Aluminokatophorite

Chemical formula: Na₂CaFe₂+₄Al(Si₇Al)O₂₂(OH)₂

Aluminocathophorite is a rare, dark-colored amphibole group mineral, primarily found in high-pressure metamorphic rocks.

## Characteristics Aluminocathophorite is a complex silicate of sodium, calcium, iron, and aluminum, belonging to the amphibole supergroup. This mineral forms prismatic or acicular crystals, but most commonly occurs as disseminated grains within rocks or as fibrous and columnar aggregates. Due to its chemical composition, its precise identification is almost exclusively possible using advanced analytical techniques, such as electron microprobe analysis. It is practically indistinguishable from other dark amphiboles with the naked eye. ## Physical Properties The hardness of aluminocathophorite on the Mohs scale is approximately 6. It exhibits a vitreous luster, which can be slightly pearly on cleavage planes. It is a brittle mineral, translucent in thin fragments, and opaque in thicker sections or in mass. Its density ranges from 3.3 to 3.4 g/cm³. ## Colors and Varieties This mineral ranges in color from dark green, through brownish-black, to black. The color variation is associated with slight fluctuations in the content of iron and other elements. No named color varieties or commercial varieties are distinguished. ## History and Name The name "aluminocathophorite" was formally approved by the International Mineralogical Association (IMA) as part of the reclassification of amphiboles. The first part of the name, "alumino-", indicates the dominant role of aluminum in a specific structural site, distinguishing it from other minerals in the cathophorite group. The name "cathophorite" comes from the Greek word *katáphero* (καταφέρω) meaning "to bring down", which may refer to its occurrence in volcanic rocks. ## Applications Aluminocathophorite has no industrial or commercial applications. Its significance is purely scientific, as an indicator of metamorphic conditions, and as a collector's item for those specializing in rare and rock-forming minerals.

Properties

Mohs hardness: 6
Luster: Vitreous
Streak: Greenish grey
Density: 3.3-3.4
Cleavage: Perfect on {110}
Fracture: Uneven
Transparency: Translucent to opaque
Crystal system: Monoclinic

Diagnostic features

## Identification Identification of aluminocathophorite in the field or based on visual characteristics is impossible. It looks identical to many common amphiboles. A characteristic feature of amphiboles is perfect cleavage in two directions, intersecting at angles of approximately 56° and 124°, which can be observed on some specimens under magnification. Definitive identification requires chemical analysis (e.g., EDS, WDS) and structural analysis (XRD). ## Distinguishing from Similar Minerals Aluminocathophorite can be confused with numerous minerals from the amphibole group, such as hornblende, arfvedsonite, barkevikite, and especially with other minerals from the cathophorite subgroup (e.g., magnesiocathophorite). The differences between them are purely chemical and cannot be determined by macroscopic observation. ## Crystal Forms It crystallizes in elongated, columnar, or acicular forms. It often forms radial, fibrous, or granular aggregates. Well-formed, isolated crystals are rare and usually small in size.

Geological environment

## Genesis Aluminocathophorite is a characteristic mineral of rocks that have undergone metamorphism under high-pressure and varying temperature conditions. It primarily forms in glaucophane schists (blueschist facies) and eclogites. It can also occur in some alkaline igneous rocks, such as nepheline syenites. ## Mineral Associations Depending on the environment of formation, it coexists with minerals such as: glaucophane, crossite, epidote, lawsonite, garnet (almandine, pyrope), omphacite, jadeite, phengite, paragonite, aegirine, nepheline, and albite. ## Localities The most important confirmed occurrences of this mineral include specific metamorphic localities worldwide. It is found, among other places, on Syros Island in Greece; in the Piedmont region of Italy; in California (USA); and in several localities in Japan. These are places known for the occurrence of blueschist facies rocks.

Rarity

Very rare

For collectors

## Quality Criteria The collector's value of aluminocathophorite is not based on its aesthetics, but on its rarity and confirmed identity. Specimens for which chemical analysis has been performed are most prized. The value of a specimen is enhanced by rich mineralization in the host rock and the presence of well-formed, even if small, crystalline forms. Association with other rare high-pressure minerals is also important. ## Market Prices The market for this mineral is very limited and mainly concerns specialized collectors of rare minerals and microminerals. Prices are usually low to moderate, with the main factor being the cost and availability of analysis confirming the chemical composition of the specimen. ## Popular Localities The most sought-after specimens by collectors come from classic high-pressure rock localities, such as Syros Island in Greece, where this mineral occurs in association with other rare amphiboles and pyroxenes.

Care and storage

## Cleaning Aluminocathophorite specimens should be cleaned mechanically, using a soft brush. Distilled water can be used, avoiding strong detergents. Cleaning in ultrasonic cleaners is risky and may lead to damage of specimens containing internal fractures. ## What to Avoid The mineral is sensitive to strong acids, especially hydrofluoric acid, which etches silicates. Sudden temperature changes, which can cause crystal fracturing, should be avoided. ## Storage Aluminocathophorite is a stable mineral. It does not require special storage conditions. It is recommended to keep it in separate, sealed, labeled boxes to protect it from dust and mechanical damage and to avoid confusion with other similar minerals.

Sources

IMA Mineral List (CNMNC)