Törnebohmite-(Ce)

Chemical formula: Ce₃+₂Al(SiO₄)₂(OH)

Törnebohmite-(Ce) is a rare cerium and aluminum silicate, forming small, vitreous grains and aggregates ranging in color from green to brown.

## Characteristics Törnebohmite-(Ce) is a rare mineral from the silicate group, belonging to the gadolinite group. Its composition is dominated by cerium, which is indicated by the -(Ce) suffix in its name. It typically occurs as very fine, irregular grains, radial or fibrous aggregates, and also as massive occurrences. Well-formed crystals are extremely rare and appear as small, prismatic columns. The mineral is most often embedded in the host rock. ## Physical Properties The mineral is characterized by a hardness of 5.5-6.5 on the Mohs scale. It has a vitreous, sometimes slightly greasy luster. It is a relatively dense mineral, with a density of approximately 4.95 g/cm³. It is brittle and exhibits an uneven or conchoidal fracture. ## Colors and Varieties Its color is variable, most often taking on shades from light green, through olive green, yellowish-green, to reddish-brown and brown. It is usually translucent, less often transparent or almost opaque in thicker fragments. No commercial or colored varieties are distinguished. ## History and Name The mineral is named in honor of Alfred Elis Törnebohm (1838-1911), a Swedish geologist and pioneer in the study of Precambrian rocks in Sweden. The -(Ce) suffix indicates the dominance of cerium as a rare-earth element in its structure. The mineral was first described in 1921 based on material from the Bastnäs deposit in Sweden. ## Uses Due to its rarity and occurrence as small grains, törnebohmite-(Ce) has no industrial applications. It is solely an object of interest for collectors specializing in rare minerals and for scientists.

Properties

Mohs hardness: 5.5 - 6.5
Luster: Vitreous to greasy
Streak: Light greenish-gray
Density: 4.9 - 4.98
Cleavage: Imperfect/poor on {001}
Fracture: Uneven to conchoidal
Transparency: Transparent to translucent
Crystal system: Monoclinic

Diagnostic features

## Identification Identifying törnebohmite-(Ce) under amateur conditions is very difficult. It relies mainly on observing the mineral association (co-occurrence with other rare-earth minerals, such as cerite or allanite), color (characteristic shades of green and brown), and mode of occurrence (small grains, radial aggregates). Definitive identification requires advanced analytical methods, such as X-ray diffraction (XRD) or chemical microanalysis (EDS/WDS). ## Distinguishing from Similar Minerals This mineral can easily be confused with other rare-earth silicates of similar color and habit, such as allanite-(Ce), cerite-(Ce), or gadolinite. Allanite often forms larger, better-formed crystals. Cerite can be more waxy or greasy in luster. Final differentiation is possible almost exclusively by laboratory methods. ## Crystal Forms Well-formed crystals are exceptionally rare and appear as short, prismatic columns with a hexagonal cross-section. It is much more commonly found as irregular grains, granular aggregates, and also as radial or fibrous aggregates, embedded in the rock.

Geological environment

## Genesis This mineral is a product of metasomatic and hydrothermal processes. It forms in skarns and carbonatites, which are carbonate rocks altered by hot, element-rich solutions. It is also found in some syenite pegmatites and Alpine veins. ## Mineral Associations It most often co-occurs with other rare-earth minerals, such as cerite-(Ce), allanite-(Ce), bastnäsite-(Ce), as well as fluorite, talc, quartz, chalcopyrite, and molybdenite. The presence of these associated minerals is an important diagnostic clue. ## Localities The most important and classic locality, from which the best specimens originate, is the Bastnäs deposit in Västmanland, Sweden. Other known localities include the Deep Bay (Qinglong) area in China; the St. Hilaire Mountains in Quebec, Canada; the Kola Peninsula in Russia; and individual occurrences in Norway and the United States (e.g., in the Mountain Pass mine in California).

Rarity

Very rare

For collectors

## Quality Criteria The collector appeal of törnebohmite-(Ce) is specific and aimed at advanced collectors. Specimens from the type locality (Bastnäs, Sweden) are most highly valued. The abundance of the mineral on the rock matrix is key – the more visible, well-colored grains or aggregates, the better. Rare, microscopic, but well-formed crystals are exceptionally sought after. Purity and intensity of color (especially vivid green) also increase the specimen's value. ## Popular Localities Specimens from the historic Bastnäs deposit in Sweden are by far the most prized by collectors. Material from this locality is considered classic and exemplary for this mineral.

Care and storage

## Cleaning Due to its occurrence as embedded grains, cleaning is limited to the host rock. A soft brush and distilled water can be used to remove dust and loose impurities. Ultrasonic cleaners should be avoided, as they can damage the brittle mineral grains. ## What to Avoid Avoid contact with strong acids, which can react with the mineral. Do not heat specimens or expose them to sudden temperature changes. Although not particularly sensitive to light, prolonged exposure to intense UV radiation is not recommended. ## Storage Specimens of törnebohmite-(Ce) are best stored under stable conditions, in closed boxes or display cases, to protect them from dust and mechanical damage. It is advisable to include a label with the exact locality, as this is a crucial element of the specimen's value.

Sources

IMA Mineral List (CNMNC)