Pyroxene weathering to smectite: Conventional and cryo-field emission scanning electron microscopy, Koua Bocca ultramafic complex, Ivory Coast

Abstract

AbstractAir-dried and high-pressure frozen/freeze-etched samples of clinopyroxene and smectite in a saprolitized clinopyroxenite from Koua Bocca, Ivory Coast, West Africa, were compared to characterize textures developed during natural weathering of chain silicates. Comparison with air-dried material allowed evaluation of high-pressure cryofixation as a technique for preserving textures of hydrated clay minerals. Air-dried pyroxene surfaces appear very smooth. Small, flat lamellae, oriented parallel to the c axis, lend a distinct splintery appearance to pyroxene surfaces in fully hydrated samples. These lamellae often display a combination of straight (110) pyroxene edges and a crinkled border, suggestive of smectite. Narrow lenticular (110) open cleavages occur in both preparations and are not a pressurization artifact. Most often these openings contain no secondary minerals. Spaces between pyroxene denticles, lined with collapsed smectite in air-dried samples, are filled with thin packets of anhedral smectite crystallites oriented face to face when hydrated. Smectite microboxwork preserves original topotactic textures developed during isovolumetric pyroxene transformation, and smaller nanoporosity appears in hydrated cryofixed examples. Occasional regions of edge-to-face ‘house of cards’ texture also occur. Elimination of sample preparation artifacts induced by surface tension during air drying demonstrates that pores actually present during a hydration reaction-driven weathering episode are smaller and more numerous than would be inferred from examining air-dried materials.