Cation order in the crystal structure of 'minasgeraisite-(Y)': Dedicated to our friend the late Luis Menezes, who brought this mineral to our attention and graciously provided a superior specimen with crystals suitable for single-crystal structure work. (28th February 2018)

• Record Type: Journal Article
• Authors: Cooper, Mark A.; Hawthorne, Frank C.
• Is Part Of: Mineralogical magazine. Volume 82:Number 2(2018)
• Page Start: 301
• Page End: 312
• Publication Date: 2018-02-28
• Subjects: minasgeraisite-(Y), -- crystal structure, -- P1 symmetry, -- cation order, -- gadolinite–datolite group; Mineralogy -- Periodicals; Mineralogy -- Great Britain -- Periodicals; 549.05
• DOI: 10.1180/minmag.2017.081.043
• Languages: English
• ISSNs: 0026-461X

Abstract

Abstract: The crystal structure of 'minasgeraisite-(Y)', triclinic P 1, 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) Å 3 and Z = 1, has been refined to an R 1 index of 2.86% for 4170 observed (| F o | > 4σ F ) reflections. Significant observed (| F o | > 40–60 σ F ) reflections violate the presence of a 21 -screw axis and an a -glide plane, negating the space group P 21 / 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 P 1 symmetry, and R psi is based on a mean data redundancy factor of > 3. Both the data merging and an | E 2 –1| value of 0.773 indicate that P 1 is the correct space group. The general formula for the gadolinite–datolite group is W 2 XZ 2 T 2 O 8 V 2 ( 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 W 1, Ca dominant at W 2, and Y dominant at W 3 and W 4. 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)4 Si4 O16 [(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.