T116 Charting Soils at the Atomic Scale II: A Tribute to the Careers of David L. Bish and Jeffrey E. Post

Monday, 10 October 2022
1:30 PM - 5:30 PM
607

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

Paper number: T116-11
Program: Technical Sessions
Time: 4:05 PM – 4:20 PM

Authors

  • Maren Pink
    Indiana University, Molecular Structure Center, Bloomington, IN
  • David Bish Presenting Author
    Indiana University, Molecular Structure Center, Bloomington, IN
  • Matteo Ardit
    University of Ferrara, Ferrara

Abstract

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 (Ca0.56Sr0.44) 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) Å3. 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 Sr0.52Ca0.48, 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.

Geological Society of America Abstracts with Programs. Vol. 54, No. 6

doi: 10.1130/abs/2022AM-378011

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