Abstract

A cobaltoan staurolite from near the original lusakite locality has the composition Fe2.05Mg1.23Co0.55Ti0.13Zn0.04Mn0.04Al17.94Si7.46O44(OH)4. It is monoclinic, C2/m, with a = 7.782 Å, b = 16.608 Å, c = 5.648 Å, and β = 90.04°. The crystal structure has been refined to R = 0.034 for 1011 reflections with I > 3σ(I), using graphite-monochromatized Mo radiation, and is only slightly different from the structure of two ferroan Fe-Mg staurolites, the only others previously refined in a monoclinic space group. Indices of refraction are α = 1.721, β = 1.730, and γ = 1.740; the measured 2Vx is 84.8°. The optical orientation is X = a, Y = b, and Z = c, with pleochroism X = blue to blue violet, Y = blue violet, and Z = violet. If Co2+ is assumed to have the same optical effect as Fe2+, then a previously established relationship between composition and optic axial angle predicts a 2Vx of 85°.

Two models of Fe-Al ordering in staurolite have been proposed on the basis of both diffraction data and Müssbauer spectra. In one model, Fe and Al are both distributed between the Fe and Al(3) sites; in the other, Fe is essentially restricted to the Fe site, and the Al(3) sites are Fe free. Our X-ray diffraction data fit the disordered model best, with 84% of the total (Fe + Co) in the Fe site and 36% of the Fe sites occupied by Al.

The causes and mechanisms of ordering in staurolite are as yet obscure. This cobaltoan staurolite is about half as ordered as (i.e., “less strongly monoclinic” than) a refined ferroan staurolite (0.17 and 0.33, respectively, by our proposed ordering parameter), but about the same as the other ferroan staurolite (0.16). Differences in ordering may be a function of detailed compositional differences (including differences in hydroxyl content) or of geologic history. The detailed geometry of the electron-density anisotropy about the tetrahedral Fe site suggests the possibility of a metal-metal attractive interaction between the atoms at the face-sharing Fe and U sites. The polyhedral distortions are qualitatively similar in this cobaltoan staurolite and the ferroan specimens, but are generally smaller in the former. The distortions in the kyanite-like layer of staurolite are remarkably similar to those in kyanite itself.

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