Marc Maderazzo1,
John M.
Hughes1, 2, M. Darby Dyar3,
George R. Rossman4,
Brandon C. Ackley5, Elizabeth C. Sklute3,
Marian V.
Lupulescu6, 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.
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. One such study,
undertaken on synthetic material of endmember composition, reports
X-ray
diffraction structure refinements that indicate a phase transition from
Pbam
to Pbnm at or just below approximately 283 K,
determined subsequently to
arise from a Peierls-like 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, 0.0186, and 0.0183,
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
Pbnm structure is
observed at 100 K and 90 K,
suggesting similar phase stability for the natural and synthetic
phases.
Comparison of Mössbauer data and X-ray site occupancies between the
natural and
synthetic phases suggests a reinterpretation of Mössbauer site
assignments. We
conclude that the Peierls-like
instability underlying the reported transition
from Pbam to Pbnm in synthetic
material occurs also in our
specimen of natural near-endmember vonsenite at temperatures between
295 K and 100
K.