Despite their importance in many petrogenetic contexts and their relevance for the release of water in subduction zones, very few data are available on the HT behaviour of amphiboles, and almost all refer to fluoro-amphiboles with very simple compositions, quite different from those of interest for Earth Sciences. Therefore, we have undertaken a systematic investigation of the HT behaviour of different nature-occurring C2/m and Pnma amphibole compositions. Experiments are done in situ on a single-crystal diffractometer in the T range 25-1050°C, and no buffering is applied. The evolution of the unit-cell parameters is followed at T steps of 25°C, and data collections are performed every 200-250°C in order to check changes in a) the geometry of the crystal structure; b) the cation partitioning between the different sites; c) the beginning and the mechanisms of dehydrogenation. The reversal path during cooling is also checked to discard the presence of some crystal-chemical hysteresis. This work reports on the first results, which were obtained on a completely dehydrogenated kaersutite with a composition preventing cation disorder (FR12) and on a partially dehydrogenated pargasite with an Fe2+ content sufficient to allow complete dehydrogenation at high temperature (DL5). Both the samples had been previously fully characterised by single-crystal structure refinement, electron (EMP) and ion (SIMS) microprobe and Mössbauer spectroscopy[1]. The two samples show linear variations of the unit-cell parameters, with the edges increasing (b > a > c) and the ? angle decreasing with T. Around 650°C, DL5 undergoes an abrupt decreases in the edges and increase in the ? angle, which are signals of dehydrogenation. After completion of the dehydrogenation process, a further annealing experiment shows a behavior similar to that of FR12. The values of the linear thermal expansion coefficients (?, o10-6) for the unit-cell and the M sites are: FR12 (50-1050°C): ?a 0.753(9), ?b 1.27(1), ?c 0.875(7), ?? -0.375(5), ?V 3.12(1), ?M(1) 2.5(2) ?M(2) 4.2(3) ?M(3) 3.7(3); DL5(OH)=0.9 (50-650°C): ?a 0.76(1), ?b 1.22(4), ?c 0.92(1), ?? -0.42(1), ?V 3.12(3), ?M(1) 2.13(7) ?M(2) 3.1(2) ?M(3)2.6(7); DL5 dehydr (50-1050°C): ?a 0.799(8), ?b 1.26(5), ?c 0.86(9), ?? -0.35(1), ?V 3.10(1), ?M(1) 2.2(2) ?M(2) 2.9(3) ?M(3) 3.3(1). Thus the presence of (OH) groups mostly affects the expansivity of c and ?. During the dehydrogenation process, Fe2+ moves from the M(4) and M(2) sites to the M(1) site, where it oxidizes to allow local electroneutrality after the loss of the proton at the coordinated O(3) oxygen atom. The complex pattern of geometrical modifications occurring both during thermal annealing and during dehydrogenation will also be discussed in detail. References 1.A. Zanetti, G. Pedrazzi, R. Oberti (2000) Plinius 24, 220-221.
HT behaviour of pargasite and kaersutite: expansivity coefficients and dehydrogenation mechanisms
2009
Abstract
Despite their importance in many petrogenetic contexts and their relevance for the release of water in subduction zones, very few data are available on the HT behaviour of amphiboles, and almost all refer to fluoro-amphiboles with very simple compositions, quite different from those of interest for Earth Sciences. Therefore, we have undertaken a systematic investigation of the HT behaviour of different nature-occurring C2/m and Pnma amphibole compositions. Experiments are done in situ on a single-crystal diffractometer in the T range 25-1050°C, and no buffering is applied. The evolution of the unit-cell parameters is followed at T steps of 25°C, and data collections are performed every 200-250°C in order to check changes in a) the geometry of the crystal structure; b) the cation partitioning between the different sites; c) the beginning and the mechanisms of dehydrogenation. The reversal path during cooling is also checked to discard the presence of some crystal-chemical hysteresis. This work reports on the first results, which were obtained on a completely dehydrogenated kaersutite with a composition preventing cation disorder (FR12) and on a partially dehydrogenated pargasite with an Fe2+ content sufficient to allow complete dehydrogenation at high temperature (DL5). Both the samples had been previously fully characterised by single-crystal structure refinement, electron (EMP) and ion (SIMS) microprobe and Mössbauer spectroscopy[1]. The two samples show linear variations of the unit-cell parameters, with the edges increasing (b > a > c) and the ? angle decreasing with T. Around 650°C, DL5 undergoes an abrupt decreases in the edges and increase in the ? angle, which are signals of dehydrogenation. After completion of the dehydrogenation process, a further annealing experiment shows a behavior similar to that of FR12. The values of the linear thermal expansion coefficients (?, o10-6) for the unit-cell and the M sites are: FR12 (50-1050°C): ?a 0.753(9), ?b 1.27(1), ?c 0.875(7), ?? -0.375(5), ?V 3.12(1), ?M(1) 2.5(2) ?M(2) 4.2(3) ?M(3) 3.7(3); DL5(OH)=0.9 (50-650°C): ?a 0.76(1), ?b 1.22(4), ?c 0.92(1), ?? -0.42(1), ?V 3.12(3), ?M(1) 2.13(7) ?M(2) 3.1(2) ?M(3)2.6(7); DL5 dehydr (50-1050°C): ?a 0.799(8), ?b 1.26(5), ?c 0.86(9), ?? -0.35(1), ?V 3.10(1), ?M(1) 2.2(2) ?M(2) 2.9(3) ?M(3) 3.3(1). Thus the presence of (OH) groups mostly affects the expansivity of c and ?. During the dehydrogenation process, Fe2+ moves from the M(4) and M(2) sites to the M(1) site, where it oxidizes to allow local electroneutrality after the loss of the proton at the coordinated O(3) oxygen atom. The complex pattern of geometrical modifications occurring both during thermal annealing and during dehydrogenation will also be discussed in detail. References 1.A. Zanetti, G. Pedrazzi, R. Oberti (2000) Plinius 24, 220-221.I documenti in IRIS sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
