Edgarite

Chemical formula: Fe³⁺Nb³⁺₃S²⁻₆

Edgarite is a rare, metallic niobium and iron mineral, found in nepheline syenites.

## Characteristics Edgarite is a rare iron niobium sulfide. It forms very small, elongated crystals with a lamellar habit, usually up to 0.2 mm long. It most often occurs as inclusions in other minerals, mainly in sodalite. Visually, it is opaque, with a metallic luster and a black color. ## Physical Properties This mineral is characterized by its black color and black streak. Its luster is metallic. Due to the small size of the crystals, precise determination of hardness and density is difficult; these values have not been accurately measured. ## History and Name Edgarite was first described in 1982 by A.D. Edgar, J.M. Charbonneau, and A.G. Paulin. Its name honors Alan Drummond Edgar (1935–2009), a professor of mineralogy at the University of Western Ontario in Canada, for his contributions to research on alkaline rocks. ## Uses Due to its extreme rarity and microscopic size, edgarite has no commercial or industrial applications. It is of purely scientific significance and is of interest to specialized collectors of rare minerals.

Properties

Mohs hardness
2-2.5
Color
Dark grey
Luster
Metallic
Streak
Black
Density
0
Cleavage
basal
Fracture
Uneven
Transparency
Opaque
Crystal system
Hexagonal

Diagnostic features

## Identification Identification of edgarite is possible only through advanced laboratory methods, such as chemical composition analysis (EDS) using a scanning electron microscope (SEM) or X-ray diffraction (XRD). Macroscopic visual identification is impossible. ## Distinguishing from similar minerals It can be confused with other small, black, metallic sulfide or oxide minerals occurring in the same environment. Definitive differentiation requires specialized analysis. ## Crystal forms Edgarite forms small, lamellar, elongated crystals, often grouped into small aggregates. It typically occurs as inclusions in other minerals.

Geological environment

## Genesis Edgarite forms in the late stages of crystallization within pegmatites associated with alkaline intrusions, such as nepheline syenites. It crystallizes under conditions of low sulfur fugacity. ## Mineral associations This mineral coexists with sodalite (in which it often forms inclusions), aegirine, eudialyte, microcline, nepheline, arfvedsonite, and other rare minerals such as rasvumite. ## Localities The only confirmed and thoroughly described locality of this mineral in the world (type locality) is the Ilímaussaq alkaline complex in Greenland.

Rarity

Extremely rare

For collectors

## Quality criteria The quality of an edgarite specimen depends solely on the confirmation of its presence through analysis and on the richness and size of its associated minerals from the Ilímaussaq complex. Edgarite itself is too small to assess its individual features, such as form or luster. ## Popular localities The only source of specimens is the Ilímaussaq complex in Greenland. Material from this locality is highly prized by collectors specializing in rare mineral species and minerals from alkaline rocks.

Care and storage

## Cleaning Specimens containing edgarite, usually as inclusions in the host rock, can only be cleaned with compressed air to remove dust. Contact with water and chemical agents should be avoided. ## What to avoid All wet cleaning methods, ultrasonics, and any chemicals that could damage both the microscopic edgarite crystals and the surrounding minerals, which are often sensitive (e.g., sodalite), should be avoided. ## Storage Specimens should be stored in stable, dry conditions, away from dust. The best solution is a closed "micromount" box, which protects against mechanical damage and contamination.

External references

Sources

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