Ferri-mottanaite-(Ce)

Chemical formula: Ca₄Ce₃+₂Fe₃+(Be₁.₅&#₉₇₄₄;₀.₅)[Si₄B₄O₂₂]O₂

Ferri-mottanaite-(Ce) is a very rare calcium, cerium, and iron borosilicate of the hellandite group, forming dark brown, prismatic crystals.

## Characteristics Ferri-mottanaite-(Ce) is a complex borosilicate belonging to the hellandite group. It forms elongated, prismatic crystals that rarely exceed a few millimeters in length. They usually occur as radial or haphazardly arranged aggregates. The mineral is characterized by a dark, reddish-brown to almost black color and is typically opaque, although the thinnest fragments may be translucent. ## Physical Properties The mineral's hardness on the Mohs scale is approximately 6. It has a vitreous luster, transitioning to resinous on some surfaces. The calculated density is about 4.11 g/cm³, which is a relatively high value, typical for minerals containing rare earth elements. ## Colors and Varieties The dominant and only known color is dark, reddish-brown to black. Due to its rarity, no color or commercial varieties have been distinguished. ## History and Name The mineral was officially recognized by the International Mineralogical Association (IMA) in 2015. Its name refers to its chemical composition – the "Ferri" prefix indicating the dominance of iron (Fe³⁺) at a specific position in the crystal structure – and its relationship to mottanaite-(Ce). The suffix "(Ce)", according to Levinson's nomenclature, signifies that cerium is the dominant rare earth element in this mineral. The type locality is Tre Croci in Italy. ## Applications Due to its extreme rarity, ferri-mottanaite-(Ce) has no industrial applications. It is solely an object of scientific research and a valuable specimen in advanced systematic and micromineral collections.

Properties

Mohs hardness: 6
Luster: Vitreous to resinous
Streak: Pale brown
Density: 4.11
Cleavage: Good on {010}
Fracture: Uneven to conchoidal
Transparency: Opaque, translucent in thin fragments
Crystal system: Monoclinic

Diagnostic features

## Identification Identifying ferri-mottanaite-(Ce) is extremely difficult and practically impossible without advanced analytical methods (EDS/WDS chemical analysis). Visual features that may suggest it include: prismatic crystal form, dark brown to black color, vitreous luster, and occurrence in a specific geological environment (eufotids in a volcanic complex). It is inactive or very weakly radioactive. ## Distinguishing from Similar Minerals It can be confused with other dark prismatic minerals, such as schorl (black tourmaline), aegirine, gadolinite, and especially with other minerals from the hellandite group. It usually differs from schorl and aegirine in its occurrence environment and associated minerals. Certain differentiation from other members of the hellandite group requires confirmation of chemical composition. ## Crystal Forms It forms elongated, columnar or prismatic crystals, often terminated by flat pinacoids. These crystals group into radial or disordered aggregates, growing into the host rock.

Geological environment

## Genesis Ferri-mottanaite-(Ce) forms under high-temperature conditions, in the late stages of magma crystallization. It occurs in miarolitic cavities within sanidinite volcanic bombs, which are part of the Cimino-Vico volcanic complex in Italy. ## Mineral Associations This mineral co-occurs with sanidine, pyroxenes from the diopside-hedenbergite series, and also with rare minerals such as vonsenite. ## Localities The only confirmed and described occurrence of ferri-mottanaite-(Ce) in the world is its type locality - Tre Croci, near Vetralla, in the province of Viterbo, Lazio region, Italy.

Rarity

Very rare

For collectors

## Quality Criteria The quality of specimens, which are almost exclusively microminerals, is assessed based on several criteria. The most important is the sharpness and degree of crystal development – the more defined the faces and terminations, the better. The size of the crystals (even 2-3 mm is a large specimen) and their abundance on the matrix also matter. Contrast with light host rock (e.g., white sanidine) and lack of damage enhance attractiveness. ## Market Prices Prices for well-formed ferri-mottanaite-(Ce) microminerals with confirmed chemical analysis range from several tens to several hundreds of euros, depending on the quality and aesthetics of the specimen. ## Popular Localities All specimens available on the collector's market come from a single location - the type locality of Tre Croci in Italy.

Care and storage

## Cleaning Specimens should only be cleaned mechanically, using a soft brush to remove dust. Due to its rarity and potential fragility, washing in water is discouraged, especially the use of ultrasonic cleaners, which can cause cracks. ## What to Avoid The mineral is potentially sensitive to strong acids. Avoid sudden temperature changes and contact with chemicals. Store away from minerals of higher hardness to prevent scratching. ## Storage The best way to store specimens is to place them in a sealed, padded "perky box", which protects them from dust, light, and mechanical damage. It is recommended to store them in stable humidity and temperature conditions.

Sources

IMA Mineral List (CNMNC)