Allanite

Allanite
Allanite from the Mt. Isa – Cloncurry area, Queensland, Australia (scale bar 1 inch)
General
Category	Sorosilicates
Formula (repeating unit)	(Ce,Ca,Y,La)2(Al,Fe+3)3(SiO4)3(OH)
Strunz classification	9.BG.05b
Crystal system	Monoclinic
Crystal class	Prismatic (2/m) (same H-M symbol)
Space group	P21/m
Unit cell	a = 8.927, b = 5.761 c = 10.15 [Å]; β = 114.77°; Z = 2
Identification
Color	Brown to black
Crystal habit	Crystals tabular, prismatic to acicular; granular, massive; commonly metamict
Twinning	Polysynthetic, common on {100}
Cleavage	Imperfect to poor
Fracture	Conchoidal to uneven
Tenacity	Brittle
Mohs scale hardness	5.5–6
Luster	Vitreous, resinous to submetallic
Streak	Grey
Diaphaneity	Translucent to opaque
Specific gravity	3.5 – 4.2
Optical properties	Biaxial (-)
Refractive index	nα = 1.715–1.791, nβ = 1.718–1.815, nγ = 1.733–1.822
Birefringence	δ = 0.018–0.031
Pleochroism	X = pale olive-green, reddish brown; Y = dark brown, brownish yellow; Z = dark reddish brown, greenish brown
2V angle	Measured: 40° to 80°
Dispersion	r > v; strong
Other characteristics	May be radioactive
References

Allanite (also called orthite) is a sorosilicate group of minerals within the broader epidote group that contain a significant amount of rare earth elements. The mineral occurs mainly in metamorphosed clay rich sediments and felsic igneous rocks. It has the general formula A₂M₃Si₃O₁₂[OH] where the A sites can contain large cations such as Ca²⁺, Sr²⁺, and rare earth elements and the M sites admit Al³⁺, Fe³⁺, Mn³⁺, Fe²⁺, or Mg²⁺ among others. However, a large amount of additional elements, including Th, U, Zr, P, Ba, Cr and others may be present in the mineral. The International Mineralogical Association lists three minerals in the allanite group, each recognized as a unique mineral: allanite-(Ce), allanite-(La) and allanite-(Y), depending on the dominant rare earth present, cerium, lanthanum or yttrium.

Allanite contains up to 20% rare earth elements and is a valuable source of them. The inclusion of thorium and other radioactive elements in allanite results in some interesting phenomena. Allanite often has a pleochroic halo of radiation damage in the minerals immediately adjacent. Also highly radioactive grains of allanite often have their structure disrupted or are metamict. The age of allanite grains that have not been destroyed by radiation can be determined using different techniques.

Allanite is usually black in color, but can be brown, brown-violet. It is often coated with a yellow-brown alteration product, likely limonite. It crystallizes in the monoclinic system and forms prismatic crystals. It has a Mohs hardness of 5.5–6 and a specific gravity of 3.5–4.2. It is also pyrognomic, meaning that it becomes incandescent at a relatively low temperature of about 95 °C.