CRYSTAL STRUCTURE OF SR-RICH CRANDALLITE; INSIGHTS INTO THE CRANDALLITE STRUCTURE

During a powder diffraction study of variscite, we required an accurate structure for the admixed crandallite, to use in Rietveld refinements. Published data reveal a disordered, 50% occupied H site, suggesting the possibility of lower symmetry and prompting us to reexamine the structure. Crandallite, CaAl₃(OH)₆(PO₄)(PO₃OH), a member of the plumbogummite group, typically occurs in cryptocrystalline masses and rarely in macroscopic single crystals. In the absence of suitable single crystals of end-member crandallite, we studied a Sr-bearing specimen (Ca_0.56Sr_0.44) from the Alto Benedito pegmatite, Paraiba, Brazil (composition from Cassedanne & Silva Nen, Min. Rec., 1991; sample provided by G. Harlow, AMNH). Data were measured on a ~123 × 135 × 191 µm crystal using a Bruker Venture D8 diffractometer with a Photon III detector at 253(2) K. 123,062 reflections to 73.88° θ (0.37 Å resolution), of which 1,796 were independent, were measured using graphite-monochromatized Mo Kα radiation. Some have proposed that the Ba analog of crandallite (gorceixite) has space group Cm, but previous studies of the Sr analog (goyazite) found space group R-3m. We evaluated other space groups, but our data also supported space group, R-3m, determined based on intensity statistics and systematic absences. The structure was solved and refined using the SHELX suite of programs, and two distinct H atoms were located using difference-Fourier maps, one set comprising OH in the XII-coordinated Sr,Ca site and the Al octahedron and the other site 50% occupied on the apical oxygen of the PO₄ tetrahedron. Unit-cell parameters are a = 6.9857(2) Å, c = 16.4393(7) Å, and V = 694.76(5) ų. Final anisotropic full-matrix least-squares refinement on F² with 29 variables converged at R1 = 3.96%. Refined occupancy of the XII-coordinated site was Sr_0.52Ca_0.48, in reasonable agreement with published data. We found no evidence of H ordering, although if H atoms are primarily responsible for a lower symmetry, this may be difficult to determine using X-rays. Ultimately we confirmed the published structures for goyazite and crandallite, including a statistical distribution of paired PO₄ and PO₃(OH) tetrahedra.

This contribution is in celebration of the career of Jeffrey Post, upon his retirement from the Smithsonian Institution.