The Natural Weathering of Staurolite: Crystal-surface Textures, Relative Stability, and the Rate-determining Step

Mineral surface-textures on naturally weathered c of staurolite [monoclinic, pseudo-orthorhombic; Fe4Al18Sis046( rtalS indicate that staurolite weathering is general1 interface-limited. Etch pits on naturally weathered staurolites are disi-shaped, extensive parallel to (OIO), and thin perpendicular to (010). (010) is the plane 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 orientatxon. 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 weatherin environments. Although staurolite contains enough Al to coat itsel f completely with a non-porous protective surface layer of Alhydroxides during weathering, protective surface layers apparently form only in some bauxites, where A1 is abundant. The capacity of staurolite to form protective surface layers around itself in most weathering environments is a parently limited by its slow weathering, which prevents the release o !' product-forming elements (especially Al) at rates sufficient to produce local supersaturation with respect to secondary minerals.