Tancaite-(Ce)

Chemical formula: Fe<sup>3+</sup>Ce<sup>3+</sup>(Mo<sup>6+</sup>O<sub>4</sub>)<sub>3</sub>·3H<sub>2</sub>O

Tancaite-(Ce) is a very rare, hydrated iron and cerium molybdate, forming small, yellow tabular crystals.

## Characteristics Tancaite-(Ce) is an extremely rare mineral from the molybdate group. It occurs as very small, hexagonal crystals with a tabular habit, reaching sizes up to 0.2 mm. It forms rosetted or spherical aggregates. Its most characteristic visual feature is its intense yellow color. ## Physical Properties Tancaite-(Ce) crystals exhibit a vitreous luster. Due to the small size of the crystals, many physical properties, such as hardness or density, have not been precisely measured. The mineral is brittle. ## Colors and Varieties This mineral occurs in a uniform, pale yellow color. No color or commercial varieties are known. ## History and Name The mineral's name comes from its type locality – the Su Seinargiu mine in the province of Sardinia, Italy, known locally as "Sa Tanca". The "-Ce" suffix in the name indicates the dominant rare-earth element in its chemical composition – cerium. The mineral was approved by the International Mineralogical Association (IMA) in 2009 (IMA2009-057).

Properties

Luster
Vitreous
Streak
White
Cleavage
Perfect on {0001}
Fracture
Brittle
Transparency
Transparent
Crystal system
Trigonal

Diagnostic features

## Identification Identifying tancaite-(Ce) under amateur conditions is practically impossible. It requires advanced analytical methods, such as Raman spectroscopy or chemical analysis using an electron microprobe (EDS/WDS). Visual characteristics that might only suggest its presence are small, yellow, hexagonal crystals in a characteristic geological environment. ## Distinguishing from Similar Minerals It can be confused with other secondary yellow molybdates, such as wulfenite or molybdofornacite. Definitive differentiation requires specialized chemical analysis, especially confirmation of cerium's presence. ## Crystal Forms It forms thin, tabular crystals with a hexagonal outline. These crystals often combine into rosetted or spherical aggregates.

Geological environment

## Genesis It is a secondary mineral, formed in the oxidation zones (gossans) of polymetallic deposits where molybdenum sulfides (molybdenite) and rare-earth element-bearing minerals originally occurred. ## Mineral Associations At the type locality, it co-occurs with quartz, muscovite, molybdenite, and other rare molybdates, such as sardignaite and ichnusaite-(Ce). ## Localities The only confirmed occurrence of this mineral in the world is its type locality: the Su Seinargiu mine (also known as Sa Tanca), Sarroch, Cagliari, Sardinia, Italy.

Rarity

Extremely rare

For collectors

## Quality Criteria As a "micromount" mineral, its collector's value depends on the quality and size of the crystals, as well as the aesthetics of the entire aggregate. The most desirable are well-formed, sharp crystals forming distinct, rosetted clusters on a contrasting matrix. Due to its extreme rarity, every authentic specimen is valuable. ## Popular Localities The only source of specimens is the Su Seinargiu mine in Sardinia. Material from this locality is highly sought after by collectors specializing in rare minerals and "micromount" specimens.

Care and storage

## Cleaning Due to its extreme rarity and small crystal size, mechanical cleaning is not recommended. Specimens should be protected from dust in specialized boxes. ## What to Avoid Avoid contact with all chemicals, ultrasound, and sudden temperature changes. The mineral is brittle and susceptible to mechanical damage. ## Storage Specimens should be stored only under stable conditions, in closed, padded "micromount" containers, away from vibrations, humidity, and direct sunlight, to prevent potential dehydration or fading.

External references

Sources

Read more