Inclusive chemical characterization of tourmaline: Mössbauer study of Fe valence and site occupancy

M. Darby Dyar 1 , Marjorie E. Taylor 1 , Timothy M. Lutz 1 , Carl A. Francis 2 , Charles V. Guidotti 3  and Michael Wise 4
  • 1 Department of Geology and Astronomy, West Chester University, West Chester, Pennsylvania 19383, U.S.A.
  • 2 Harvard Mineralogical Museum, 24 Oxford St., Cambridge, Maine 02138, U.S.A.
  • 3 Department of Geological Sciences, University of Maine, Orono, Maine 04469, U.S.A.
  • 4 Department of Mineral Sciences NHB 119, National Museum of Natural History, Washington, D.C. 20560, U.S.A.


We report here the results of a series of inclusive chemical characterizations, including all elements except oxygen, for a suite of 54 tourmaline samples. A combination of analytical techniques was used to analyze for major and light elements (electron microprobe), Fe3+ and Fe2+ (Mossbauer spectroscopy), H (U extraction), and B, Li, and F (ion microprobe, or SIMS). The B content of the tourmalines studied ranges from 2.86 to 3.26 B per formula unit (pfu) with 31 anions; excess boron is believed to reside in the Si site. Li ranges from 0.0 to 1.44 Li pfu and F contents are 0.0-0.91 pfu. H contents range from nearly anhydrous up to 3.76 H pfu and do not correlate simply with Fe3+ content.

Mössbauer results show that tourmaline exhibits the entire range of Fe3+∑Fe from 0.0-1.0. Fe2+ is represented in the spectra by three doublets, with occupancy in at least three distinct types of Y sites (with different types of nearest and next nearest neighbors). Fe3+ was found in 26 of the 54 samples studied. Although Mössbauer data do not allow the distinction between [Y]Fe3+ and [Z]Fe3+ site occupancies to be made, XRD data on these samples suggest that the majority of Fe3+ is also in Y. Of the samples studied, [4]Fe3+ occurs in nine; five of those were either olenite or uvite with extensive Na substitution. A mixed valence doublet corresponding to delocalized electrons shared between adjacent octahedra was observed in 14 of the samples studied. Projection pursuit regression analysis shows that distribution of Fe among doublets is a function (albeit a complex one) of bulk composition of the tourmaline and supports the interpretation of doublets representing different populations of neighbors. Variations in Fe3+/ Fe2+ ratio cannot be directly related to variations in charge in any single site of the structure. Fe3+/Fe2+ ratio is probably controlled by the prevailing oxidation state in the bulk rock assem­blage, rather than by any particular crystal chemical substitution.

If the inline PDF is not rendering correctly, you can download the PDF file here.


Journal + Issues