Clinofergusonite-(Ce)

Chemical formula: Ce₃+Nb₅+O₄

Clinofergusonite-(Ce) is a rare, brownish-black niobium and cerium mineral, occurring as small, resinous crystals in pegmatites.

## Characteristics Clinofergusonite-(Ce) is an oxide mineral belonging to the fergusonite series. It forms very small, tabular or prismatic crystals, usually less than 1 mm in size. Most often, it occurs as granular aggregates or as single grains embedded in rock. Its surface can be dull or covered with a coating. ## Physical Properties This mineral is characterized by a hardness of 5-6 on the Mohs scale and a high density of approximately 5.46 g/cm³. It has a luster described as resinous to vitreous. It is brittle, and its fracture is uneven. Due to the content of rare earth elements and niobium, it may exhibit weak radioactivity. ## Colors and Varieties Clinofergusonite-(Ce) ranges in color from brown to black, often with a reddish or yellowish tint. The streak is brown. No distinct color or commercial varieties are recognized. ## History and Name The mineral's name refers to its monoclinic crystal symmetry and to the dominant rare earth element – cerium (Ce). It is the monoclinic (clino) analogue of tetragonal fergusonite-(Ce). It was first described in 1987 based on material from pegmatites in the Lake Baikal region of Russia.

Properties

Mohs hardness: 5-6
Luster: Resinous
Streak: Brown
Density: 5.46
Cleavage: None
Fracture: Uneven
Transparency: Opaque
Crystal system: Monoclinic

Diagnostic features

## Identification Identification of clinofergusonite-(Ce) under amateur conditions is practically impossible and requires advanced analytical methods, such as X-ray diffraction (XRD) and chemical composition analysis (EDS/WDS). Preliminary identification is based on its color (brownish-black), resinous luster, high density, and occurrence in granitic pegmatites. ## Distinguishing from Similar Minerals It can be confused with other dark minerals from the fergusonite group (e.g., fergusonite-(Y), fergusonite-(Ce)), euxenite, samarskite, or columbite. Differentiation without specialized equipment is impossible and relies mainly on chemical analysis to determine the dominant rare earth element and on crystal structure analysis. ## Crystal Forms It forms very small, poorly developed crystals with a tabular or prismatic habit. It usually occurs as irregular grains and their aggregates.

Geological environment

## Genesis Clinofergusonite-(Ce) is a magmatic mineral, crystallizing in the late stages of the formation of granitic pegmatites, especially those enriched in rare earth elements (REE). ## Mineral Associations It co-occurs with minerals typical of pegmatites, such as quartz, microcline, albite, biotite, as well as with other REE minerals, e.g., allanite-(Ce), monazite-(Ce), and bastnäsite-(Ce). ## Localities The most important confirmed localities for this mineral are in Russia, in the Lake Baikal region (type locality) and in the Ilmen Mountains in the Urals. It has also been reported in Norway (pegmatites in Arendal) and Sweden (Östanmossa mine).

Rarity

Very rare

For collectors

## Quality Criteria The quality of a specimen primarily depends on the degree of crystal development and size, which is rare. Specimens with visible, even if small, crystals with defined faces, set against a contrasting rock matrix (e.g., on white albite or quartz), are most valued. Due to its microscopic size, it is a mineral mainly for collectors specializing in systematics or "micromount" specimens. ## Popular Localities Specimens from the type locality in Russia and from Norwegian pegmatites are the most classic examples of this mineral in collections.

Care and storage

## Cleaning Specimens should only be cleaned mechanically, using a soft brush to remove dust. The use of water or any chemical agents that could react with the mineral's surface or the surrounding rock is not recommended. ## What to Avoid Avoid contact with acids and other chemicals. The mineral is brittle, so it must be protected from impacts and falls. It does not require special protection from light, but prolonged and intense exposure is never advisable for collector specimens. ## Storage It is recommended to store specimens in stable conditions, in closed display boxes, to protect them from dust and mechanical damage. Due to the small size of the crystals, they are often stored in special "micromount" boxes.

Sources

IMA Mineral List (CNMNC)