Mössbauer effect study of the crystal chemistry of tourmaline

Abstract

One of the main features of the tourmaline group of minerals is that the six coordinatedY andZ sites, possessing different point group symmetries and sizes, share edges. The ME parameters are, therefore, controlled by the mechanisms by which the tourmaline structure obtains congruency along the common edge between the two octahedra. For example, if the size of theY site in elbaite is increased by replacing Li and Al by Fe2+, then, for small values of Fe2+, size congruency is maintained by simultaneously replacing Al by Fe2+ at theZ site. The ME spectra of Fe-bearing elbaite are therefore characterised by two doublets assigned to Fe2+ in theY andZ sites. For higher values of Fe2+ at theY site, however, the tourmaline structure reduces the misfit further by oxidation of Fe2+ at theY site. The effect of the second mechanism is to increase Fe2+-Fe3+ interaction centres and thereby promote charge delocalisation. These Fe-Fe charge interactions introduce three more peaks with ME parameters intermediate between Fe2+ and Fe3+. On the other hand, the replacement of Fe2+ by Mg2+ reduces the density of Fe2+-Fe3+ centres, blocks Fe-Fe charge interactions and increases the intensity of Fe3+.