『Abstract
The distribution of four trace metals - nickel (Ni), chromium
(Cr), manganese (Mn) and cobalt (Co) - and some major elements
is studied in a soil profile derived from a natural trace metals-bearing
rock (serpentinite), at the scales of (1) the weathering profile
(C and B horizons) and (2) its related weathering microsystems
and clay minerals. All trace elements are relatively enriched
in weathered horizons compared to the rock, in the same way as
iron and aluminum whereas magnesium is largely leaching. An evolutive
sequence of trace metals host phases is formed along the weathering
profile from magnesian at the bottom to iron rich clay minerals,
and then aluminous clay minerals and oxy-hydroxides at the top.
Different chemical forms of trace metals in weathering profile
have been quantified by chemical extractions. Ni and Cr remain
mainly in the silicates. In contrast, Mn and Co are mainly distributed
into amorphous oxy-hydroxides. In addition, trace metals contents
are measured (using electron microprobe) in individual rock-forming
minerals and in their specific weathering microsystem each with
its specific clay mineral. Each rock-forming mineral serves as
the host phase for a specific trace metal. Silicates (serpentine
and clinochlore) contain Cr, Ni and Mn whereas oxides concentrate
Co and Cr. In the first weathering stages, Ni and Cr are concentrated
in secondary clays, especially in Fe-montmorillonite (derived
from mesh serpentine minerals) and in trioctahedral vermiculite
(derived from chlorite) respectively, whereas Mn and Co are mainly
used to constitute secondary oxides. At the top of the profile,
Mn, Co and Ni host secondary oxy-hydroxides have developed due
to iron segregation.
Keywords: Serpentinite; Geochemical background; Trace metal; Weathering;
Nickel; Chromium』
1. Introduction
2. Materials and methods
2.1. Materials
2.2. Methods
3. Results
3.1. Trace metals distribution at the scale of the weathering
profile
3.1.1. Bulk chemistry of the serpentinite and weathered horizons
3.1.2. Estimation of the trace metals mobility by chemical extractions
3.2. Trace metals distribution at the scale of the weathering
minerals
3.2.1. Geochemical background of the serpentinite
3.2.2. Trace metals from rock-forming minerals and their weathered
products
4. Discussion
4.1. Distribution and various forms of trace metals in the
weathered horizons
4.2. Trace metals distribution at the microscale according the
weathering rate
4.2.1. C horizon
4.2.2. B horizon
5. Conclusion
Acknowledgements
References