During the last 20 years, an impressive amount of work has been done in Pavia on the chemical variability and crystal-chemistry of amphiboles. One of the most important recent achievements is the detection and exploration of the new realm of light and volatile elements in amphiboles. This work showed that Li-rich and OH-deficient amphiboles are far more widespread than expected, and gave rise to the identification of many new root-names and species. Li and H detection and quantification was first made possible thanks to the willingness and efforts of the colleagues of the SIMS lab, and distribution and crystal-chemical mechanisms of incorporation were then cross-checked based on structure refinement (SREF) and Mössbauer and FTIR spectroscopy. In order to avoid these long-lasting and expensive analyses, methods were derived to accurately estimate the amount of Li (and its partitioning between the B and C sites) and of OH based solely on more or less complex relations of selected SREF results. Amazingly, in this way structure refinement turns out to be a very powerful analytical tool even for low-Z elements. Other new features of amphibole crystal-chemistry were treated in the same way. These procedures joined those already available to calculate and optimise the crystal-chemical formulae of "standard" amphiboles, which have been however implemented based on the presently available dataset. At the end of this long-lasting and strongly iterative work, we now have the pleasure to make available to the scientific community COMAMPH, a simple and straightforward tool to calculate the crystal-chemical formula of almost any C2/m amphibole based solely on SREF results. This program allows researchers to understand in a fast and efficient way whether or not a given crystal contains Li or F, is partially or totally dehydrogenated (and clarifying the crystal-chemical mechanism ruling during-crystallization or post-crystallization dehydrogenation), or presents other crystal-chemical features such as Mg and Fe2+ at the B sites, Ca at the A site, Ti and Al at the T(2) sites. All these substitutions had been so far considered as oddities, but are actually not so rare and rather crucial for amphibole crystal-chemistry. Also, the program is useful to calculate a reliable unit formula and group-site distribution starting from EMP analyses, in that it provides sound constraints such as the number of A cations, the amount of Li at the C sites or that of Al at the T sites, the presence and amount of the oxo-component. Failures to calculate a reasonable and neutral formula usually highlight either mistakes in the refined model or interesting new crystal-chemical features to be investigated further. The program also allows classification according to the new IMA amphibole nomenclature (cf. Hawthorne and Oberti 2007 for some discussion), which will be hopefully published soon.
COMAMPH: a snapshot of the crystal-chemical formula of amphiboles based on structure refinement
OBERTI ROBERTA;
2009
Abstract
During the last 20 years, an impressive amount of work has been done in Pavia on the chemical variability and crystal-chemistry of amphiboles. One of the most important recent achievements is the detection and exploration of the new realm of light and volatile elements in amphiboles. This work showed that Li-rich and OH-deficient amphiboles are far more widespread than expected, and gave rise to the identification of many new root-names and species. Li and H detection and quantification was first made possible thanks to the willingness and efforts of the colleagues of the SIMS lab, and distribution and crystal-chemical mechanisms of incorporation were then cross-checked based on structure refinement (SREF) and Mössbauer and FTIR spectroscopy. In order to avoid these long-lasting and expensive analyses, methods were derived to accurately estimate the amount of Li (and its partitioning between the B and C sites) and of OH based solely on more or less complex relations of selected SREF results. Amazingly, in this way structure refinement turns out to be a very powerful analytical tool even for low-Z elements. Other new features of amphibole crystal-chemistry were treated in the same way. These procedures joined those already available to calculate and optimise the crystal-chemical formulae of "standard" amphiboles, which have been however implemented based on the presently available dataset. At the end of this long-lasting and strongly iterative work, we now have the pleasure to make available to the scientific community COMAMPH, a simple and straightforward tool to calculate the crystal-chemical formula of almost any C2/m amphibole based solely on SREF results. This program allows researchers to understand in a fast and efficient way whether or not a given crystal contains Li or F, is partially or totally dehydrogenated (and clarifying the crystal-chemical mechanism ruling during-crystallization or post-crystallization dehydrogenation), or presents other crystal-chemical features such as Mg and Fe2+ at the B sites, Ca at the A site, Ti and Al at the T(2) sites. All these substitutions had been so far considered as oddities, but are actually not so rare and rather crucial for amphibole crystal-chemistry. Also, the program is useful to calculate a reliable unit formula and group-site distribution starting from EMP analyses, in that it provides sound constraints such as the number of A cations, the amount of Li at the C sites or that of Al at the T sites, the presence and amount of the oxo-component. Failures to calculate a reasonable and neutral formula usually highlight either mistakes in the refined model or interesting new crystal-chemical features to be investigated further. The program also allows classification according to the new IMA amphibole nomenclature (cf. Hawthorne and Oberti 2007 for some discussion), which will be hopefully published soon.I documenti in IRIS sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
