Magnesio-fluoro-arfvedsonite

Chemical formula: NaNa₂(Mg₄Fe₃+)Si₈O₂₂F₂

A silicate from the amphibole group, characterized by its black color and occurrence in rare alkaline rocks.

## Characteristics Magnesio-fluoro-arfvedsonite is a mineral from the sodium amphibole group. It forms elongated, prismatic or acicular crystals, often gathered in radial or disordered aggregates. It is usually black, sometimes with a greenish or bluish tint. It rarely occurs as well-formed, isolated crystals, more often intergrown in the rock mass. ## Physical Properties Its Mohs hardness is approximately 6. It has a vitreous luster, and on fracture surfaces, it can be dull. It is translucent (on thin edges) to completely opaque. It exhibits good, two-directional cleavage, typical of amphiboles, with angles between planes of approximately 56° and 124°. ## Colors and Varieties Black is the dominant color. Other shades, such as dark green or blue-black, are rarer and depend on subtle variations in chemical composition. No distinct color varieties or commercial names are recognized. ## History and Name The mineral's name is tripartite and precisely describes its chemical composition in relation to other minerals in the arfvedsonite group. The prefix "magnesio-" indicates the dominance of magnesium over iron, "fluoro-" indicates the dominance of fluorine over the hydroxyl (OH) group, and "arfvedsonite" refers to its belonging to the arfvedsonite series. The mineral was formally defined during the revision of the amphibole supergroup nomenclature in 2012. ## Applications Magnesio-fluoro-arfvedsonite has no industrial applications. It is solely an object of scientific and collector interest due to its rarity and occurrence in specific mineral parageneses.

Properties

Mohs hardness: 6
Luster: Vitreous
Streak: Greenish gray
Density: 3.2-3.3
Cleavage: Good on {110}
Fracture: Uneven
Transparency: Translucent to opaque
Crystal system: Monoclinic

Diagnostic features

## Identification Preliminary identification is based on its black color, vitreous luster, prismatic crystal habit, and amphibole-typical cleavage. A key indicator is its environment of occurrence – nepheline syenites and associated pegmatites. ## Distinguishing from Similar Minerals Macroscopic differentiation of magnesio-fluoro-arfvedsonite from other black amphiboles (e.g., arfvedsonite, riebeckite) or pyroxenes (e.g., aegirine) is practically impossible without advanced chemical analyses (e.g., electron microprobe) to confirm the dominance of magnesium and fluorine. Aegirine differs from it in cleavage angles (approximately 87°/93°). Black tourmaline (schorl) can be similar but has a different crystal habit (often with a characteristic trigonal cross-section) and does not exhibit cleavage. ## Crystal Forms It most often forms columnar, prismatic crystals with an elongated shape. It also occurs as acicular, fibrous aggregates and granular aggregates intergrown in the rock.

Geological environment

## Genesis It is a characteristic mineral of silica-undersaturated igneous rocks, especially peralkaline nepheline syenites and their associated pegmatites. It forms in the late stages of crystallization of magma rich in sodium, fluorine, and rare elements. ## Mineral Associations It often co-occurs with other rare minerals typical of this environment, such as eudialyte, aegirine, nepheline, albite, microcline, lorenzenite, astrophyllite, or lomonosovite. ## Localities The most important localities worldwide, from which the best-studied specimens originate, are alkaline complexes such as Ilímaussaq in Greenland (type locality), Mont Saint-Hilaire in Canada, and the Khibiny and Lovozero massifs on the Kola Peninsula in Russia.

Rarity

Very rare

For collectors

## Quality Criteria Specimens with well-formed, sharp, and lustrous crystals, clearly contrasting with the rock matrix, are most highly valued. Paragenesis is of great importance – specimens with accompanying, rare minerals (e.g., pink eudialyte) are much more sought after. The size of individual crystals also increases the value of the specimen. ## Popular Localities Localities in Russia (Kola Peninsula), Canada (Mont Saint-Hilaire), and Greenland (Ilímaussaq) are considered classic and provide the best collector specimens. Specimens from these locations are the benchmark for this mineral.

Care and storage

## Cleaning Specimens should only be cleaned with lukewarm distilled water and a soft brush. Avoid strong rubbing, which could damage the crystal surfaces or exploit cleavage planes. ## What to Avoid The mineral is sensitive to acids. Ultrasonic cleaners should not be used, as they can cause fracturing along cleavage planes. Protect it from impacts and sudden temperature changes. ## Storage Store in stable conditions, away from dust and chemicals. It is best to store it in separate display boxes to avoid scratching by other, harder minerals.

Sources

IMA Mineral List (CNMNC)