Sellaite

Chemical formula: MgF<sub>₂</sub>

Sellaite is a rare magnesium fluoride, forming colorless or white, vitreous crystals, valued by collectors for its rarity and classic localities.

## Characteristics Sellaite is a magnesium fluoride belonging to the rutile group. It occurs as well-formed, prismatic or acicular crystals, often gathered in radial or stellate aggregates. It is also found in granular masses or fibrous aggregates. It is a brittle mineral with a vitreous luster. Pure crystals are colorless and transparent, but impurities can give it a whitish, yellowish, or grayish tint. ## Physical Properties Sellaite is characterized by a relatively high hardness, ranging from 5 to 6 on the Mohs scale. Its density varies from 3.13 to 3.18 g/cm³. The luster is typically vitreous. Crystals can be transparent to translucent. It exhibits perfect cleavage in two directions. ## Colors and Varieties This mineral is most commonly colorless or white. Rarer specimens may be yellowish, gray, or even pink or violet. No named color varieties or commercial varieties are distinguished. ## History and Name Sellaite was first described in 1868 by the Italian engineer and mineralogist Quintino Sella. The mineral was named in honor of his brother, the geologist and politician, Quintino Sella (1827-1884), who was also Prime Minister of Italy. Type specimens come from the Gébroulaz Glacier in Savoy, France. ## Uses Due to its rarity, sellaite has no industrial applications. It is solely an object of interest for mineral collectors and scientific institutions.

Properties

Mohs hardness: 5-6
Luster: Vitreous
Streak: White
Density: 3.13-3.18
Cleavage: Perfect on {110} and {100}
Fracture: Conchoidal to uneven
Transparency: Transparent to translucent
Crystal system: Tetragonal

Diagnostic features

## Identification Sellaite can be identified by its tetragonal, prismatic or acicular crystals, often forming radial aggregates. Characteristic features include its vitreous luster, relatively high hardness (scratches glass), and occurrence in specific geological environments. Under ultraviolet light, it may exhibit weak, yellowish or creamy fluorescence. ## Distinguishing from Similar Minerals Sellaite is sometimes confused with quartz, from which it differs by its higher density and perfect cleavage (quartz does not possess cleavage). From other fluorides, such as fluorite, it differs by its higher hardness and different crystal form (fluorite crystallizes in the isometric system, forming cubes and octahedra). From minerals of the zeolite group, it is distinguished by greater hardness and lack of reaction with acids. ## Crystal Forms Sellaite crystals are typically elongated, prismatic, with a square cross-section, terminated by a bipyramid. They often occur as slender needles. It forms radial, stellate, fibrous aggregates, as well as granular and massive occurrences.

Geological environment

## Genesis Sellaite forms in several different environments. It is most commonly found in hydrothermal veins, where it crystallizes from solutions rich in magnesium and fluorine. It also occurs in contact metamorphic zones where magma interacts with dolomitic and magnesitic rocks. It can be found in marine evaporites (salt deposits) and as a product of volcanic exhalations (sublimates). ## Mineral Associations This mineral often co-occurs with fluorite, anhydrite, gypsum, celestine, sulfur, magnesite, dolomite, and in volcanic environments with various other fluorides. ## Localities The most important and classic localities for sellaite are the Gébroulaz Glacier in Savoy (France) and the salt mine in Bleicherode in the Harz Mountains (Germany). Significant specimens also come from the Brumado massif in Bahia (Brazil), where it occurs in association with magnesite and quartz. Other known occurrences include the Vesuvius region in Italy, Hunan province in China, and some mines in Karlovy Vary in the Czech Republic.

Rarity

Rare

For collectors

## Quality Criteria The most prized sellaite specimens by collectors are well-formed, transparent, and colorless crystals, especially those forming aesthetic, radial aggregates. Specimens from classic, historical localities, such as Gébroulaz, are highly valued. Crystal size is also important – large, undamaged prisms are rare. A contrasting association with the matrix mineral (e.g., with magnesite in specimens from Brumado) significantly increases the attractiveness of the specimen. ## Popular Localities Collectors primarily seek specimens from three localities: Brazil (Brumado, Bahia) for beautiful, transparent crystals on magnesite; Germany (Bleicherode) for historical significance and classic aggregates; and Italy (Vesuvius) and France (Gébroulaz) as type localities.

Care and storage

## Cleaning Sellaite specimens should be cleaned very carefully. It is best to use a soft brush to remove dust. For heavier soiling, distilled water and a soft brush can be used, followed by thorough drying of the specimen. Ultrasonic cleaners should be avoided, as they can damage crystals due to cleavage. ## What to Avoid Sellaite is sensitive to strong acids. It should be protected from sudden temperature changes and prolonged exposure to direct sunlight, although the latter is not considered particularly harmful. Due to its perfect cleavage, it is sensitive to impact and pressure. ## Storage Sellaite specimens are best stored in separate, padded boxes or display cases to prevent scratching by harder minerals and protect them from mechanical damage. Stable environmental conditions, without high humidity and temperature fluctuations, should be ensured.

External references

https://en.wikipedia.org/wiki/https%3A%2F%2Fen.wikipedia.org%2Fwiki%2FSellaite