Singolarita' cristallochimiche di melaniti italiane messe in evidenza dalla spettroscopia di assorbimento dei raggi-X in luce di sincrotrone alla soglia K dell'alluminio.

The aluminium 1s X-ray-absorption spectra of a number of melanitic garnets from two volcanic complexes, Alban Hills in Latium and Mount Vulture in Basilicata, have been recorded at SSRL using the novel YB66 crystal monochromator. Substantial differences occur both at the edge and in the intermediate multiple-scattering region. They appear not to be differentiating among the volcanic complexes; rather, they occur within the same complex at different levels, in samples from different pyroclastic units. These differences 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: those essentially related to grossular, with their Al in octahedral coordination, and those related to andradite, having a substantial part of their Al in tetrahedral coordination. As a local probe, XANES spectroscopy at the K-edge of low Z elements appears to be a perfect tool to investigate the role played by atoms located in different coordination shells.