DISTRIBUTION OF METALLIC TRACE ELEMENT (MTE) ACCORDING TO THE WEATHERING SEQUENCES OF ROCK-FORMING MINERALS IN A SERPENTINITE

摘要

The pedogeochemical background from a serpentinite was studied at the scale of the weathering sequences of rock-forming minerals (microsystems) [1]. The weathering of a serpentinite rock, constituted of serpentine (70-85 percent), magnesian chlorite (10-15 percent) associated with magnetite and chromite, leads to the complete replacement of rock-forming minerals by 2:1 layer silicates and produces new Fe-oxides. The serpentine minerals crystallize under different habits: (i) mesh, hourglass and (ii) thin bladed pseudomorphic textures, and (iii) non-pseudomorphic crosscutting serpentine veins. Specific weathering microsystem habits with clay mineral crystallisations originate from these different habits: a poorly aluminous saponite in thin bladed textures, two Fe-rich montmorillonites in mesh, hourglass textures (《 MH 》), and in veins (《 V 》) which differentiate on Al, Mg, and Fe contents. Magnesian chlorites are found to weather into trioctahedral vermiculite. Magnetite and chromite are replaced by newly formed Fe-oxides: maghemite, goethite and hematite.