The Al Is X-ray absorption spectra of a number of melanitic garnets from two volcanic complexes in Italy (Alban Kills 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(O-2) 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.
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|Titolo:||Crystal chemical peculiarities of Italian melanites pointed out by XAS at the Al K-edge|
|Data di pubblicazione:||1997|
|Appare nelle tipologie:||Articolo|