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Published November 1984 | public
Journal Article

Ferric Iron in Tourmaline

Abstract

Red Fe^(3+)-rich and black Fe^(3+), Fe^(2+)-rich tourmalines have been studied by optical and Mössbauer spectroscopies to determine the optical characteristics of Fe^(3+) in tourmaline. Prominent optical absorption features at 485 and 540 nm are assigned to transitions of multiple exchange-coupled Fe^(3+) pairs in several site combinations. These transitions are more intense than those of isolated Fe^(3+) and are polarized along the vector between the interacting ions, thus permitting site assignments. The 485 nm band occurs at an unusually low energy for Fe^(3+) in silicate minerals. Similar behavior has been observed in the spectrum of coalingite, Mg_(10)Fe_2^(3+) (OH)_(24)CO_(3)·2H_2O, in which Fe^(3+) occurs in related pairs in edge-shared sheets. These lower energies are proposed to result from magnetic exchange in edge-shared geometries. Antiferromagnetic exchange has been confirmed by a variable temperature magnetic susceptibility study of a Kenyan dravite with 3.36 wt percent Fe. The Mössbauer spectrum of this sample is unusual in that it shows a pronounced decrease in width of component peaks from 298 K to 5 K.

Additional Information

© 1984 Springer-Verlag. Received April 16, 1984. Special thanks are given to Pete Dunn (Smithsonian Institution), Pete Flusser (Los Angeles), Robert Gaal and Peter Keller (Gemmological Institue of America), Julius Petsch (Idar-Oberstein), and Roger Burns (MIT) for providing samples. We would like to thank Wayne Dollase (UCLA), Georg Amthauer (Marburg), and Roger Burns (MIT) for providing Mossbauer spectra and helpful discussions. Our appreciation is also extended to Peter Boyd (USC) for assistance in obtaining the magnetic susceptibility data and Edward Stolper (Caltech) for the use of controlled atmosphere ovens. This study was funded in part by NSF grants EAR 8212540 and EAR 7904801.

Additional details

Created:
August 22, 2023
Modified:
October 26, 2023