Allanite-(La)

Chemical formula: CaLaFe₂+Al₂(Si₂O₇)(SiO₄)O(OH)

A rare mineral from the epidote group, a lanthanum-dominant variety of allanite, black in color and occurring in pegmatites.

## Characteristics Allanite-(La) is a rare mineral belonging to the allanite group, within the epidote supergroup. It is a silicate with a complex chemical composition, in which lanthanum is the dominant rare earth element. It forms tabular or prismatic crystals, but most often occurs as irregular, embedded grains or aggregates within the rock mass. Its color is usually black to brownish-black. Due to the possible presence of radioactive elements (such as thorium), it can be metamict, meaning its crystal structure has been partially or completely destroyed by its own radiation. ## Physical Properties This mineral is characterized by a hardness of approximately 5.5 on the Mohs scale. The luster is vitreous to resinous, and dull on weathered surfaces. It is an opaque mineral; only in very thin splinters can it be translucent. Its density is relatively high, about 3.89 g/cm³, which is a result of the presence of heavy elements such as lanthanum and iron. Metamictization affects its physical properties, lowering hardness and density and changing the fracture to conchoidal. ## Colors and Varieties Allanite-(La) is by definition a variety of allanite, distinguished by the dominance of lanthanum. Its color is uniform, in shades from black to dark brown. It does not form color varieties or have separate trade names. ## History and Name The mineral's name refers to its chemical composition – dominant lanthanum (La) – and its belonging to the allanite group. This group was named after the Scottish mineralogist Thomas Allan (1777-1833), who first described its main representative, allanite-(Ce). Allanite-(La) was formally recognized as a separate mineral species in 1987 by the International Mineralogical Association (IMA) to standardize nomenclature within the allanite group. The type locality, from which the mineral's type specimen originates, is Buer in Norway. ## Applications Due to its rarity, allanite-(La) has no industrial applications. It is an object of scientific interest as an indicator of geochemical processes in igneous rocks and as a source of information about the concentration of rare earth elements. It is also valued by collectors specializing in rare and systematic minerals.

Properties

Mohs hardness: 5.5
Luster: Vitreous, Resinous
Streak: Gray, grayish-brown
Density: 3.89
Cleavage: Poor/Indistinct on {001}
Fracture: Conchoidal to Uneven
Transparency: Translucent to Opaque
Crystal system: Monoclinic

Diagnostic features

## Identification Identifying features include: black color, resinous or vitreous luster, high density, and occurrence in alkaline rock pegmatites. Potential weak radioactivity, detectable with a Geiger counter, is a strong diagnostic indicator for minerals of the allanite group. ## Distinguishing from similar minerals Allanite-(La) is visually indistinguishable from other minerals of the allanite group, especially the much more common allanite-(Ce). Definitive differentiation requires advanced chemical analyses (e.g., EDS, WDS). It differs from black tourmaline (schorl) by the absence of striations on crystal faces and a different habit. It can also be confused with gadolinite, euxenite, or samarskite, which have similar color, density, and are often metamict. In these cases, identification also requires specialized studies. ## Crystal Forms It crystallizes in the monoclinic system. Crystals, if well-formed, take the shape of thick tablets or elongated prisms. More often, however, it is found as embedded, irregular grains and massive aggregates.

Geological environment

## Genesis Allanite-(La) is an igneous mineral, crystallizing as an accessory mineral in the late stages of formation of silica- and alkali-rich rocks. Its formation is associated with environments of elevated rare earth element concentrations. ## Mineral Associations It most commonly co-occurs with minerals from alkaline and syenitic pegmatites, such as microcline, albite, quartz, aegirine, zircon, titanite, and other REE minerals. ## Localities The most important and type locality (localitas typica) is the Buer farm in Bjørkedalen valley, near Porsgrunn in Telemark county, Norway. This is a classic locality for many rare pegmatite minerals.

Rarity

Rare

For collectors

## Quality Criteria The most desired by collectors are specimens with well-formed, sharp crystals, which are extremely rare for allanite-(La). Rich aggregates in the host rock are also highly valued, especially from an analytically confirmed type locality. The value of a specimen is enhanced by co-occurrence with other rare minerals. ## Popular Localities Undoubtedly, the most valued and sought-after specimens come from the type locality in Buer, Norway. Finds from other localities primarily have scientific value until they are thoroughly studied and described.

Care and storage

## Cleaning Allanite-(La) specimens can be cleaned with a soft brush and distilled water. Strong vibrations should be avoided, so ultrasonic cleaners are not recommended, especially for metamict or fractured specimens. ## What to avoid The mineral is sensitive to strong acids. Avoid sudden temperature changes. As a potentially radioactive mineral, it requires careful handling – avoid inhaling dust during any processing. It does not require special protection from light. ## Storage It is recommended to store specimens in separate, labeled boxes, which is standard practice for radioactive minerals. This also protects other minerals in the collection from potential long-term radiation exposure. The label should include information about potential radioactivity.

External references

https://en.wikipedia.org/wiki/https%3A%2F%2Fen.wikipedia.org%2Fwiki%2FAllanite