Johnsomervilleite

Chemical formula: Na<sub>3</sub>CaFe<sup>2+</sup><sub>11</sub>(PO<sub>4</sub>)<sub>9</sub>

Johnsomervilleite is a rare sodium, calcium, and iron phosphate, forming small, hexagonal, brown crystals.

## Characteristics Johnsomervilleite is a complex phosphate belonging to the fillowite group. It occurs as small, hexagonal, tabular crystals, usually not exceeding 1 mm. It most commonly forms granular aggregates or rosette-like clusters. Its characteristic features include a brown to reddish-brown color and a vitreous luster. ## Physical Properties This mineral has a hardness of approximately 4 on the Mohs scale. It exhibits a vitreous luster and is translucent. The density of johnsomervilleite is about 3.46 g/cm³. ## Colors and Varieties Johnsomervilleite occurs in shades of brown, from light brown through reddish-brown to dark brown. No varieties have been distinguished. ## History and Name The mineral is named in honor of John Somerville (1904-1992), a British chemist and micromounter of minerals, who first discovered this mineral. It was described and approved as a new mineral species in 1980 by A. M. Clark, E. E. Fejer, and D. G. Couper. ## Uses Johnsomervilleite has no industrial applications. It is solely an object of interest for collectors specializing in rare minerals and micromounts.

Properties

Mohs hardness
4
Luster
Vitreous
Streak
Light brown
Density
3.46
Cleavage
Good on {0001}
Fracture
Uneven
Transparency
Translucent
Crystal system
Trigonal

Diagnostic features

## Identification Johnsomervilleite can be identified by its characteristic hexagonal, tabular crystal habit, brown color, and vitreous luster. It occurs in a specific geological environment – in granitic pegmatites. Due to the small size of the crystals, a microscope is often necessary for its identification. ## Distinguishing from Similar Minerals It can be confused with other brown phosphates found in pegmatites, such as members of the fillowite group. Definitive differentiation requires advanced analytical methods, such as chemical analysis (EDS) or X-ray diffraction (XRD), which allow for the determination of its unique composition and crystal structure. ## Crystal Forms It forms small, tabular crystals with a hexagonal outline. These often occur as rosette-like or granular aggregates.

Geological environment

## Genesis Johnsomervilleite is a mineral of magmatic origin. It forms in the late stages of crystallization of granitic pegmatites, where high concentrations of phosphorus, sodium, and iron are present. ## Mineral Associations This mineral co-occurs with other phosphates typical of pegmatites, such as triplite, especially in its altered form known as heterosite. It is also associated with vivianite, ludlamite, and strengite. ## Localities The most important and type locality (locus typicus) is the Loch Quoich quarry, near Invergarry in Scotland, United Kingdom. This is the only confirmed and well-documented locality for this mineral worldwide.

Rarity

Very rare

For collectors

## Quality Criteria Specimens with well-formed, sharp crystals with a distinct hexagonal outline are most valued by collectors. The intensity and attractiveness of the brown color are also important. Since the crystals are small, specimens with larger, well-isolated crystals or aesthetic rosettes on matrix are more highly prized. Contrast with associated minerals, e.g., lighter quartz, enhances visual appeal. ## Popular Localities The only source of johnsomervilleite specimens is its type locality in Scotland. Material from this location is extremely limited, making it a great rarity in the collector's market.

Care and storage

## Cleaning Specimens should be cleaned very carefully, using a soft brush to remove dust. Due to its rarity and small crystal size, wet cleaning is not recommended to avoid damaging delicate aggregates. ## What to Avoid Avoid contact with acids and other chemicals that may react with phosphates. The mineral is relatively soft, so it must be protected from scratches and impacts. It should not be heated or subjected to ultrasound. ## Storage The best way to store specimens is to place them in a sealed "micromount" box, which protects them from dust, mechanical damage, and sudden changes in humidity.

Sources

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