TRACE METAL SPECIATION IN SOILS DERIVED FROM ULTRAMAFIC ROCK, NORTHWESTERN COSTA RICA

In dry tropical forest of northwestern Costa Rica, Cretaceous ultramafic rocks enriched in Co, Cr and Ni outcrop as peridotite on the Santa Rosa peninsula and as komatiite near Tortugal. Unweathered rock from both areas contains 10 – 15 % olivine; enstatite comprises 5 – 8% of peridotite at Santa Rosa and 20-25% of komatiite at Tortugal. Santa Rosa peridotite contains 55 – 60% serpentine; Tortugal komatiite contains 45% serpentine. Both areas receive approximately 1500 mm of annual precipitation. This study makes use sequential chemical extraction, electron microscopy and X-ray diffraction to assess weathering products and trace metal speciation in soils derived from rocks with 100 mg/kg Co, 3000 mg/kg Cr and 2000 mg/kg Ni.

Kaolinite/smectite and halloysite are the dominant clay minerals in A and B soil horizons on Santa Rosa. In the Tortugal region, Mg-rich trioctahedral smectite (saponite) is the main weathering product; K/S is also present but far less abundant. Additional weathering products include goethite, hematite and amorphous hydroxides and aluminosilicates. In both regions, serpentine persists during chemical weathering to the point that it is the dominant mineral in C horizons. Enstatite is locally present in B and C horizons but is absent from A horizons. Olivine is absent from all soils but is present in some weathering rinds.

Co, Cr and Ni are all concentrated in these soils by weathering and leaching of more soluble elements. At Santa Rosa, Co in bulk soil ranges from 230 - 415 mg/kg, Cr from 4200 - 11800 mg/kg, and Ni from 4670 - 8290 mg/kg. At Tortugal, pedogenic enrichment of Co, Cr and Ni is less pronounced, attaining maximum values of 120, 6430 and 4120 mg/kg, respectively. Exchangeable Ni ranges from 190 mg/kg to 740 mg/kg in Santa Rosa soil and is more abundant than exchangeable Ca (180 - 220 mg/kg). Co and Cr are much less available as exchangeable cations, with ranges of 2.5 to 27 mg/kg and 0.22 to 11 mg/kg, respectively. Analysis of quantitative XRD and sequential chemical extraction data indicates that, in Santa Rosa soils, most Ni is contained in exchange sites, primary serpentine or poorly crystalline hydroxides and organic matter. At Tortugal, most Ni occurs in saponite with less Ni in primary serpentine, hydroxides, organic matter and exchange sites. Chromium mainly occurs in refractory oxides.