Site Splitting at Site M3 in Allanite-(Ce) - 2022

Rare Earth Deposits & Mining

Shen and 6 co-authors reported in 2022 that the crystal structure of allanite-(Ce), with composition (Ca_1.0REE_0.9-0.1)_Σ2.0(Al_1.46 Fe³⁺ _0.52Fe²⁺_0.76Mg_0.12Ti_0.15)_Σ3.01Si₃O₁₂(OH) from the Xinfeng rare earth element (REE)-bearing granite in Guangdong Province, China.

                                                             (a) Molecular structure of allanite-(Ce) showing the split sites;
                                                             (b) refined polyhedral style structure of allanite-(Ce) projected down the b axis.

It has the unit cell a = 8.9550(4) Å, b =5.77875(16) Å, c = 10.2053(4) Å, β = 114.929(5)° and Z = 2 in space group P₂1/m and is characterised by site splitting at M3 into M3a and M3b, at a distance of 0.38(3) Å, which are occupied partially by Fe_0.764Mg_0.12 and Ti_0.15, respectively.

The structure was determined by single-crystal X-ray diffraction and refined with anisotropic full-matrix least-squares refinement on F² to R₁ = 2.82%, wR₂ = 7.77% for 1856 independent reflections (8772 collected reflections). However, M3 splitting is not present in either ferriallanite-(Ce) or epidote, in
which M3 is almost fully occupied either by Fe²⁺ or by Fe³⁺.

Comparisons of bond lengths and volumes in cation polyhedra among allanite-(Ce), ferriallanite-(Ce) and epidote tend to indicate that the essential factor that facilitates site splitting of M3 in allanite- (Ce) is heterovalent substitution and occupation of a crystallographic site between Fe²⁺(Mg²⁺/Mn²⁺)–Al³⁺(Ti⁴⁺), a common phenomenon in minerals, such as the plagioclase series. Fine structure analysis of the M3 split model revealed that deformation of A2 is related closely to distorted M3, which is consistent with Fe²⁺ incorporation following REE substitution.

Based on crystal chemistry and structure refinement of allanite-(Ce), ferriallanite-(Ce) and epidote under the constraint of their compositions, tjhe authors concluded that the M3 site splitting model helps to obtain a better structure that accommodates heterovalent substitution in allanite subgroup minerals. A higher distortion index implies that both different ionic radii and the site split contribute to crystal lattice distortion.