Ferriallanite-(Ce)

Chemical formula: CaCe₃+Fe₂+Fe₃+Al(SiO₄)(Si₂O₇)O(OH)

Ferriallanite-(Ce) is a rare mineral of the epidote group, characterized by high iron, cerium, and calcium content.

## Characteristics Ferriallanite-(Ce) is a complex silicate belonging to the epidote group. It typically forms small, prismatic crystals with a tabular or columnar habit, often terminated by steep faces. Specimens are usually black, opaque, with a vitreous or resinous luster. This mineral is brittle and exhibits an uneven fracture. Due to its rarity and small crystal size, it is primarily of interest to collectors specializing in rare minerals and to scientists. ## Physical Properties Ferriallanite-(Ce) crystals are characterized by a hardness of 5.5-6 on the Mohs scale. They have a vitreous to resinous luster. The mineral's density is relatively high, approximately 3.85 g/cm³. It is a brittle mineral with an uneven fracture. ## Colors and Varieties Ferriallanite-(Ce) occurs in a uniform, black color. Its streak is grayish-brown. No color varieties or commercial varieties are distinguished. ## History and Name The name "ferriallanite" refers to its chemical composition – a high iron content (Latin: *ferrum*) – and its relation to the allanite structure. The "-(Ce)" suffix indicates the dominant rare-earth element in its composition, which is cerium. The mineral was formally described and approved by the International Mineralogical Association (IMA) in 2000. ## Uses Ferriallanite-(Ce) has no industrial applications. Its significance is purely scientific and collectible.

Properties

Mohs hardness: 5.5-6
Luster: Vitreous to resinous
Streak: Greyish-brown
Density: 3.85
Cleavage: Imperfect on {001}
Fracture: Uneven
Transparency: Opaque
Crystal system: Monoclinic

Diagnostic features

## Identification Ferriallanite-(Ce) is identified by its black color, vitreous to resinous luster, and characteristic tabular or prismatic crystals. An important diagnostic feature is its high density and its co-occurrence with other pegmatitic minerals. However, definitive identification requires advanced analytical methods, such as chemical composition analysis (EDS/WDS). ## Distinguishing from Similar Minerals This mineral can be confused with other black minerals of the epidote group, such as allanite-(Ce), as well as with tourmaline (schorl), ilmenite, or hornblende. It differs from allanite-(Ce) by its higher trivalent iron content. It is distinguished from schorl by its lower hardness and different crystal habit (schorl often forms crystals with a triangular cross-section). It differs from ilmenite by its lower, non-submetallic luster and lack of magnetic properties. ## Crystal Forms Crystals are usually small, up to several millimeters in length. They adopt a tabular form, flattened parallel to the (100) plane, or short prisms. They often occur as embedded grains or irregular aggregates within the rock.

Geological environment

## Genesis Ferriallanite-(Ce) is a mineral of magmatic origin. It forms in the late stages of crystallization in granite pegmatites, rich in rare-earth elements. It forms under conditions of elevated pressure and temperature. ## Mineral Associations This mineral co-occurs with typical pegmatitic minerals such as quartz, feldspars (mainly microcline and albite), biotite, zircon, and other minerals containing rare-earth elements. ## Localities The most important and well-documented occurrences of ferriallanite-(Ce) are found in Italy. The type locality is the pegmatites in the Alpe di Siusi (Seiser Alm) area in South Tyrol. It is also known from pegmatites in Val Vigezzo in Piedmont.

Rarity

Very rare

For collectors

## Quality Criteria The most prized specimens by collectors are those with well-formed, sharp, and lustrous crystals, clearly contrasting with the host rock (matrix). The absence of mechanical damage is important. Due to the small size of the crystals, even specimens a few millimeters in size, if well-formed, are considered valuable. Contrast with a light matrix (e.g., white feldspar) significantly enhances the visual appeal of the specimen. ## Popular Localities The most known and valued specimens come from the Italian Alps, especially from Alpe di Siusi in South Tyrol and from Val Vigezzo in Piedmont. Specimens from these localities are considered classic for this mineral.

Care and storage

## Cleaning Specimens should only be cleaned mechanically, using a soft brush or brush to remove dust. Compressed air may be used. For heavier soiling, a damp (not wet) cloth with distilled water can be used, followed by immediate drying of the specimen. ## What to Avoid Ultrasonic cleaners and all chemical agents, including acids and detergents, which can damage the mineral surface, should be absolutely avoided. Ferriallanite-(Ce) is brittle, so it should be protected from impacts and falls. ## Storage It is recommended to store specimens in separate, padded collector's boxes to avoid scratches and mechanical damage. It should be protected from moisture and sudden temperature changes.

Sources

IMA Mineral List (CNMNC)