Allanite Group

A group of the Epidote Supergroup(A1A2)(M1M2M3)O4[Si₂O₇][SiO₄]O10. The allanite group was recently defined by Armbruster (2006).

Members of this group are REE-rich minerals typified by the eponymous mineral allanite.
This group may be derived from clinozoisite by homovalent substitutions and one coupled heterovalent substitution of the type ^A2(REE)³⁺ + ^M3M²⁺ → ^A2Ca²⁺ + ^M3M³⁺.

The valences on the key sites are: A1 = M²⁺, A2 = M³⁺, M1 = M³⁺, M2 = M³⁺, M3 = M²⁺, O4 = O^2-, O10 = (OH)^- (Armbruster et al., 2006). Some unnamed species are, however, thought to be dominantly oxidised, with M3 = M³⁺, balanced by O replacing OH.

A1=Ca, Mn²⁺
A2=Ce, La, Nd, Y
M1=Al, Fe³⁺, V
M2=Al, Cr?
M3=Fe²⁺, Mn²⁺, Mg

Fleischer (1985) has studied analyses of 506 allanites and presents the average content of each of the REE elements (including Y) in various rock types:

Average REE content in % of total REE content in allanites sorted by rock type REE granite pegmatite granite granodiorite carbonatites alkaline rocks alkaline pegmatites %La 22.2 25.3 29.3 32.4 33.3 34.9 %Ce 43.2 48.1 48.4 49.5 49.6 50.8 %Pr 5.7 5.0 4.3 4.1 4.3 3.8 %Nd 15.9 15.4 13.5 12.1 11.1 9.1 %Sm 4.1 2.4 1.8 0.9 0.7 0.6 %Eu 0.2 0.1 - 0.1 - - %Gd 3.8 1.3 2.0 0.3 0.2 0.3 %Tb 0.4 0.1 0.1 - - - %Dy 2.1 1.2 0.3 0.3 0.4 0.2 %Ho 0.3 0.3 - - - 0.1 %Er 0.9 0.3 0.3 0.1 0.1 0.1 %Tm 0.1 - - - - - %Yb 0.9 0.4 0.1 0.2 0.2 0.1 %Lu 0.2 0.1 - - - - %Y/(Y+Ln) 9.6 6.8 2.2 - 0.3 1.1 Number of samples analysed 114 139 29 11 13 24