Hawthorne, Frank C. Department of Geological Sciences, University of Manitoba, Winnipeg, Canada.
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A nesosilicate mineral occurring in metamorphic rocks. The chemical formula of staurolite may be written as A4B4C18D4T8O40X8, where A = Fe2+, Mg, □ (□ = vacancy, □ > 2); B = Fe2+, Zn, Co, Mg, Li, Al, Fe3+, Mn2+, □; C = Al, Fe3+, Cr, V, Ti; D = Al, Mg, □ (□ > 2); T = Si, Al; X = OH, F, O. Staurolite is an order–disorder series between an ordered monoclinic structure (with β = 90.64°) and a disordered orthorhombic structure (with β = 90°). The structure of staurolite consists of slabs of the kyanite structure [Al2SiO5] and layers of Fe-Al oxy-hydroxide [∼ Al0.67Fe2 (OH)2] alternating along , the monoclinic symmetry axis. The kyanite slab consists of edge-sharing chains of AlO6 octahedra (Fig. 1a, regular dots) extending along the c direction, flanked by AlO6 octahedra (random dots) and cross-linked by SiO4 tetrahedra (crosses); only small amounts of Mg and Fe substitute for Al in this slab. The oxide-hydroxide layer consists of edge-sharing chains of (Al,□)O6 octahedra (Fig. 1b, regular dots) extending along the c axis, alternating with chains of (Al,□)O6 octahedra (random dots) and (FeO4) tetrahedra (crosses). In the latter chains, the (Al,□)O6 octahedra share edges; the (FeO4) tetrahedra share faces with adjacent (Al,□)O6 octahedra, and polyhedra adjacent across a shared face cannot both be locally occupied. The extensive coupling of substitutions involving the cations Al, Fe2+, H and □ (vacancy) gives rise to extensive short-range order, and local bond-valence arrangements are optimal for the formula [Fe2+4Al18-Si8O44(OH)4]2+. However, at the long-range level, a crystal cannot have a net charge, and hence the principal heterovalent substitutions in staurolite [Al HaRR; Si, Mg Al, □ H, Li HaRR; Fe2+, □ hArr; Fe2+] all act such as to reduce this charge to zero. Thus the chemical complexity of staurolite results from the interaction of long-range and short-range charge-balance requirements.
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