Barroisite

Chemical formula: &#9744;NaCa[Mg<sub>3</sub>Al<sub>2</sub>](Si<sub>7</sub>Al)O<sub>22</sub>(OH)<sub>2</sub>

Barroisite is a rare amphibole group mineral, forming prismatic crystals ranging in color from blue-green to black, found in metamorphic rocks.

## Characteristics Barroisite is a complex silicate of sodium, calcium, magnesium, and aluminum, belonging to the amphibole supergroup. It forms elongated, prismatic crystals, often terminated by steeply sloped faces. It also occurs as fibrous, radial aggregates or as granular masses within the host rock. Its crystals rarely reach large sizes, and well-formed specimens are prized by collectors. ## Physical Properties This mineral is characterized by a hardness of 5-6 on the Mohs scale. It has a vitreous luster and is typically translucent to opaque. The density of barroisite ranges from 3.11-3.26 g/cm³. ## Colors and Varieties Barroisite most commonly exhibits colors ranging from blue-green, through dark green, gray-green, to black. It displays pleochroism, meaning a change in hue depending on the observation direction, usually in yellowish-green to blue-green tones. No named varieties are distinguished. ## History and Name The mineral's name honors Charles Barrois (1851-1939), a French geologist and paleontologist, professor of geology at the University of Lille. The mineral was first described in 1897. ## Uses Barroisite has no industrial significance. It is solely an object of scientific and collecting interest.

Properties

Mohs hardness
5-6
Luster
Vitreous
Streak
Grayish-green
Density
3.11-3.26
Cleavage
Perfect on {110}
Fracture
Uneven
Transparency
Translucent to opaque
Crystal system
Monoclinic

Diagnostic features

## Identification Barroisite can be identified by its prismatic crystal habit, characteristic blue-green to black color, and occurrence in metamorphic rocks. An important diagnostic feature is its perfect cleavage in two directions, intersecting at angles of approximately 56° and 124°, typical of amphiboles. ## Distinguishing from Similar Minerals Barroisite is difficult to distinguish from other dark amphiboles, such as hornblende, glaucophane, or actinolite, without advanced studies (chemical analysis, optical methods). Glaucophane usually has a more blue hue, and actinolite is typically green. Definitive identification requires specialized equipment. ## Crystal Forms Barroisite crystals are typically prismatic, elongated along one axis. They often form columnar, fibrous, radial aggregates or granular masses embedded in the rock.

Geological environment

## Genesis Barroisite is a characteristic mineral of metamorphic rocks that have undergone transformation under conditions of high pressure and moderate to high temperatures. It forms mainly in blueschists, eclogites, and amphibolites. ## Mineral Associations This mineral often co-occurs with other minerals typical of the blueschist and eclogite facies, such as glaucophane, omphacite, garnet (especially almandine and pyrope), epidote, phengite, paragonite, lawsonite, rutile, and quartz. ## Localities Key barroisite localities worldwide include the Western Alps, particularly in the Piedmont region of Italy and Corsica (France). It also occurs in Greece (Cyclades), Japan (Sanbagawa region), Cuba, Venezuela, and California (USA).

Rarity

Rare

For collectors

## Quality Criteria The most prized specimens by collectors are those with well-formed, sharply terminated crystals of intense, blue-green color. Contrasting host rock, such as light quartz or phengite, enhances their appeal. Large, single crystals or aesthetic groups of radial aggregates are rare and sought after. ## Popular Localities The best quality collector specimens, known for their sharp crystals, come from Alpine localities in Italy, especially the Aosta Valley and Piedmont. Specimens from the island of Syros in Greece are also valued in the market.

Care and storage

## Cleaning Barroisite specimens should be cleaned carefully, using a soft brush and distilled water. Ultrasonic cleaners should be avoided, as they can damage fragile crystals or fibrous aggregates. ## What to Avoid The mineral is sensitive to strong acids, which can etch it. It should be protected from sudden temperature changes and prolonged exposure to direct sunlight, although the latter is not considered particularly harmful. ## Storage Barroisite specimens, especially those with a delicate, fibrous structure, should be stored in separate boxes or display cases to prevent abrasion and mechanical damage. They should be protected from dust, which can be difficult to remove from aggregates.

Sources

Read more