Fluorcalciomicrolite
Chemical formula: (Ca,Na)₂Ta⁵⁺₂O₆F
Fluorcalciomicrolite is a rare mineral from the microlite group, an oxide of tantalum and calcium, found as small, colorless, octahedral crystals.
Properties
- Mohs hardness
- 4-5
- Luster
- Vitreous to resinous
- Streak
- White
- Density
- 6.160
- Cleavage
- None
- Fracture
- Conchoidal
- Transparency
- Translucent
- Crystal system
- Isometric
Diagnostic features
## Identification Characteristic features of fluorcalciomicrolite include its typical octahedral crystals, high density (the specimen is surprisingly heavy for its size), lack of cleavage, and conchoidal fracture. It occurs in a specific geological environment – granitic pegmatites. ## Distinguishing from Similar Minerals It can be confused with other minerals from the microlite group, from which differentiation is usually impossible without advanced chemical analyses (e.g., EDS). From other colorless minerals with regular crystal forms (e.g., fluorite, spinel), it is distinguished by its significantly higher density. ## Crystal Forms It crystallizes in the isometric system, forming almost exclusively crystals in the shape of octahedra, often with sharp edges and smooth faces. Crystals are usually small, rarely exceeding a few millimeters.
Geological environment
## Genesis Fluorcalciomicrolite is a mineral of magmatic origin. It forms in the late stages of crystallization of granitic pegmatites, rich in rare earth elements, tantalum, and niobium. ## Mineral Associations It often co-occurs with other pegmatite minerals, such as quartz, albite, microcline, lepidolite, spodumene, as well as other tantalum and niobium minerals. ## Localities The most important and typical occurrence (type locality) of this mineral is the Volta Grande (Mibra) mine in Nazareno, Minas Gerais state, Brazil. This is the main locality from which the best-known specimens originate.
Rarity
Very rare
For collectors
## Quality Criteria Specimens with sharp, well-formed, undamaged crystals with an ideal octahedral habit are most valued by collectors. Transparency and lack of inclusions are also important. Specimens where fluorcalciomicrolite crystals are embedded on a contrasting rock matrix (e.g., on albite or lepidolite) are rated higher than isolated crystals. ## Popular Localities The only known and significant locality from a collector's perspective is the Volta Grande mine in Brazil. Specimens from this locality are the standard for this mineral.
Care and storage
## Cleaning Specimens should only be cleaned with lukewarm distilled water and a very soft brush or paintbrush. Ultrasonic cleaners should be avoided, as they can cause cracks in brittle crystals. ## What to Avoid The mineral is sensitive to strong acids. Rapid temperature changes, impacts, and scratches should be avoided due to its moderate hardness and brittleness. ## Storage Collectible fluorcalciomicrolite specimens, especially those with well-formed crystals, should be stored in separate, padded boxes (e.g., "perky boxes") to protect them from mechanical damage and contact with harder minerals.