The natural weathering of staurolite: crystal-surface textures, relative stability, and the rate-determining step

Abstract

Mineral surface-textures on naturally weathered crystals of staurolite [monoclinic, pseudo-orthorhombic; Fe₄Al₁₈Si₈O₄₆(OH)₂] indicate that staurolite weathering is generally interface-limited. Etch pits on naturally weathered staurolites are disk-shaped, extensive parallel to (010), and thin perpendicular to (010). (010) is thep lane of weak bonding and weak cleavage and the orientation of common stacking defects in the staurolite lattice, any of which could account for preferential dissolution in this orientation. Staurolite weathers very slowly relative to most other silicate minerals; this may be due to presence of stable kyanite-like "ribbons" in the staurolite structure or to the low site-energy of the Fe-site in the staurolite structure (compared to other orthosilicates). Staurolite weathering is interface-limited in most weathering environments. Although staurolite contains enough Ai to coat itself completely with a non-porous protective surface layer of Alhydroxides during weathering, protective surface layers apparently form only in some bauxites, where Al is abundance. The capacity of staurolite to form protective surface layers around itself in most weathering environments is apparently limited by its slow weathering, which prevents the release of product-forming elements (especially Ai) at rates sufficient to produce local supersaturation with respect to secondary minerals.

Citation

Velbel, Michael A.; Basso, Charles L., Jr.; Zieg, Michael J. 1996. The natural weathering of staurolite: crystal-surface textures, relative stability, and the rate-determining step. American Journal of Science, Vol. 296: 453-472