in Italy. Material from this locality is the reference point for this mineral.
Care and storage
## Cleaning It is recommended to clean only with a soft, dry brush to remove dust. The use of water, especially with detergents, is not advisable due to the risk of unforeseen chemical reactions. ## What to Avoid Avoid contact with all chemicals, including acids and bases, which can damage the mineral's surface. It should not be heated or subjected to ultrasound. Due to its dark color, prolonged exposure to intense sunlight may not be harmful, but caution is advised. ## Storage Specimens should be stored in stable conditions, away from moisture and chemical contaminants. It is best to keep them in separate, padded boxes to prevent abrasions and mechanical damage, especially if they are part of a larger collection.