Thermal expansion and dehydrogenation processes in kaersutites: II - models at the atomic scale

Amphiboles are hydrous double-chain silicates, and are a major mineral phase of the Earth's deep-crust and upper mantle. They are considered to play a crucial role in the hosting of water in cold subduction zones to depths of 350-450 km, extending into the Mantle Transition Zone. Accurate models of the HP-HT behavior of amphiboles and of the mechanisms and parameters ruling the release of H are crucial to understand their contribution to the water budget and constrain their role in geophysical models. Quantitative estimates of the non-ambient behavior of unit-cell parameters in kaersutites are reported in Comodi et al. and Zema et al. (this volume), who show that the presence of OH groups favors the expansion of the c edge and the shrinking of the ? angle, but reduces the expansivity of the a edge. Single-crystal structure refinements (SREF) done at T 25, 250, 500, 600, 750 and 900 °C shed light on the atomic-scale mechanisms ruling expansion and dehydrogenation. The loss of H in sample DL5 starts around 600 °C, and thus its behavior is modeled until 500 °C, whereas that of the other samples is modeled in the whole T range. During annealing, the individual tetrahedra do not change significantly; distances corrected for riding motion show that only T(1)-O(7) and T(2)-O(6) expand slightly. In contrast, the octahedra expand significantly. Linear thermal coefficients (?, o10-6) are: ?M(1): DL5 with (OH)0.9 2.13(7); DL5 dehydr. 2.2(2); FR12 2.5(2) ?M(2): DL5 with (OH)0.9 3.1(2); DL5 dehydr. 2.9(3); FR12 4.2(3) ?M(3): DL5 with (OH)0.9 2.6(7); DL5 dehydr. 3.3(1); FR12 3.7(3) In order to keep the space between the double-chain of tetrahedra and the strip of octahedra, both the geometry of the 6-membered rings and the stacking of the double chains of tetrahedra change, yielding a stretching along c and an increase in asin?. The hinge site M(4) expands [?M(4): DL5 with (OH)0.9 3.8(2); DL5 dehydr. 4.4(4); FR12 4.5(2)] in an anisotropic way, because M(4)-O(2) and M(4)-O(4) lengthen slightly, M(4)-O(6)s shortens and M(4)-O(6)l lengthens significantly. The same is true for the A site [?A: DL5 with (OH)0.9 6.2(4); DL5 dehydr. 6.0(4); FR12 5.6(3)], where the A-O(5) distance shortens with T. The loss of H and the concomitant oxidation of Fe decrease the volumes of all polyhedra but T and M(2). Most significant is the abrupt shrinking of the bonds connecting the M(1) and M(3) cations with the O(3) oxygen atoms, which contracts and distorts the M(1) and M(3) sites. From the unit-cell perspective, all the edges shorten with increasing dehydrogenation (a in a quite dramatic way) whereas ? increases. Hence, the volumetric effects of dehydrogenation are somewhat opposite to those of thermal expansion, a possible further factor increasing the HT stability of amphiboles. As observed in comparative studies of amphiboles, the occurrence of Fe3+ at M(1) can be easily detected from its distortion. The M(1)-M(2) distance, which can be used to estimate dehydrogenation (cf. [1] for discussion), lengthens only slightly with T, and with the same slope in the two samples. Thus, the ongoing of dehydrogenation can be estimated also at high T with a simple correction of the standard equation. The refined site-scattering values clearly show that at the ongoing of dehydrogenation, Fe2+ migrates from M(4) and M(2) to the M(1) site, where it oxidizes to Fe3+, a behavior that is opposite to that observed in cpx during annealing. Both the fully dehydrogenated samples have solely Ca and Mg at the M(4) site. No further change in cation ordering was observed during this study. Insights into the intriguing discrepancies with thermal behaviors of cpx and micas will be also discussed. References [1] Oberti, Hawthorne, Cannillo, Camara(2007) Rev Mineral Geochem 67, 125-171