On the symmetry and crystal chemistry of britholite: new structural and microanalytical data

Combination of structural and micro-chemical (electron- and ion-probe analysis for intermediate Z and low and high-Z elements, respectively) analysis of two britholites samples gave the following compositions (Mn0.04Ca4.75REE4.37Th0.72U0.12)10.00(Si5.57P0.25B0.16)5.98O24 (OH0.23F1.77)2.00 and (Na0.98Ca2.01REE6.97)9.96 (Si5.07P0.75)5.82 O24 (OH0.53F1.47)2. Structure refinements indicate that the best approximation to the real symmetry is the P63 space group. In britholite, the lowering of symmetry with respect to the P63/m space group of apatite makes the O3 and O3a oxygen atoms no longer equivalent and allows a rotation of the tetrahedron of up to ~ 4° around the Si-O1 bond. Consequently, the O3a oxygen moves closer to the REE1a site, whereas the O3 oxygen moves farther from the REE1 site and closer to the REE2 site, which thus assumes a [7+1] coordination. The infrared spectrum of britholite shows a unique and very broad band in the OH-stretching region at 3437 cm-1, which is consistent with the ordering of trivalent REE cations at the REE2 site. A remarkable constancy in the unit-cell volume along the whole apatite-britholite compositional range is observed for values of the aggregate ionic radius at the REE sites shorter than 1.12 and longer than 1.15 Å, whereas large variations are observed for intermediate values; this behaviour suggests constraints due to the rigidity of the tetrahedral group, that is further enhanced at high symmetry.