J. Phys. IV France
Volume 7, Numéro C2, Avril 1997
Proceedings of the 9th International Conference on X-Ray Absorption Fine Structure
Page(s) C2-501 - C2-502
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. Wu1, 5

1  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

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