We report the crystal structure of allanite-(Ce), with composition (Ca1.0REE0.9□0.1)Σ2.0(Al1.46Fe3+0.52Fe2+0.76Mg0.12Ti0.15)Σ3.01Si3O12(OH) from the Xinfeng rare earth element (REE)-bearing granite in Guangdong Province, China. 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 Fe0.764Mg0.12 and Ti0.15, 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.

A study of mine dump material from Koberg, Bergslagen, Sweden has revealed a varied mineral assemblage, including Zr-, REE-, Th- and U-bearing minerals associated with the sulphide ore deposit. Detailed descriptions and microprobe analyses are presented of the rare earth minerals yttrian zirconolite and allanite-(Ce), and the zirconium oxide baddeleyite. Yttrian zirconolite is patchily zoned in the actinide elements Th and U, and the low analytical totals suggest the presence of several weight per cent of H2O. The implications of these features with respect to synthetic zirconolite in SYNROC and to radioactive waste management are briefly discussed. Chemical zoning of individual REE in yttrian zirconolite is described and the REE distribution compared with those of zirconolites from other localities. Allanite-(Ce) is generally unzoned, but contains REE-rich rims which are considered to be sub-micron intergrowths of allanite plus a bastnäisite-type mineral phase. The MgO content of allanite-(Ce) is unusually high and appears to be a characteristic feature of allanite from other cerium and mineral occurrences of Central Sweden. The rare earth minerals at Koberg are interpretated as having been formed as a result of localized remobilization of elements including Ti and Zr with H2O- and CO2-rich fluids.

A recent study of granitic gneisses in the ultrahigh-pressure (UHP) metamorphic terrane from Dabie Shan, central China reveals that allanite-(Ce) is a widespread REE-bearing accessory phase. The microprobe analyses show that allanites-(Ce) have nearly uniform chemical compositions, with total REE contents ranging from 23 to 28 wt.% and a La/Ce ratio of 0.57–0.73. All allanites-(Ce) are replaced or rimmed by epidotes and commonly form a relatively REE-poor rim near the interfaces. The textural relations and mineral chemistry suggest that allanite-(Ce) crystallized prior to metamorphism and therefore was of igneous origin, whereas the surrounding epidote was the product of metamorphism. Granitic gneiss, composed chiefly of sodic plagioclase, microcline and quartz with minor amounts of Mn-rich garnet, biotite, low Si phengitic mica, epidote, chlorite and sometimes amphibole, was formed at 495°C and ∼5 kbar, corresponding to the retrograde metamorphism of UHP rocks. Accordingly, it is believed that granitic gneisses have not experienced UHP metamorphism. This implies that a large scale tectonic juxtaposition developed before the epidote-amphibolite facies retrograde metamorphism of UHP rocks during the late continental collision in Dabie Shan.

Allanite-(La) (containing up to 7.80 wt.% V2O3 and with La/ΣREE and La/Ce atomic ratios up to 0.54 and 1.45, respectively) and allanite-(Ce) (up to 8.46 wt.% V2O3) occur in close association with vanadian muscovite, barian tomichite and vanadian titanite in the main ore zone of the Hemlo gold deposit, Ontario, Canada. Allanite-(Ce) generally occurs as a minor constituent in cross-cutting veins or along foliation planes, whereas allanite-(La) invariably occurs in direct contact with titanite. The high V concentrations in the allanites from Hemlo are readily attributable to an adequate local source of this element, and are most likely controlled mainly by a simple substitution of V for Al in octahedral coordination. Vanadian allanite-(La) and vanadian allanite-(Ce), without any systematic differences in other constituents, are clearly distinct in REE composition, in respect to both the relative concentrations of La and Ce and abundances of other REE. The formation of both allanites (Ce- and La-rich) indicates very localised remobilisation and concentration of REE during a late hydrothermal alteration. The unusual REE composition of vanadian allanite-(La) directly reflects partitioning of REE with coexisting titanite, and the formation of this unusual phase may be attributable to replacement of earlier titanite with redistribution of REE in the solid state.