Törnebohmite-(La)

Chemical formula: La₂Al(SiO₄)₂(OH)

Törnebohmite-(La) is a rare lanthanum and aluminum silicate, forming small, olive-green aggregates in syenitic pegmatites.

## Characteristics Törnebohmite-(La) is a rare silicate mineral. It typically occurs as small, granular or massive aggregates, rarely forming discernible crystals. Its most characteristic feature is its color, ranging from green to olive-green. Specimens are usually opaque or at most translucent on thin edges. ## Physical Properties This mineral is characterized by a Mohs hardness of 4.5, placing it in the group of medium-hard minerals. Its density is relatively high, approximately 4.8 g/cm³. The luster is described as vitreous to resinous. The fracture is uneven. ## Colors and Varieties The dominant and only recorded color of törnebohmite-(La) is green, most often an olive shade. No color or commercial varieties are distinguished. ## History and Name The mineral's name honors Alfred Elis Törnebohm (1838-1911), a Swedish geologist and director of the Geological Survey of Sweden. The suffix "-(La)" indicates lanthanum as the dominant rare earth element in its chemical composition. The mineral was first described in 1921. ## Uses Due to its extreme rarity, törnebohmite-(La) has no industrial applications. It is solely an object of interest for collectors specializing in rare earth minerals and systematic collections.

Properties

Mohs hardness
4.5
Luster
Vitreous to resinous
Streak
Light green
Density
4.805
Cleavage
Imperfect on {001}
Fracture
Irregular/Uneven
Transparency
Translucent
Crystal system
Monoclinic

Diagnostic features

## Identification Amateur identification of törnebohmite-(La) is very difficult. It relies mainly on its specific olive-green color, relatively high density, and occurrence in a characteristic geological environment – syenitic pegmatites. Definitive identification requires advanced analytical methods, such as X-ray diffraction (XRD) or chemical composition analysis (EDS/WDS). ## Distinguishing from Similar Minerals This mineral can be confused with other green minerals found in similar parageneses, such as epidote, allanite, or some amphiboles. Differentiation is based on the lack of distinct cleavage (unlike epidote and amphiboles) and on hardness and density. Allanite is often metamict (damaged by its own radioactivity), which is not a typical feature of törnebohmite. ## Crystal Forms Well-formed crystals are extremely rare. This mineral most often forms massive, fine-grained, or fibrous aggregates embedded in the host rock.

Geological environment

## Genesis It is a hydrothermal mineral, forming in the late stages of crystallization of alkaline pegmatites, especially syenitic and nepheline-syenitic types. It forms under conditions enriched in rare earth elements, particularly lanthanum. ## Mineral Associations It co-occurs with other minerals typical of alkaline pegmatites, such as fluorite, allanite, bastnäsite, cerite, zircon, aegirine, microcline, and minerals from the titanosilicate group. ## Localities The most important and historical occurrence (type locality) is Mochalin Log in the Kyshtym area, Chelyabinsk Oblast, Russia. Other known localities include the Mountain Pass deposit in California (USA) and some localities in Sweden and Norway.

Rarity

Very rare

For collectors

## Quality Criteria The collector's appeal of törnebohmite-(La) specimens is primarily determined by the richness of the aggregates and their color intensity against the host rock (matrix). Since well-formed crystals are practically nonexistent, the main criterion is the quantity and quality of massive aggregates. Specimens from a well-documented type locality are particularly prized. ## Popular Localities Specimens from the type locality in Russia (Mochalin Log) are by far the most sought after by collectors. Material from other localities, such as Mountain Pass in the USA, is also found in collections but holds less historical value.

Care and storage

## Cleaning It is recommended to clean specimens only with a soft brush to remove dust. The use of water, especially with detergents, is inadvisable due to the lack of complete data on its reactivity. ## What to Avoid Avoid contact with acids and other chemicals that may damage the mineral's surface. Due to its medium hardness, the mineral is susceptible to scratching by harder minerals (e.g., quartz), so contact with them should be avoided. There is no detailed data on its sensitivity to light or temperature; however, it is generally recommended to avoid prolonged exposure to strong sunlight and extreme temperatures. ## Storage Törnebohmite-(La) specimens are best stored in separate, padded display boxes to protect them from mechanical damage and scratching. Display in a closed cabinet, away from direct sunlight and heat sources, is permissible.

External references

Sources

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