Alumino-ferrobarroisite

Chemical formula: &#₉₇₄₄;NaCa(Fe₂+₃Al₂)(Si₇Al)O₂₂(OH)₂

A very rare amphibole of the barroisite group, forming dark green to black, prismatic crystals in metamorphic rocks.

## Characteristics Alumino-ferrobarroisite is an amphibole group mineral, belonging to the barroisite series. It occurs as prismatic, often acicular crystals, dark green, greenish-black, or black in color. It usually forms small grains or radial aggregates embedded in the host rock. Well-formed, isolated crystals are rarely found, making it a mineral valued mainly by specialized collectors. ## Physical Properties Its Mohs hardness is 5-6, which is typical for amphiboles. It has a vitreous luster, and on cleavage planes, it can be slightly pearly. It is translucent in thin fragments and opaque in larger specimens. The calculated density is approximately 3.23 g/cm³. ## Colors and Varieties Dark green to black is the dominant color. Color variability depends on slight variations in chemical composition, mainly the iron to magnesium and aluminum ratio. No named color varieties or commercial varieties are distinguished. ## History and Name The mineral's name refers to its chemical composition – the presence of aluminum (alumino) and iron (ferro) – and its relation to the structurally similar mineral, barroisite. It was officially recognized as a new mineral species by the International Mineralogical Association (IMA) in 2016. ## Uses Alumino-ferrobarroisite has no industrial application. Its significance is purely scientific, as an indicator of metamorphic conditions, and collectible, as a rare mineralogical species.

Properties

Mohs hardness: 5-6
Luster: Vitreous
Streak: Pale greenish grey
Density: 3.23
Cleavage: Perfect on {110}
Fracture: Uneven
Transparency: Translucent to Opaque
Crystal system: Monoclinic

Diagnostic features

## Identification Helpful identification features include: dark green or black color, prismatic crystal habit, and occurrence in metamorphic rocks. The characteristic amphibole cleavage in two directions (angles of approximately 56° and 124°) is a key feature, although difficult to observe in fine grains. ## Distinguishing from Similar Minerals Alumino-ferrobarroisite is macroscopically indistinguishable from other dark amphiboles, such as hornblende, ferro-hornblende, or other minerals of the barroisite group. It can also be confused with pyroxenes (e.g., augite), which, however, have cleavage at an angle close to 90°. Definitive identification requires advanced chemical (EDS) and structural (XRD) analyses. ## Crystal Forms It forms elongated, columnar, or acicular crystals. They often occur as disordered or radial granular and fibrous aggregates within the rock.

Geological environment

## Genesis This is a mineral typical of metamorphic rocks that have undergone transformations under high-pressure and medium to high-temperature conditions. It forms mainly in amphibolites, eclogites, and glaucophane schists. ## Mineral Associations It often co-occurs with minerals such as garnet (especially almandine), omphacite, epidote, rutile, phengite, quartz, and plagioclase. ## Localities As a rare mineral, it is known from few localities worldwide. The type locality (the place of first finding and description) is in the Pierrepont area, St. Lawrence County, New York, USA. Its occurrence has also been confirmed in Austria (Styria) and Japan.

Rarity

Very rare

For collectors

## Quality Criteria The most highly valued specimens are those that contain well-formed, sharply terminated crystals with a distinct luster. Due to its rarity, even microscopic but well-formed crystals are sought after. The value of a specimen is enhanced by its association with other rare metamorphic minerals. ## Popular Localities Specimens from the type locality in New York State, USA, are most desired by collectors specializing in mineral systematics, as they serve as reference material for this species.

Care and storage

## Cleaning Alumino-ferrobarroisite specimens are relatively durable. They can be cleaned with a soft brush using distilled water. Ultrasonic cleaners should be avoided, as they could damage the specimen along cleavage planes. ## What to Avoid The mineral is sensitive to strong acids. Avoid sudden temperature changes, impacts, and pressure, which can cause fracturing along the perfect cleavage. ## Storage Store in stable conditions, away from dust and direct sunlight, which, however, does not cause it to fade. Microscopic specimens are best stored in special "micromount" boxes to protect delicate crystals from mechanical damage.

Sources

IMA Mineral List (CNMNC)