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Crystal-structure refinement of a highly ordered staurolite

1996; Mineralogical Association of Canada; Volume: 34; Issue: 5 Linguagem: Inglês

1499-1276

Roberta Oberti, F. C. Hawthorne, Alberto Zanetti, Luisa Ottolini, Juan I. Soto,

Layered Double Hydroxides Synthesis and Applications

The crystal structure of zinc-rich staurolite, Zn 240 Fe (sub 1.41) Mg (sub 1.27) Mn (sub 0.01) Ni (sub 0.01) Ti (sub 0.09) Li (sub 0.01) Cr (sub 0.02) Al (sub 16.80) Si (sub 7.99) O 45 (OH) 3 , a 7.877(3), b 16.609(5), c 5.665(1) Aa, beta 90.68(2) degrees , V 741.0 Aa 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 MoKalpha 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 (sub M(3)) [= (X (sub M(3A)) -X (sub M(3B)) )/(X (sub M(3A)) +X (sub 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.