Aluminobarroisite

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>

Dark green to black amphibole group mineral, found in high-pressure metamorphic rocks.

## Characteristics Aluminobarroisite is a sodium-calcium amphibole group mineral, closely related to barroisite. It forms prismatic, elongated crystals, often fibrous or asbestiform in habit, which rarely reach large sizes. It typically occurs as small grains embedded in rock or as acicular aggregates. Its color is dark green, greenish-black, or almost black. ## Physical Properties This mineral is characterized by a hardness of 6 on the Mohs scale and a vitreous luster. It is translucent, and its density, calculated from the chemical formula, is approximately 3.16 g/cm³. It exhibits perfect cleavage in two directions, typical of amphiboles. ## Colors and Varieties Aluminobarroisite occurs in shades from dark green to black. No color varieties or trade names are distinguished for it, as it is a mineral defined primarily by its chemical composition rather than visual characteristics. ## History and Name The mineral's name refers to its chemical composition – high aluminum content – and its relation to the previously known mineral barroisite. It was formally recognized as a distinct mineral species by the International Mineralogical Association (IMA) in 2003. ## Applications Aluminobarroisite has no industrial applications. Its significance is purely scientific, as an indicator of high-pressure metamorphic conditions, and for collectors, for specialists and systematic mineral collectors.

Properties

Mohs hardness
6
Luster
Vitreous
Streak
Pale brownish green
Density
3.16
Cleavage
Perfect on {110}
Fracture
Uneven
Transparency
Translucent
Crystal system
Monoclinic

Diagnostic features

## Identification Identifying aluminobarroisite is extremely difficult and practically impossible without specialized equipment. In the field, its dark color, prismatic habit, and occurrence in high-pressure metamorphic rocks, such as eclogites, can be indicators. The streak is pale greenish-brown. ## Distinguishing from Similar Minerals Aluminobarroisite is visually indistinguishable from many other dark amphiboles, such as hornblende, arfvedsonite, glaucophane, riebeckite, or barroisite itself. Definitive differentiation requires advanced analytical methods, such as electron microprobe (EDS/WDS) or X-ray diffraction (XRD), to precisely determine the chemical composition. ## Crystal Forms It most often forms columnar, prismatic, or acicular crystals, which can form radial, fibrous, or tangled aggregates. It usually occurs as small grains within the rock mass.

Geological environment

## Genesis Aluminobarroisite is a characteristic mineral of rocks that have undergone metamorphism under high-pressure and medium-temperature conditions. It forms in the eclogite facies and glaucophane schist (blueschist) facies, mainly as a result of the alteration of basaltic rocks. ## Mineral Associations This mineral often co-occurs with other high-pressure indicator minerals, such as omphacite, garnet (especially pyrope and almandine), phengite, epidote, quartz, and rutile. ## Localities The most important and best-studied occurrence is the type locality in Tyrol, Austria (Sesknöser Schiefer, Defereggen Valley). It has also been reported from other high-pressure metamorphic areas worldwide, for example, from the Dominican Republic.

Rarity

Rare

For collectors

## Quality Criteria As aluminobarroisite is mainly a microscopic mineral or forms small crystals, for collectors, the most important aspect is the quality of the crystal itself – its habit, luster, and lack of damage. Specimens where aluminobarroisite is clearly visible and occurs in association with other rare metamorphic minerals, such as omphacite, are highly valued. Reliable analysis confirming identification is crucial. ## Popular Localities The most sought-after specimens by specialized collectors are those from the type locality in Austria, as these are the best-characterized materials in the world.

Care and storage

## Cleaning Specimens should be cleaned gently, using a soft brush and distilled water. Due to its possible fibrous structure and perfect cleavage, ultrasonic cleaners are not recommended, as they can cause damage or disintegration of aggregates. ## What to Avoid The mineral is sensitive to strong acids. Avoid sudden temperature changes. Despite its general stability, delicate, fibrous specimens are susceptible to mechanical damage. ## Storage Store in stable conditions, away from dust. Specimens, especially those of micromount character, should be stored in sealed, labeled boxes to prevent damage and identification errors.

Sources

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