The crystal structure of zinc-rich staurolite, Zn2.40Fe1.41Mg1.27Mn0.01Ni0.01Ti0.09Li0.01Cr0.02Al16.80Si7.99O45(OH)(3), a 7.877(3), b 16.609(5), c 5.665(1) Angstrom, beta 90.68(2)degrees, V 741.0 Angstrom(3), Z = 1, from the Nevado-Filabride complex, Lubrin, Almeria province, Spain, has been refined to an R index of 2.0% based on 986 observed reflections collected with MoK alpha X-radiation. This crystal is poor in Al, as reflected in the high Si content and the large amount of [6]-coordinated Mg. The beta value is the largest yet measured for staurolite, and is slightly larger than the maximum beta-angle predicted for staurolite (Hawthorne et al. 1993b). Site-scattering refinement shows that the M(3) cations are highly, but not completely, ordered: Q(M(3)) [= (X(M(3A)) - X(M(3B)))/X(M(3A)) + X(M(3B)))] is equal to 0.89, rather than to the value of 1.0 expected for the beta angle of this crystal. However, the refined M(3A) and M(3B) site-scattering values are compatible with complete order of Al at M(3A) but disorder of Mg over M(3A) and M(3B). The results of this work quantitatively support the order-disorder model for staurolite proposed by Hawthorne et al. (1993b), but suggest that the quantitative details can be affected by the presence of [6]-coordinated Mg because it behaves differently from [6]-coordinated Al.
Crystal-structure refinement of a highly ordered staurolite
Oberti R;Zanetti A;Ottolini L;
1996
Abstract
The crystal structure of zinc-rich staurolite, Zn2.40Fe1.41Mg1.27Mn0.01Ni0.01Ti0.09Li0.01Cr0.02Al16.80Si7.99O45(OH)(3), a 7.877(3), b 16.609(5), c 5.665(1) Angstrom, beta 90.68(2)degrees, V 741.0 Angstrom(3), Z = 1, from the Nevado-Filabride complex, Lubrin, Almeria province, Spain, has been refined to an R index of 2.0% based on 986 observed reflections collected with MoK alpha X-radiation. This crystal is poor in Al, as reflected in the high Si content and the large amount of [6]-coordinated Mg. The beta value is the largest yet measured for staurolite, and is slightly larger than the maximum beta-angle predicted for staurolite (Hawthorne et al. 1993b). Site-scattering refinement shows that the M(3) cations are highly, but not completely, ordered: Q(M(3)) [= (X(M(3A)) - X(M(3B)))/X(M(3A)) + X(M(3B)))] is equal to 0.89, rather than to the value of 1.0 expected for the beta angle of this crystal. However, the refined M(3A) and M(3B) site-scattering values are compatible with complete order of Al at M(3A) but disorder of Mg over M(3A) and M(3B). The results of this work quantitatively support the order-disorder model for staurolite proposed by Hawthorne et al. (1993b), but suggest that the quantitative details can be affected by the presence of [6]-coordinated Mg because it behaves differently from [6]-coordinated Al.I documenti in IRIS sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
