Themal expansivity and dehydrogenation in amphiboles

Few data exist on the thermal expansivity of amphiboles, and they refer to rather exotic compositions which either are not useful to model geologic processes or undergo phase transition. Indeed, amphiboles are not crystal-chemically inert during annealing. Whereas symmetry changes involve only compositions with Mg2 or (NaMg) at the B sites, important changes in cation distribution are frequent during annealing. Moreover, at high T dehydrogenation processes coupled with oxidation of Fe2+ may become significant, and may induce crystal collapse. Both these processes strongly affect the evolution of the unit-cell parameters, and thus quantification of expansivity in amphiboles is tricky. On the other end, amphiboles play a crucial role in the deep crust and upper-mantle regions, especially concerning constraints to their water budget. Thus information of HT-HP behaviour of amphiboles is crucial. This work is part of a systematic project aimed at modelling the HT-HP behaviour of amphiboles, which is based on a long-term expertise in amphibole crystal-chemistry and in situ HT crystallography of anhydrous minerals acquired in Pavia. We started with amphiboles peculiar of upper mantle geological contexts, such as kaersutite and pargasite. Progressive in situ annealing up to the experimental limit of 1050 °C was done on well-characterised crystals with similar overall composition but with different OH and Fe2+ contents, as well as on a fully dehydrogenated Fe2+-free kaersutite where cation disorder is impossible. Measurements of the evolution of the unit-cell parameters were coupled with HT structure refinement to monitor changes in cation ordering and site geometries as well as crystal-chemical signals of increasing dehydrogenation. Thus the different processes could be discriminated, and their effects accurately evaluated by difference. The unit-cell edges of the kaersutite increase linearly up to 1050 °C (b > c > a), while the beta angle decreases linearly; the unit-cell volume increases by 3%. Those of the partially to totally hydrogenated amphiboles have a similar behaviour until 600-700 °C (depending on composition). When the dehydrogenation process starts, the unit-cell edges decrease (a in a quite dramatic way), and the beta angle increases. When the isothermal reaction is complete, the reversal path measured during cooling has quite similar expansivity values. Structure refinements results confirm that: a) during annealing, the tetrahedra do not expand, but 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 asinbeta); b) with ongoing dehydrogenation, a rearrangement of the double chains around a more compressed A site is observed, as well as a increased transfer of Fe2+ from the B to the C sites; c) dehydrogenation is coupled with oxidation of Fe at the M(1) and M(3) sites (which both shrink significantly), and its amount can be estimated based on proper geometrical parameters even at high T.