Folvikite
Chemical formula: Sb⁵⁺Mn³⁺(Mg,Mn²⁺)₁₀O₈(BO₃)₄
Folvikite is an extremely rare borate and oxide of antimony, manganese, and magnesium, found as tiny, black crystals in marbles.
Properties
- Mohs hardness
- 6
- Color
- Black to dark reddish-brown
- Luster
- Metallic
- Streak
- Reddish-brown
- Density
- 4.14
- Cleavage
- Perfect on {100}
- Fracture
- Uneven
- Transparency
- Opaque
- Crystal system
- Monoclinic
Diagnostic features
## Identification Identification of folvikite is based on its characteristic appearance at the occurrence site. Typical features include very small, black, lustrous, bladed crystals forming radial aggregates on white marble. Identification is practically only possible with material from the type locality. Definitive identification requires advanced analytical methods, such as Raman spectroscopy or chemical analysis (EDS). ## Distinguishing from Similar Minerals Folvikite can be confused with other black, fine-grained manganese minerals found in similar environments, such as piemontite, braunite, or hausmannite. "Hand specimen" differentiation is practically impossible. Folvikite is distinguished by its unique chemical composition (it is the only known antimony and manganese borate) and specific crystal structure. ## Crystal Forms Folvikite crystals are very small, typically up to 0.5 mm long and up to 20 micrometers thick. They have the form of thin tablets or blades, often flattened. They usually form radial or stellate aggregates, less commonly occurring as single, isolated crystals.
Geological environment
## Genesis Folvikite forms as a result of metamorphic processes. It was found in lenses of manganese-rich rocks occurring within Proterozoic calcitic marbles. Its formation is associated with contact or regional metamorphism under amphibolite facies conditions. ## Mineral Associations This mineral co-occurs with calcite (which constitutes the main mass of the rock), as well as with other manganese minerals such as piemontite, hausmannite, braunite, manganhumite, magnussonite, sarkinite, berzeliite, and also with hematite and barite. ## Localities The only confirmed locality of folvikite in the world is its type locality: the Folvik marble quarry, located north of the village of Folvik in the municipality of Fauske, Nordland county, Norway.
Rarity
Extremely rare
For collectors
## Quality Criteria The quality of folvikite specimens is assessed primarily based on the richness and aesthetics of the crystal aggregates. Most valued are specimens with numerous, well-formed, radial clusters of black crystals, creating a strong contrast with the white marble background. The size of individual aggregates is also important, although even the largest of them are still very small. ## Popular Localities The only source of folvikite specimens is the Folvik quarry in Norway. Material from this location is extremely difficult to obtain and appears on the collector's market very sporadically, usually in small quantities directly after discovery.
Care and storage
## Cleaning Folvikite specimens should be cleaned very carefully, preferably using compressed air to remove dust. Due to the small size and fragility of the crystals, mechanical cleaning, including brushes, should be avoided. Contact with water is not recommended without certainty about the stability of associated minerals. ## What to Avoid Contact with all chemicals, especially acids, which can damage both folvikite and the host rock (marble), should be avoided. Specimens should be protected from ultrasound and sudden temperature changes. ## Storage Specimens should be stored in stable conditions, away from dust and moisture. The best solution is a sealed "micromount" box, which protects delicate crystals from mechanical damage and contamination.