Allanite - Ca,(REE)³⁺Al₂Fe²⁺[Si₂O₇][SiO₄]O(OH)

Allanite, also known as "orthite" in Europe, was named in 1810 by Thomas Thomson for Thomas Allan, Scottish banker and mineralogist, who first observed the mineral. The structure of allanite is similar to that of the other monoclinic members of the epidote group. The connection of allanite with the common FeAl₂-bearing epidotes may be expressed by the coupled substitution Ca²⁺ + Fe³⁺ ↔ REE³⁺ + Fe²⁺, and allanite is the only member of the group in which Fe²⁺ is an essential constituent. Thorium and uranium, in amounts up to 5 ThO₂ and 0.5 U₃O₈ wt.% respectively, are present in the majority of allanites. Allanite is one of the most common rare earth minerals; up to 20% of allanite's weight could be composed of these rare earth elements, making allanite a potentially valuable ore. Because of these rare earth metals, especially thorium, which are frequently radioactive, allanite is frequently slightly radioactive.

Allanite in the strictest sense is broken into three officially recognized minerals:

Allanite-(Ce) the cerium rich allanite, also the most common and in general the one most often referred to as just allanite or orthite.
Allanite-(La) the lanthanum rich allanite.
Allanite-(Y) the yttrium rich allanite.

These minerals share the same structures and vary slightly in only a few of their properties. Chemical test would be required to differentiate them.

Allanite often occurs in the metamict state due to the destruction of the crystalline structure by the bombardment of α (alfa) particles emitted by the radioactive constituents. The effect can destroy a crystal lattice completely while leaving the outward appearance unchanged. The complete destruction of the allanite structure will produce a glassy hydrated substance. The hydration is facilitated by the metamictation. The alteration and hydration are accompanied by expansion, and allanite is commonly surrounded by anastomosing cracks that radiate into the adjacent minerals; brown haloes are sometimes observed in the surrounding rocks due to radiation damage. The non-metamict allanites are distinguished from other epidotes by their brownish colour, the metamict specimens by their isotropic character and the presence of anastomosing cracks.

Allanite is a characteristic accessory mineral in many granites, granodiorites, monzonites and syenites; it occurs in larger amounts in some limestone skarns and in pegmatites.

Optical properties:

• Color: In a fresh stage pale grey-yellow to orange-brown, with red-brown pleochroism.
• Form: Tends to be in the form of rounded to irregular-shaped grains, rarely acicular.
• Cleavage: (001) good appearing as traces parallel to the prism edge in prismatic sections but intersecting at right angle in basal section.
• Interference colors: Medium-high birefringence (interference colours are masked by the crystal colour; in case of complete metamictization isotropic).
• Alteration: Can alter into epidote; during weathering it is replaced by carbonate and limonite.
• Special characteristics: Almost always zoned; occasionally it is metamictic-isotropic and it is commonly surrounded by anastomosing cracks that radiate into the adjacent minerals

Bibliography

• Cox et al. (1979): The Interpretation of Igneous Rocks, George Allen and Unwin, London.
• Howie, R. A., Zussman, J., & Deer, W. (1992). An introduction to the rock-forming minerals (p. 696). Longman.
• Le Maitre, R. W., Streckeisen, A., Zanettin, B., Le Bas, M. J., Bonin, B., Bateman, P., & Lameyre, J. (2002). Igneous rocks. A classification and glossary of terms, 2. Cambridge University Press.
• Middlemost, E. A. (1986). Magmas and magmatic rocks: an introduction to igneous petrology.
• Shelley, D. (1993). Igneous and metamorphic rocks under the microscope: classification, textures, microstructures and mineral preferred-orientations.
• Vernon, R. H. & Clarke, G. L. (2008): Principles of Metamorphic Petrology. Cambridge University Press.

Photo
Allanite crystals (surrounded by radial cracks), biotite (brown) and feldspar (colorless) in a granite from Amparo, Brazil. PPL image, 2x (Field of view = 7mm)	Allanite crystals (isotropic), biotite and microcline feldspar in a granite from Amparo, Brazil. XPL image, 2x (Field of view = 7mm)	Allanite crystal (surrounded by radial cracks) and feldspar (colorless) in a granite from Amparo, Brazil. PPL image, 2x (Field of view = 7mm)
Allanite crystal (surrounded by radial cracks), biotite (brown) and feldspar (colorless) in a granite from Amparo, Brazil. PPL image, 10x (Field of view = 2mm)	Allanite crystal (surrounded by radial cracks), biotite (brown) and feldspar (colorless) in a granite from Amparo, Brazil. PPL image, 10x (Field of view = 2mm)	Allanite crystal (surrounded by radial cracks), biotite (brown) and feldspar (colorless) in a granite from Amparo, Brazil. PPL image, 10x (Field of view = 2mm)
Allanite crystals (surrounded by radial cracks), biotite (brown) and feldspar (colorless) in a granite from Amparo, Brazil. PPL image, 10x (Field of view = 2mm)	Allanite crystal (surrounded by radial cracks), biotite (brown) and feldspar (colorless) in a granite from Amparo, Brazil. PPL image, 10x (Field of view = 2mm)	Allanite crystal (surrounded by radial cracks), biotite (brown) and feldspar (colorless) in a granite from Amparo, Brazil. PPL image, 10x (Field of view = 2mm)
Allanite crystal (surrounded by radial cracks), biotite (brown) and feldspar (colorless) in a granite from Amparo, Brazil. PPL image, 10x (Field of view = 2mm)	Allanite crystal (isotropic), biotite and microcline feldspar in a granite from Amparo, Brazil. XPL image, 10x (Field of view = 2mm)	Allanite crystal (zoned), biotite (brown) and feldspar (colorless) in a granite. PPL image, 2x (Field of view = 7mm)
Allanite crystal (with low birefringence), biotite and feldspars in a granite. XPL image, 2x (Field of view = 7mm)	Allanite crystal (zoned), biotite (brown) and feldspar (colorless) in a granite. PPL image, 10x (Field of view = 2mm)	Allanite crystal (with low birefringence), biotite and feldspars in a granite. XPL image, 10x (Field of view = 2mm)
Allanite crystal (zoned and twinned), biotite (brown) and feldspar (colorless) in a granite. PPL image, 10x (Field of view = 2mm)	Twinned allanite crystal (with low birefringence), biotite and feldspars in a granite. XPL image, 10x (Field of view = 2mm)	Allanite crystal (zoned), biotite (brown) and feldspar (colorless) in a granite. PPL image, 10x (Field of view = 2mm)
Allanite crystal (zoned), biotite (brown) and feldspar (colorless) in a granite. PPL image, 10x (Field of view = 2mm)	Allanite crystal (with low birefringence), biotite and feldspars in a granite. XPL image, 10x (Field of view = 2mm)	Allanite crystal (zoned), biotite (brown) and feldspar (colorless) in a granite. PPL image, 10x (Field of view = 2mm)
Allanite crystals (zoned), biotite (brown) and feldspar (colorless) in a granite. PPL image, 10x (Field of view = 2mm)	Allanite crystals (with low birefringence), biotite and feldspars in a granite. XPL image, 10x (Field of view = 2mm)