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

Abstract

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 par- allel 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 preferen- tial 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 weath- erin environments. Although staurolite contains enough Al to coat itsel f completely with a non-porous protective surface layer of Al- hydroxides 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 weath- ering 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 second- ary minerals.