Fluor-arfvedsonite

Chemical formula: Na₃F₂+₄Fe₃+Si₈O₂₂F₂

Fluor-arfvedsonite is a rare, fluorine-rich sodium amphibole, forming dark, prismatic crystals with a vitreous luster.

## Characteristics Fluor-arfvedsonite is a mineral from the sodium amphibole group, belonging to the arfvedsonite series. It forms elongated, prismatic crystals, often terminated by steep faces. It typically occurs as granular aggregates or radiating clusters. Its crystals are characterized by a dark color, from black to dark green or bluish-green, and exhibit strong pleochroism. ## Physical Properties This mineral has a hardness of 5-6 on the Mohs scale and a density ranging from 3.37-3.55 g/cm³. It has a vitreous luster on crystal faces and a greasy luster on fracture surfaces. It is brittle and exhibits perfect cleavage in two directions, typical for amphiboles. It is an opaque mineral, only translucent on thin edges. ## Colors and Varieties Fluor-arfvedsonite occurs in shades from black, through dark green, bluish-green, to greenish-black. No named color varieties or commercial varieties are distinguished. The color variability is mainly a result of strong pleochroism, visible under a polarizing microscope (from dark blue through grayish-violet to light yellowish-brown). ## History and Name The mineral's name refers to its chemical composition – the dominance of fluorine over the hydroxyl (OH) group – and its relation to standard arfvedsonite. Arfvedsonite was named after the Swedish chemist Johan August Arfwedson (1792-1841), the discoverer of lithium. Fluor-arfvedsonite was formally described as a distinct mineral species much later, after the role of fluorine in its structure was established. ## Uses Fluor-arfvedsonite has no industrial applications. Its significance is limited to the scientific and collecting spheres, as a rare and well-formed representative of the amphibole group.

Properties

Mohs hardness: 5-6
Luster: Vitreous
Streak: Bluish-gray
Density: 3.37-3.55
Cleavage: Perfect on {110}
Fracture: Uneven
Transparency: Translucent to opaque
Crystal system: Monoclinic

Diagnostic features

## Identification Fluor-arfvedsonite can be identified by its dark color (black, dark green), vitreous luster, and the characteristic long, prismatic crystal form typical of amphiboles. A key feature is perfect cleavage in two directions, intersecting at an angle of approximately 124 degrees, which is visible in cross-sections of the crystals. ## Distinguishing from Similar Minerals It can be confused with other dark amphiboles, such as hornblende, riebeckite, or standard arfvedsonite, as well as pyroxenes (e.g., aegirine). It is distinguished from aegirine by its cleavage angle (for pyroxenes, it is approximately 90 degrees). Distinguishing it from other sodium amphiboles (arfvedsonite, riebeckite) is extremely difficult without advanced chemical analyses (EDS/WDS) to confirm the dominance of fluorine. ## Crystal Forms Crystals are typically elongated, prismatic, often with a hexagonal cross-section. They occur as single, well-formed crystals embedded in the host rock, as well as in radiating or granular aggregates.

Geological environment

## Genesis Fluor-arfvedsonite is an igneous mineral, crystallizing in the late stages from alkaline, sodium- and fluorine-rich melts. It forms mainly in nepheline syenites and their associated pegmatites. It is a typical mineral of agpaitic rocks, where the ratio of alkali metals to aluminum is greater than one. ## Mineral Associations It often co-occurs with other alkaline minerals, such as nepheline, albite, microcline, aegirine, eudialyte, lorenzenite, as well as zircon and titanite. Depending on the location, it may be accompanied by many rare zirconium and titanium minerals. ## Localities The most important and well-known localities for fluor-arfvedsonite worldwide are the alkaline massifs on the Kola Peninsula in Russia (especially the Khibiny and Lovozero massifs). Significant specimens also come from the Ilímaussaq complex in Greenland and Mont Saint-Hilaire in Quebec, Canada. It also occurs in Norway (Langesundsfjorden) and the USA (e.g., Pikes Peak, Colorado).

Rarity

Rare

For collectors

## Quality Criteria The most prized specimens by collectors are those with well-formed, sharply terminated, lustrous crystals that exhibit a distinct prismatic habit. Crystals contrastingly set on a light rock matrix (e.g., on albite or nepheline) are highly valued. Large size of individual crystals and their partial translucency on the edges increase the specimen's value. Aesthetic, radiating aggregates are also sought after. ## Popular Localities The most famous collector specimens, often large and perfectly formed, come from the pegmatites of the Khibiny and Lovozero massifs on the Kola Peninsula in Russia. Classic, though usually smaller, crystals are found in the Ilímaussaq complex in Greenland. Mont Saint-Hilaire in Canada provides microscopic but perfectly formed crystals, prized by micromounters.

Care and storage

## Cleaning Specimens should only be cleaned mechanically, using a soft, dry brush or paintbrush to remove dust. For heavier soiling, compressed air can be used. The use of water is not recommended due to the risk of reaction with inclusions or associated minerals. ## What to Avoid Avoid contact with all chemicals, especially acids, which can damage the mineral's surface. The mineral is brittle, so it should be protected from impacts and falls. It should not be heated or subjected to sudden temperature changes. ## Storage Fluor-arfvedsonite specimens are best stored in separate, padded boxes or display cases to prevent scratches and mechanical damage. Well-formed crystals are worth displaying in showcases, away from direct sunlight and sources of vibration.

Sources

IMA Mineral List (CNMNC)