『Abstract
Air-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.
Key Words: Etch pits; FEG-SEM; Isovolumetric weathering; Pyroxene;
SEM; Smectite; Weathering』
Introduction
Materials
Methods
Results
Conventional SEM
FEG-SEM
Air-dried samples
Freeze-etched samples
Reactant-product molar volume relations
Discussion
Relative persistence of different pyroxenes to natural weathering
Textures of clay minerals formed by pyroxene weathering, and
implications
Conclusions
Acknowledgments
References