The crystal chemistry of gedrite-group amphiboles and its relations with their monoclinic BCa counterparts have been only recently clarified.1 This work provides information on the behaviour of gedrite at high-T conditions, in terms of thermal expansivity, cation order and dehydrogenation processes. The only data now available on thermal expansivity of orthorhombic amphiboles concern anthophyllite, ideally AoBMg2CMg5TSi8O22(OH)2,2 where dehydrogenation is prevented by the lack of Fe2+ (in amphiboles, the reaction proceeds via oxidation of Fe2+ at the M1 and M3 sites which are both bonded to the involved O3 oxygen atom). Gedrite sample A(26), with reported composition ANa0.46B(Mg1.11Fe2+0.83Mn0.02Ca0.04) C(Mg3.44 Fe2+0.33 Al1.18Ti0.05)T(Si6.26 Al1.74)O22OH2,1 was annealed up to 1273 K at intervals of 25 K using a micro furnace mounted on the diffractometer. Unit-cell parameters were measured in situ at each step, and structure refinements were done from single-crystal data collected at 533, 723 and 973 K during both annealing and reversal (thus on the partially dehydrogenated phase). Unit-cell parameters increase almost linearly up to 973 K (?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), where an abrupt contraction in all the edges (much stronger in a) indicates the start of the dehydrogenation process. Dehydrogenation proceeds by oxidation of all Fe, which migrates mainly from the M4 to the M1 and M3 sites, thus causing significant changes in site geometry. 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 (for anthophyllite, ?a = 1.49 ?10-5 K-1)2, and is related to the presence of a half-filled A site in gedrite. In the dehydrogenated sample, Na disorders into two different positions within the A cavity, the second of which is closer to the A double-chain of tetrahedra and particularly closer 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. 1Schindler, M.; Sokolova, E.; Abdu, Y.; Hawthorne, F.C.; Evans, B.E.; and Ishida, K., Mineral. Mag. 2008, 72, 703-730. 2 Cámara, F.; Welch, M.D.; and Oberti, R., IMA2010, Abstracts 2010.
High-T studies of orthorhomic amphiboles: the dehydrogenation process and its effect on cation ordering and thermal expansivity in Gedrite
Oberti R;
2010
Abstract
The crystal chemistry of gedrite-group amphiboles and its relations with their monoclinic BCa counterparts have been only recently clarified.1 This work provides information on the behaviour of gedrite at high-T conditions, in terms of thermal expansivity, cation order and dehydrogenation processes. The only data now available on thermal expansivity of orthorhombic amphiboles concern anthophyllite, ideally AoBMg2CMg5TSi8O22(OH)2,2 where dehydrogenation is prevented by the lack of Fe2+ (in amphiboles, the reaction proceeds via oxidation of Fe2+ at the M1 and M3 sites which are both bonded to the involved O3 oxygen atom). Gedrite sample A(26), with reported composition ANa0.46B(Mg1.11Fe2+0.83Mn0.02Ca0.04) C(Mg3.44 Fe2+0.33 Al1.18Ti0.05)T(Si6.26 Al1.74)O22OH2,1 was annealed up to 1273 K at intervals of 25 K using a micro furnace mounted on the diffractometer. Unit-cell parameters were measured in situ at each step, and structure refinements were done from single-crystal data collected at 533, 723 and 973 K during both annealing and reversal (thus on the partially dehydrogenated phase). Unit-cell parameters increase almost linearly up to 973 K (?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), where an abrupt contraction in all the edges (much stronger in a) indicates the start of the dehydrogenation process. Dehydrogenation proceeds by oxidation of all Fe, which migrates mainly from the M4 to the M1 and M3 sites, thus causing significant changes in site geometry. 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 (for anthophyllite, ?a = 1.49 ?10-5 K-1)2, and is related to the presence of a half-filled A site in gedrite. In the dehydrogenated sample, Na disorders into two different positions within the A cavity, the second of which is closer to the A double-chain of tetrahedra and particularly closer 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. 1Schindler, M.; Sokolova, E.; Abdu, Y.; Hawthorne, F.C.; Evans, B.E.; and Ishida, K., Mineral. Mag. 2008, 72, 703-730. 2 Cámara, F.; Welch, M.D.; and Oberti, R., IMA2010, Abstracts 2010.I documenti in IRIS sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
