Cation order in the crystal structure of ‘minasgeraisite-(Y)’

Abstract

The crystal structure of ‘minasgeraisite-(Y)’, triclinic P1, a = 9.994(4), b = 7.705(3), c = 4.764(2) Å, α = 90.042(9), β = 90.218(14), γ = 90.034(9) (°), V = 366.8(5) ų and Z = 1, has been refined to an R₁ index of 2.86% for 4170 observed (|F_o| > 4σF) reflections. Significant observed (|F_o| > 40–60 σF) reflections violate the presence of a 2₁-screw axis and an a-glide plane, negating the space group P2₁/a previously found for minerals of the gadolinite–datolite group. Averaging of the X-ray data in Laue groups 2/m and $\bar 1$ gives the following agreement indices: 2/m (9.68%) and $\bar 1$ (5.68%). The internal agreement index from averaging of identical reflections collected at multiple positions along the diffraction vector is significantly lower than that for the Laue group $\bar 1$: R_psi = 2.40%, where 13,109 reflections were collected, 4288 are unique for P1 symmetry, and R_psi is based on a mean data redundancy factor of > 3. Both the data merging and an |E²–1| value of 0.773 indicate that P1 is the correct space group. The general formula for the gadolinite–datolite group is W₂XZ₂T₂O₈V₂ (Z = 2) which we have expanded to 20 anions (Z = 1) to show the W-site cation ordering present in ‘minasgeraisite-(Y)’. Bismuth, Ca and REE are ordered over four W sites, with Bi dominant at W1, Ca dominant at W2, and Y dominant at W3 and W4. The dominant constituent at the X sites is a vacancy, and Ca does not occur at the X sites. Significant B and Si are assigned to the Be-dominant Z sites, and the T sites are occupied by Si. The simplified ‘minasgeraisite-(Y)’ formula (Z = 1) is BiCa(Y,Ln)₂(□,Mn)₂(Be,B,Si)₄Si₄O₁₆ [(OH),O]₄. ‘Minasgeraisite-(Y)’ should be assigned to a triclinic subgroup of the gadolinite–datolite group, and its lower symmetry suggests that Ca-substituted gadolinites and hingganites should be examined for evidence of triclinic symmetry associated with cation order at the W sites.