Thermal expansivity and dehydrogenation process in gedrite

The crystal chemistry of gedrite-group amphiboles has been recently discussed by [1,2], who also clarified relations among bond topology, bond valence and site preference and identified similarities and differences between monoclinic and orthorhombic amphiboles. The present work is aimed at providing information on the behaviour of gedrite at high-T conditions, in terms of thermal expansivity and cation order and dehydrogenation processes. The only data available on thermal expansivity of orthorhombic amphiboles concern anthophyllite [3], and no dehydrogenation occurs due to the lack of Fe2+. Sample A(26) in [1] (code AMNH136484), with reported composition ANa0.46 B(Mg1.11Fe2+0.83Mn0.02Ca0.04) C(Mg3.44 Fe2+0.33 Al1.18Ti0.05) T(Si6.26 Al1.74) O22OH2, was annealed up to 1273 K at intervals of 25 K using a micro furnace. Unit-cell parameters were measured in situ at each step, and structure refinement was done from single-crystal diffraction data collected at 533, 723 and 973 K during both annealing and reversal (thus on the partially dehydrogenated phase). The almost linear increase in the unit-cell parameters (?V = 3.15 ?10-5 K-1, ?a = 1.10 ?10-5 K-1, ?b = 0.94 ?10-5 K-1, ?c = 1.19 ?10-5 K-1) ends around 973 K, where an abrupt contraction in all the edges (much stronger in a) indicates the start of the dehydrogenation process, which proceeds by migration and oxidation of all Fe from the M4 to the M1 and M3 sites and significant changes in site geometry. When dehydrogenation stops (as monitored through unit-cell variations), reversal experiments down to RT allowed calculation of thermal expansion coefficients of the partially dehydrogenated phase: ?V = 3.51 ?10-5 K-1, ?a = 1.15 ?10-5 K-1, ?b = 1.09 ?10-5 K-1, ?c = 1.26 ?10-5 K-1. The crystal was then heated at 1273 K and finally cooled to room T to check for the completeness (as far as allowed by the Fe2+ content) of the dehydrogenation process. A significant difference in thermal expansivity with respect to anthophyllite is observed only for the much greater stiffness of the a edge (?a = 1.49 ?10-5 K-1 in [3]), and is related to the presence of a half-filled A site in gedrite. Na disorders into two different positions within the A cavity in the dehydrogenated sample, the second of which is closer to the A chain and particularly to the O3A oxygen atom. Hence, the dehydrogenated samples has a different bond valence arrangement. The behaviour of the different cation polyhedra and structure moduli during annealing and dehydrogenation, and differences with respect to anthophyllite and monoclinic amphiboles will be discussed. [1] Schindler, M. et al. (2008) Mineral. Mag., 72, 703-730. [2] Hawthorne, F.C. et al. (2008) Mineral. Mag., 72, 731-745. [3] Cámara F. et al. (2010), this meeting.