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Clay mineralogy of the nickel laterite profiles associated with the Western Vardar ophiolite zone

Two laterite profiles associated with the Western Vardar ophiolite zone have been investigated in order to estimate the formation conditions of the secondary minerals and the trace element behaviour during tropical weathering of serpentinite from the ophiolitic complex. Nickel laterite deposit in the village Ba (44.15°N, 20.20°E), Serbia, has been formed by weathering of serpentinite thrust over the Triassic limestone. The weathering of serpentinite in karstic environment produced a number of nickel-bearing minerals including nickeliferous clay minerals. According to X-ray powder diffraction (XRPD) analysis the principal clay mineral is montmorillonite with cation exchange capacity (CEC) in the range between 70 and 73 mEq/100g. Gibbsite and minor amount of 7Å-halloysite occur as well. The Ni-laterite weathering crust at Gornje Orešje locality (46.01°N, 16.22°E), Croatia, comprises, from the base up, the bed rocks (serpentinized harzburgite, massive apoharzburgitic serpentinite, apodunitic serpentine and schistose serpentinite) ; the saprolite zone (highly fractured and oxidized serpentinite) and the clayey zone. Montmorillonite represents the most dominant clay mineral. The following reaction illustrates breakdown of serpentine (lizardite) into Mg-montmorillonite: 11 Mg3Si2O5(OH)4 + 14 Al3+ + 22 H+ ↔ 6 [Al2(Si3.67Al0.33)O10(OH)2]Mg0.167 + 32 Mg2+ + 27 H2O. Logarithmic activity diagram, constructed for standard conditions (1 bar, 25°C) according thermodynamic data published by Woods & Garrels (1987) and Brookins (1988), indicates stability of montmorillonite at log a(Al3+)/ a(H+)3 and log a(Mg2+)/ a(H+)2 above 10 and 0, respectively. Mobility of aluminium in form of Al3+ required decreased pH (a(Al) = 10-6 ; pH < 4.4).

clay mineralogy; nickel laterite; Western Vardar ophiolite zone

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