Proceedings of the 9th International Conference on X-Ray Absorption Fine Structure

J. Phys. IV France 7 (1997) C2-501-C2-502
DOI: 10.1051/jp4/1997069

Crystal-Chemical Peculiarities of Italian Melanites as Pointed out by XAS at the Al K Edge

A. Marcelli¹, A. Mottana², G. Giuli³, F. Scordari⁴ and Z.Y. Wu<sup>1, 5

1</sup>  IMFN, Laboratori Nazionali di Frascati, P. O. Box 13, 00044 Frascati, Italy
²  Dipartimento di Scienze Geologiche, Terza Università dì Roma, Via Ostiense 169, 00154 Roma, Italy
³  Dipartimento di Scienze della Terra, Università di Camerino, 62032 Camerino, Ita
⁴  Dipartimento Geomineralogico, Università di Bari, ViaOrabona4, 70124 Bari, Italy
⁵  Institut des Materiata de Nantes, UMR 110 du CNRS, Laboratoire de Chimie des Solides, 2 rue de laHoussinière, 44072 Nantes cedex 03, France

Abstract
The Al Is X-ray-absorption spectra of a number of melanitic garnets from two volcanic complexes in Italy (Alban Hills and Mount Vulture) were recorded at SSRL using the YB₆₆ crystal monochromator. Substantial differences occur which appear not to be differentiating between volcanoes; rather they occur within the same complex at different levels, in samples from different pyroclastic units, and reflect local changes in the P-T-f(O₂) conditions. They mainly involve changes in the Al coordination, from octahedral (prevalent) to tetrahedral (occasional), probably matched by opposite changes in the coordination of Fe³⁺.Thus two melanite families are revealed: (i) those essentially related to grossular, with their Al in octahedral coordination, and (ii) those related to andradite, with their Al in tetrahedral coordination.

© EDP Sciences 1997