Notwithstanding our present detailed knowledge of amphibole crystal-chemistry, HT behaviour in amphiboles is still a difficult issue to be addressed. Indeed, thermal annealing generally induces three simultaneous processes: thermal expansion, cation disordering, and deprotonation; hence, understanding the details of the different crystal-chemical mechanisms is not straightforward. The relations and the relative importance of the three processes, as well as their onset temperatures, vary as a function of composition; hence, a systematic work has been undertaken and is still in progress. First, the simplest compositions were studied, where both cation disordering and deprotonation processes are excluded. Then, the study was extended to more complex compositions, which are however of major relevance to petrology. In order to build the most accurate model as possible, different techniques were applied, and all were optimized in order to take into account the peculiarities of both the experimental setting and the mineral matrix. Indeed, it soon turned out that each of the techniques applied would have profited of a careful inter-calibration. On the structural (crystallographic) side, in operando measurement of unit-cell parameters was combined with single-crystal structure refinement done at temperature values chosen to monitor the crucial step of the irreversible deprotonation/oxidation process. The same approach was applied on cooling, in order to investigate the behavior of the deprotonated phase. The available crystal-chemical knowledge was used to model changes in site-scattering, site and chain geometry, as well as in atomic displacement parameters. To make a very long story short, we concluded that oxidation proceeds by M(4)Fe2+ migration (when available) to the M(1,3) sites and by its preferential oxidation at the M(1) site. However, the structural response varies significantly as a function of composition.
Combining structure refinement and spectroscopies: hints and warnings for more efficient tools to decipher the mechanisms of deprotonation in amphiboles
2015
Abstract
Notwithstanding our present detailed knowledge of amphibole crystal-chemistry, HT behaviour in amphiboles is still a difficult issue to be addressed. Indeed, thermal annealing generally induces three simultaneous processes: thermal expansion, cation disordering, and deprotonation; hence, understanding the details of the different crystal-chemical mechanisms is not straightforward. The relations and the relative importance of the three processes, as well as their onset temperatures, vary as a function of composition; hence, a systematic work has been undertaken and is still in progress. First, the simplest compositions were studied, where both cation disordering and deprotonation processes are excluded. Then, the study was extended to more complex compositions, which are however of major relevance to petrology. In order to build the most accurate model as possible, different techniques were applied, and all were optimized in order to take into account the peculiarities of both the experimental setting and the mineral matrix. Indeed, it soon turned out that each of the techniques applied would have profited of a careful inter-calibration. On the structural (crystallographic) side, in operando measurement of unit-cell parameters was combined with single-crystal structure refinement done at temperature values chosen to monitor the crucial step of the irreversible deprotonation/oxidation process. The same approach was applied on cooling, in order to investigate the behavior of the deprotonated phase. The available crystal-chemical knowledge was used to model changes in site-scattering, site and chain geometry, as well as in atomic displacement parameters. To make a very long story short, we concluded that oxidation proceeds by M(4)Fe2+ migration (when available) to the M(1,3) sites and by its preferential oxidation at the M(1) site. However, the structural response varies significantly as a function of composition.I documenti in IRIS sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
