Ottrélite

Chemical formula: Mn₂+Al₂O(SiO₄)(OH)₂

Ottrelite is a rare, greenish or blackish mineral from the chloritoid group, forming small, tabular crystals in metamorphic rocks.

## Characteristics Ottrelite is a mineral from the chloritoid group, belonging to the silicates. It most often occurs as small, scattered, hexagonal or rounded tabular crystals within the host rock. It rarely forms rosette-like or stellate aggregates. Its crystals are usually very small, rarely exceeding 1-2 mm, which makes identification with the naked eye difficult. It is most commonly observed as greenish or blackish, shiny flakes within schists. ## Physical Properties This mineral is characterized by a hardness in the range of 6-7 on the Mohs scale, making it relatively resistant to scratching. It has a vitreous, somewhat pearly luster on cleavage surfaces. It is a rather brittle mineral. Its density is approximately 3.4 g/cm³. ## Colors and Varieties Ottrelite occurs in shades from grayish-green and pistachio-green to almost black. Its color is closely related to its manganese and iron content. There are no named commercial or gemological varieties, and its value is purely scientific and collectible. ## History and Name The mineral's name comes from the location of its first description – the village of Ottré in the municipality of Vielsalm, in the Ardennes (Belgium). It was first described in 1819. ## Uses Ottrelite has no industrial applications. Its significance is limited to its role as an indicator of metamorphic conditions in geological studies and as an interesting mineral for specialized collectors of rare silicates.

Properties

Mohs hardness: 6-7
Luster: Vitreous
Streak: Grayish-green
Density: 3.41
Cleavage: Good on {001}
Fracture: Uneven
Transparency: Translucent to opaque
Crystal system: Triclinic

Diagnostic features

## Identification Characteristic features of ottrelite include its small, most often pseudohexagonal in outline, tabular crystals of greenish or black color, embedded in metamorphic rocks such as phyllites and schists. A vitreous to pearly luster on cleavage surfaces and relatively high hardness are also helpful in identification. ## Distinguishing from Similar Minerals Ottrelite is sometimes confused with other platy minerals such as chlorite or biotite. It is distinguished from chlorite by its significantly higher hardness (chlorite is very soft, about 2-2.5 on the Mohs scale). It differs from biotite in color (biotite is usually brown to black) and often a more regular, hexagonal crystal outline. Final differentiation from other members of the chloritoid group requires advanced chemical analysis. ## Crystal Forms Crystals are typically thin- to thick-tabular, with a pseudohexagonal outline. They often occur as single, scattered flakes in the rock. Less commonly, they form rosette-like or stellate aggregates.

Geological environment

## Genesis Ottrelite is a mineral typical of rocks that have undergone low- to medium-grade regional metamorphism, under greenschist and glaucophane facies conditions. It forms in aluminum- and manganese-rich pelitic sediments (shales, siltstones). It is an indicator mineral, helping to determine the pressure and temperature prevailing during metamorphic processes. ## Mineral Associations This mineral often co-occurs with quartz, sericite (fine-grained muscovite), chlorite, chloritoid, pyrophyllite, almandine, spessartine, carpholite, and diaspore. ## Localities The most important occurrences of ottrelite include the historic locality in Ottré, Ardennes (Belgium). It is also found in the Alps (Switzerland, Italy), in Greece (Euboea island), in the United States (Massachusetts), and in Japan.

Rarity

Rare

For collectors

## Quality Criteria The collector appeal of ottrelite specimens primarily depends on the size and development of the crystals. Most valued are specimens with distinct, sharply defined, pseudohexagonal crystals that contrast colorfully with the host rock. Large crystals for this mineral (above a few millimeters) or rare rosette-like aggregates are particularly sought after. Purity and lack of mechanical damage are crucial. ## Popular Localities Classic and most prized specimens come from the type locality in Ottré, Belgium. Specimens from Alpine localities in Switzerland and Italy also provide good quality collector material.

Care and storage

## Cleaning Ottrelite specimens, most often found in host rock, should be cleaned very carefully. It is best to use a soft brush to remove dust. If necessary, the specimen can be rinsed with distilled water and then thoroughly dried. Ultrasonic cleaners should be avoided, as they can damage delicate crystals or cause them to detach from the matrix. ## What to Avoid Contact with strong acids and bases, which can react with the mineral, should be avoided. Despite its relatively high hardness, it is brittle, so it should be protected from impacts and falls. It is not sensitive to sunlight, but prolonged storage in humid conditions is not recommended. ## Storage Ottrelite specimens are best stored in separate, padded collector boxes to prevent abrasions and mechanical damage. Since the crystals are usually small and embedded in rock, the entire specimen should be handled gently to avoid chipping off small flakes of the mineral.

Sources

IMA Mineral List (CNMNC)