Crystal-structure refinement of a highly ordered staurolite

CNR, Centro di Studio Per la Cristallochimica e la Cristallografia, Pavia, Italy

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.

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