Weathering sequences of rock-forming minerals in a serpentinite: Influence of microsystems on clay mineralogy

Abstract

Under closed geochemical conditions, the weathering of a serpentinite rock composed of serpentine (70–85%) and magnesian chlorite (10–15%) associated with magnetite and chromite leads to the complete replacement of serpentine and chlorite by 2:1 layer silicates and produces new Fe oxides. The serpentine minerals crystallize under different habits issued from the serpentinization processes: mesh and hourglass pseudomorphic textures were formed from olivine, and thin-bladed pseudomorphic textures from pyroxene and amphibole crystals. Serpentine veins crosscut the whole rock with locally non-pseudomorphic interpenetrating and interlocking serpentines.Specific weathering microsystem habits with specific clay mineral crystallizations originate from these different habits: a poorly aluminous saponite in thin-bladed textures, two Fe-rich montmorillonites in mesh and hourglass (MH) textures, and in veins (V) which differentiate on Al, Mg and Fe contents. Magnesian chlorites, isolated from serpentine by hand-picking under a stereomicroscope, are found to weather to trioctahedral vermiculite. Magnetite and chromite extracted from the bulk samples are replaced by newly formed Fe oxides, maghemite, goethite and hematite, which give way to specific Fe accumulation habits in the regolith zone of the weathering profile.

Linking Ni and Cr concentrations to soil mineralogy: does it help to assess metal contamination when the natural background is high?

“…No correlation was in fact found between Ni E or Ni ex and the total Fe oxides that are mostly of lithogenic origin as indicated by the low variability among soil horizons and should therefore have a low specific surface area (Schwertmann and Taylor 1989). Smectites contribute to Ni sorption and form during the very first steps of serpentine weathering (Caillaud et al 2006), but only seldom did we find swelling minerals in these Entisols and Inceptisols. The presence of swelling minerals, with high cation exchange capacity, apparently did not induce any difference in the Ni E and Ni ex concentrations (p=0.538 and 0.873, respectively, n=8, oneway Anova), thus indicating that also the extractable forms are linked to the composition of the parent material in these poorly developed soils.…”

A novel mechanism of spheroidal weathering: a case study from the Monchepluton layered complex, Kola Peninsula, Russia

“…Spheroidal weathering is quite a common result of chemical weathering; it involves corestones (i.e., spheroids of relict bedrock) of various compositions and sizes, which are surrounded by concentric shells, i.e., rindlets, arranged in zones. Though mechanisms and agents of chemical and spheroidal weathering are still discussed, these processes are fairly well characterized from various geological settings (e.g., Claridge & Campbell, 1984;Pokrovsky et al, 2005;SoubrandColin et al, 2005;Caillaud et al, 2006;Velbel, 2009).…”

Trace metals distribution from a serpentinite weathering at the scales of the weathering profile and its related weathering microsystems and clay minerals

“…Additional results (Caillaud et al, 2006) showed that such mineralogical complexity arises from the heterogeneity of the serpentine textures in the rock, the occurrence of additional rockforming minerals (chlorite and oxides) and the diversity of their corresponding weathering microsites (with specific morphological, mineralogical and chemical characteristics) in the profile.…”