Fluor-uvite
Chemical formula: CaMg<sub>3</sub>(Al<sub>5</sub>Mg)(Si<sub>6</sub>O<sub>18</sub>)(BO<sub>3</sub>)<sub>3</sub>(OH)<sub>3</sub>F
A silicate from the tourmaline group, characterized by fluorine dominance, forming prismatic crystals of variable color, most often green or brown.
Properties
- Mohs hardness
- 7.5
- Luster
- Vitreous
- Streak
- White
- Density
- 3.06
- Cleavage
- Indistinct
- Fracture
- Uneven, Conchoidal
- Transparency
- Transparent to Translucent
- Crystal system
- Trigonal
Diagnostic features
## Identification Fluor-uvite is most easily recognized by its typical tourmaline crystal form – elongated prisms with a characteristic rounded triangular cross-section and vertical striations. Hardness (7.5 on the Mohs scale) and vitreous luster are also important features. Many specimens exhibit strong pleochroism (color change when rotating the crystal). However, definitive differentiation from other tourmalines requires advanced chemical analysis. ## Distinguishing from Similar Minerals Fluor-uvite is visually indistinguishable from other members of the tourmaline group with similar colors, such as dravite or uvite. Dravite often occurs in similar geological environments. It is distinguished from minerals like epidote or hornblende by its lack of cleavage and typical crystal cross-section. Identification based solely on visual characteristics is uncertain. ## Crystal Forms It most commonly forms columnar, prismatic crystals, ranging from short and stout to long and acicular. The prism faces are usually strongly striated along the c-axis. Crystals can occur individually or form radial, fan-like aggregates. It is also found in granular aggregates and embedded in the host rock.
Geological environment
## Genesis Fluor-uvite is a mineral typical of metamorphic rocks. It forms under conditions of contact and regional metamorphism, mainly in calcium- and magnesium-rich rocks, such as marbles (metamorphosed limestones) and skarns. It is also found in some granitic pegmatites and crystalline schists. ## Mineral Associations It often co-occurs with minerals typical of its formation environment. The most common associations include: diopside, tremolite, phlogopite, calcite, dolomite, quartz, scapolite, and other tourmaline group minerals, such as dravite. ## Localities Significant fluor-uvite localities worldwide include: - United States: Gouverneur and De Kalb area in St. Lawrence County (New York) - type locality; Franklin (New Jersey). - Brazil: Brumado in Bahia state, from where beautiful, green, often gem-quality crystals originate. - Myanmar: Mogok area, known for the occurrence of various gemstones. - Nepal: various localities supplying well-formed crystals. - Russia: Southern Urals.
Rarity
Not very common
For collectors
## Quality Criteria The most highly prized fluor-uvite specimens are characterized by intense, attractive color (e.g., deep green, rare blue), high transparency, and absence of internal fractures or inclusions. Ideal crystal habit is crucial, with sharp edges, smooth, lustrous faces, and an undamaged termination. Specimens on matrix, especially in aesthetic contrast with other minerals (e.g., white calcite or diopside), command higher prices. Crystal size also significantly affects its value. ## Popular Localities Collectors particularly seek specimens from Brumado, Brazil, which are famous for their beautiful, emerald-green color and often gem-quality clarity. Classic, historical specimens come from localities in New York State (USA), which are valued for their scientific significance. Specimens from Nepal and Myanmar are also sought after in the collector's market.
Care and storage
## Cleaning Fluor-uvite specimens are best cleaned with lukewarm water, a mild soap, and a soft brush (e.g., a toothbrush). After washing, rinse thoroughly with distilled water and allow to air dry completely. Ultrasonic cleaners are not recommended, especially for specimens with inclusions or fractures. ## What to Avoid The mineral is sensitive to rapid temperature changes, which can cause internal fractures (thermal shock). Avoid contact with strong acids and bases. Although relatively hard, it should be protected from impacts and contact with harder minerals (e.g., diamond, corundum) to prevent scratches. ## Storage Fluor-uvite specimens should be stored in separate boxes or pouches to prevent rubbing and scratching by other minerals. Displayed specimens should be protected from dust and direct, prolonged exposure to strong sunlight, although its color is generally stable.