Select site splitting at M3 in allanite–(Ce)

[Marco E. Ciriotti]

Prossima pubblicazione.

Referenza:
▪ Shen, C., Shu, Z., Gu, X., Dick, J., Feng, Y., Zheng, H., Lu, A. (2022): Select Site splitting at M3 in allanite–(Ce). Mineralogical Magazine, 86, (in press).

Abstract:
We report the crystal structure of allanite-(Ce), with chemistry (Ca1.0REE0.9☐0.1)2.0(Al1.46Fe3+0.52Fe2+0.76Mg0.12Ti0.15)3.01Si3O12(OH) from the Xinfeng 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), Z=2 in space group P21/m and is characterized by site split at M3 into M3a and M3b, at a distance of 0.38(3) Å, which are partially occupied 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, the split of M3 is not present in either ferriallanite-(Ce) or epidote, in which M3 is almost fully occupied either by Fe2+ or by Fe3+. Comparisons of bonding lengths and volumes in cation polyhedra among allanite-(Ce), ferriallanite-(Ce) and epidote tend to indicate that the essential factor that facilitates the site split 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 closely related to distorted M3, which is consistent with the Fe2+ incorporated following REE substitution.

[Marco E. Ciriotti]

Pubblicazione effettuata.

Referenza:
▪ Shen, C., Shu, Z., Gu, X., Dick, J., Feng, Y., Zheng, H., Lu, A. (2022): Select Site splitting at M3 in allanite–(Ce). Mineralogical Magazine, 86, 134-140.