Magnesio-arfvedsonite

Chemical formula: NaNa<sub>2</sub>(Mg<sub>4</sub>Fe<sup>3+</sup>)Si<sub>8</sub>O<sub>22</sub>(OH)<sub>2</sub>

Magnesio-arfvedsonite is a rare sodium amphibole with a dark green to black color, forming prismatic crystals in syenites and related rocks.

## Characteristics Magnesio-arfvedsonite is a mineral from the sodium amphibole group, belonging to the arfvedsonite series. It forms elongated, prismatic, or acicular crystals, often aggregated into radial or fibrous masses. Its color is typically dark green, blue-green, gray-green, or even black. It is opaque or translucent only on thin edges. It is an important component of some alkaline igneous rocks. ## Physical Properties This mineral is characterized by a hardness of 5-6 on the Mohs scale and a density of approximately 3.26-3.38 g/cm³. The luster is vitreous, and on cleavage surfaces, it can be pearly. It exhibits perfect cleavage in two directions, typical of amphiboles, intersecting at angles of approximately 56° and 124°. ## Colors and Varieties The dominant colors range from dark green through blue-green to almost black. The color variation depends on the magnesium to iron content ratio. There are no distinct commercial or gemological varieties, and the mineral is primarily of scientific and collector interest. ## History and Name The name "Magnesio-arfvedsonite" refers to its chemical composition – it is the magnesium (Magnesio-) analogue of arfvedsonite. The name arfvedsonite was given in honor of the Swedish chemist Johan August Arfwedson (1792-1841), the discoverer of lithium. The mineral was formally described and recognized by the International Mineralogical Association (IMA) as part of the amphibole nomenclature revision. ## Uses Due to its rarity and generally small crystal sizes, magnesio-arfvedsonite has no industrial applications. However, it is valued by collectors specializing in rock-forming minerals, mineral systematics, and minerals from rare localities.

Properties

Mohs hardness
5-6
Luster
Vitreous
Streak
Grayish green
Density
3.26-3.38
Cleavage
Perfect on {110}
Fracture
Uneven
Transparency
Translucent to opaque
Crystal system
Monoclinic

Diagnostic features

## Identification Characteristic features of magnesio-arfvedsonite include its dark green to black color, prismatic crystal habit, vitreous luster, and perfect amphibole-type cleavage. It often exhibits weak pleochroism in shades of green and blue, visible under a petrographic microscope. ## Distinguishing from Similar Minerals Magnesio-arfvedsonite is difficult to distinguish from other dark amphiboles, such as arfvedsonite, riebeckite, or hornblende, without advanced chemical analyses (e.g., EDS, WDS) or detailed optical studies. Arfvedsonite is typically richer in iron and may exhibit stronger pleochroism. Hornblende occurs in a wider range of rocks, not just alkaline ones. ## Crystal Forms Crystals are usually columnar, prismatic, often elongated or acicular. It occurs as granular, fibrous, radial aggregates, or as single crystals embedded in rock with a hexagonal cross-section.

Geological environment

## Genesis Magnesio-arfvedsonite is a rock-forming mineral that originates under magmatic conditions. It crystallizes from alkaline magmas, rich in sodium and poor in silica. It is a typical component of some nepheline syenites, phonolites, and carbonatites. ## Mineral Associations It co-occurs with other minerals characteristic of alkaline rocks, such as nepheline, albite, microcline, eudialyte, aegirine, lorenzenite, neptunite, and lamprophyllite. ## Localities Significant occurrences of this mineral are found in alkaline rock complexes. The most important include: the Khibiny and Lovozero massifs on the Kola Peninsula in Russia, the Ilímaussaq complex in Greenland, Mont Saint-Hilaire in Quebec, Canada, and some localities in Norway and the USA (e.g., Magnet Cove, Arkansas).

Rarity

Rare

For collectors

## Quality Criteria The most prized specimens are those with sharply terminated, well-formed, lustrous crystals of intense, dark green or blue-green color. The attractiveness is enhanced by a contrasting background of light matrix rock, such as white albite or nepheline. Large, single crystals or aesthetic, radial aggregates are sought after by collectors. ## Popular Localities The best quality specimens come from classic localities for alkaline minerals, such as Mont Saint-Hilaire in Canada and the Khibiny and Lovozero massifs in Russia. Crystals from these locations often serve as a benchmark for the species in terms of form and mineral associations.

Care and storage

## Cleaning Specimens should be cleaned carefully, using a soft brush and distilled water. A brief ultrasonic bath can be used, but with great caution, especially for specimens with fragile, acicular crystals or on a delicate matrix. ## What to Avoid Avoid contact with strong acids, which can damage the mineral. It is not sensitive to light, but sudden temperature changes should be avoided, as they can cause fractures along cleavage planes. ## Storage Store in stable conditions, away from dust and chemicals. Specimens with well-formed crystals are best displayed in closed display cases or perky boxes to protect them from mechanical damage.

Sources

Read more