Allanite

Monoclinic  Images    
   
   
  Formula (Ca,Ce,La)2(Al,Fe3+,Fe2+)3O(Si2O7)(SiO4)OH
  Optic class & sign Biaxial positive or negative
  Optical orientation X∧c and Z∧a vary widely, Y = b
  Optical plane (010); may also be normal to (010)
  Relief High
  Refractive indices nx = 1.690 -1.791
ny = 1.700 -1.815
nz = 1.706 -1.828
n increases with increasing Fe and REE content
  Birefringence   (max.)0.013  - 0.036
   Δn increases with increasing Fe and REE content
  Optic Angle 2Vx = 40 - 90°
2Vz = 90 - 57°
  Sign of elongation Length-slow, l (+), and length-fast, l (-), for crystals elongate in b
  Interference figure Strong colour can mask interference colours. Strong optic axis dispersion. Hence, interference figures may not be useful for mineral identification.
  Colour /   pleochroism Various shades of brown; green is less common. Distinct pleochroism, usually Z ≥ Y > X.
  Zoning Colour zoning is widespread. Δn zoning may occur. Overgrowths of epidote on allanite are common.

  Form  Habit Granular, columnar, bladed, acicular. Crystals are elongate parallel to b.
  Surface Euhedral to anhedral
  Cleavage Cleavages are generally not well developed. Most pronounced is the (001) set.
  Twinning (100) rare
  Extinction Straight to (h0k) faces and to (001) cleavage traces in elongate sections (long axis = b). X-Z sections of max Δn show inclined extinction to all morphological elements.

  Reaction textures  
  Alteration /   decomposition More susceptible to weathering than other members of the epidote group.

  Occurence     Ign Felsic to intermediate igneous rocks
  Met Schists, gneisses, amphibolite, metamorphosed carbonate-silicate rocks
  Sed 
  Hyd 
  Other 

  Distinctive   properties               High relief, colour and pleochroism. Pleochroic halos around allanite as inclusions in, or bordering, minerals like biotite, chlorite, amphiboles.
  Additional   comments The optical properties of allanite vary over a large n-Δn range reflecting the multitude of possible substitutions, not only REE for Ca, but also on the octahedral M(1) and M(3) positions. The ranges of n listed above apply to the more common allanites. The n-Δn chart shows the main fields for the common allanite compositions, plus nX-nZ pairs for selected REE-rich members of the epidote group. Trends of n and Δn for the clinozoisite-epidote solid solution series are included for reference.
Aln-La - allanite-(La): Orlandi & Pasero (2006), Can. Min. 44, 63-68; Aln-Nd - allanite-(Nd): Skoda et al., Am. Min. 97, 983-988; Dis-Ce – dissakisite-(Ce), Dls-Ce – dollaseite-(Ce), Faln-Ce – ferriallanite-(Ce), Kh-Ce – khristovite-(Ce): Giere & Sorensen (2004), Rev. Min. & Geochem. 56, 431-493; Dis-La – dissakisite-(La): Tumiati et al. (2005), Am. Min. 90, 1177-1185; Faln-La – ferriallanite-(La): Kolitsch et al. (2012), Eur. J. Min. 24, 741-747.
Metamict allanite is common. Metamict-isotropic varieties: n = 1.54 – 1.72.
Androsite is the Mn-rich equivalent of allanite, with Mn2+ replacing Ca, and Mn3+ as well as Mn2+ in octahedral positions. The refractive indices (mean n = 1.877) are similar to those of ferriallanite. Androsite has a very pronounced pleochroism from X = pale orange-brown to Z = deep brown-red.