The atomic arrangement of vonsenite at 295, 100, and 90 K


Marc Maderazzo1, John M. Hughes1,2, M. Darby Dyar3, George R. Rossman4,
Brandon Ackley5, Elizabeth C. Sklute3, Jeffrey Chiarenzelli7

1Department of Geology, University of Vermont, Burlington, Vermont, 05405, U.S.A.

2Department of Geology and Environmental Earth Sciences, Miami University, Oxford, Ohio 45056, U.S.A.

3Department of Astronomy, Mount Holyoke College, South Hadley, Massachusetts 01075, U.S.A.

4Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, California 91125-2500, U.S.A.

5Department of Chemistry, University of Vermont, Burlington, Vermont, 05405, U.S.A. 

6New York State Museum, Research and Collections, 3140 CEC, Albany, New York 12230, U.S.A.

7Department of Geology, St. Lawrence University, Canton, New York 13617, U.S.A.

ABSTRACT

Vonsenite, Fe2+2Fe3+O2BO3, has been the subject of many studies in the materials-science and condensed-matter-physics communities, due to interest in the electronic and magnetic properties and ordering behavior of the phase. All such studies, undertaken on synthetic material of endmember composition, report a structural phase transition from Pbam to Pbnm at or below approximately 283 K, determined subsequently to arise from a Peierls instability. To compare the stability of the natural phase with that of synthetic material, we performed high-precision X-ray crystal structure analyses at 295, 100, and 90 K (R1 = 0.0119, 0.0126, and 0.0124, respectively), Mössbauer spectroscopy at 295, 220, 150, 80, and 4.2 K, and wavelength-dispersive electron microprobe analysis on a vonsenite of near-endmember composition from Jayville, New York, U.S.A. The 283 K structural transition is not observed in the natural specimen, indicating a difference in structural stability between natural and synthetic phases. Comparison of x-ray site occupancies and Mössbauer data suggests a new interpretation of the Mössbauer site assignments. We conclude that the Peierls instability underlying the reported transition from Pbam to Pbnm in synthetic material does not occur in our specimen of natural near-endmember vonsenite.

last revised 22 -Jun-2018