INCLUSIVE CHEMICAL CHARACTERIZATION OF TOURMALINE - MOSSBAUER STUDY OF FE VALENCE AND SITE OCCUPANCY

We report here the results of a series of inclusive chemical character izations, including all elements except oxygen, for a suite of 54 tour maline samples. A combination of analytical techniques was used to ana lyze for major and light elements (electron microprobe), Fe3+ and Fe2 (Mossbauer spectroscopy), H (U extraction), and B, Li, and F (ion mic roprobe, or SIMS). The B content of the tourmalines studied ranges fro m 2.86 to 3.26 B per formula unit (pfu) with 31 anions; excess boron i s 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 anhydro us up to 3.76 H pfu and do not correlate simply with Fe3+ content. Mos sbauer results show that tourmaline exhibits the entire range of Fe3+/ Sigma Fe from 0.0-1.0. Fe2+ is represented in the spectra by three dou blets, with occupancy in at least three distinct types of Y sites (wit h different types of nearest and next nearest neighbors). Fe3+ was fou nd in 26 of the 54 samples studied. Although Mossbauer data do not all ow the distinction between Fe-[Y](3+) and Fe-[Z](3+) site occupancies to be made, XRD data on these samples suggest that the majority of Fe3 + is also in Y. Of the samples studied, Fe-[4](3+) occurs in nine; fiv e of those were either olenite or uvite with extensive Na substitution . A mixed valence doublet corresponding to delocalized electrons share d 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 compositi on of the tourmaline and supports the interpretation of doublets repre senting different populations of neighbors. Variations in Fe3+/Fe2+ ra tio cannot be directly related to variations in charge in any single s ite of the structure. Fe3+/Fe2+ ratio is probably controlled by the pr evailing oxidation state in the bull; rock assemblage, rather than by any particular crystal chemical substitution.