Allanite - Encyclopedia

    Class : Silicates
    Subclass : Sorosilicates
    Crystal System : Monoclinic
    Chemistry : (Ce, Ca)2 (Al, Fe)3 (SiO4)3 (OH)
    Rarity : Uncommon

Allanite is a member of the group of epidote in which it represents the iron and rare earth elements rich term (cerium, lanthanum, neodymium, etc...). According to the dominant rare earth element there are several species : allanite-Ce, allanite-Nd, allanite-La, etc... It owes its name to the Scottish mineralogist Thomas Allan who discovered the species. Allanite crystallized in thick tablets, sometimes sticks or needles of brown, black, or brown-yellow. Because of its content of uranium and thorium (up to 2%), the crystals are generally metamict (disordered on the atomic scale). It is an accessory mineral of granites, pegmatites and syenites. It is found more rarely in metamorphic rocks, pyrometasomatic deposits or in some lavas (dacites). It is a common mineral but often occurs in very accessory and in very small crystals. Allanite can be used as cerium and rare earth ore. The macro-samples are only for the collection but because of its content of radioactive elements, allanite is also used by geologists for rocks datations.

Allanite in the World

The locality producing the largest crystals is Arendal in Norway with a record of more than 1.10 m. There are also spectacular acicular crystals in Finnbo, Sweden, in Bancroft, Ontario, Canada, where 70 cm crystals are associated with molybdenite, but also in the granitic pegmatites of Pacoima Canyon in California with individuals over 45 cm in size. 

Allanite in France

Allanite is known in France in the pegmatites of the pink granites of Ploumanac'h in the Côtes d'Armor where crystals can reach 11 cm, and in beautiful brown gemmy crystals (up to 3 cm) on dolomite of the large talcum deposit of Trimouns in Ariège.

Large monocrystal (23.5 mm) of allanite-dissakisite from Trimouns, Ariège, France

Gemmy allanite-dissakisite on dolomite from Trimouns, Ariège, France

Group of allanite-dissakisite crystals on dolomite from Trimouns, Ariège, France

Group of allanite-dissakisite crystals on dolomite from Trimouns, Ariège, France

Twinning

Allanite commonly presents polysynthetic twins on {100}, impossible to identify with the naked eye.

Fakes and scams

No scam known for this mineral.



Hardness : 5,5 à 6
Density : 3,5 to 4,2
Fracture : Conchoidal to uneven
Trace : Grey




TP : Opaque to transparent
RI : 1,715 à 1,791
Birefringence : 0,018 to 0,031
Optical character : Biaxial -
Pleochroism : Green to brown
Fluorescence : None


Solubility : Hydrochloric acid

Magnetism : Paramagnetic
Radioactivity : Weak

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