Abstract

Laterite develops from intense weathering, making it abundant in tropical latitudes. The mineralogy and the variation in geochemistry of these soil systems is complex and is largely dependent on parent composition and variation in weathering intensity. Important resources such as nickel and cobalt can be concentrated in laterites that developed from mafic and ultramafic rocks. Laterites exist in western Puerto Rico and have been historically studied but details regarding fine scale mineralogy and geochemical variation are poorly constrained. This study reports results from a variability study of tropical laterite from a road cut south of Mayagüez, Puerto Rico. Selected samples were analyzed using bulk X-ray fluorescence (XRF), powder X-ray diffraction (XRD), reflective spectroscopy (RS), and transmission electron microscopy energy dispersive spectroscopy (TEM-EDS). XRF data is generally consistent with previous historical surveys of laterites and indicates metal concentrations of an average of 33.8 wt.% for Fe, 0.576 wt.% for Mn, 0.330 wt.% for Ni, 1.07 wt.% for Cr, and 0.161 wt.% for Co. TEM-EDS mapping on selected sample material indicates oxyhydroxide mineral particles are uniformly intergrown and create a bird's nest texture. Individual crystallites of oxyhydroxide minerals observed have an average length of approximately 500 nanometers and width of approximately 50 nanometers. TEM-EDS data shows Fe was the most dominant element found, with minor amounts of Mn, Cr, Ni, and Al, and is consistent with XRF data. Preliminary XRD data is complex, and for laterite, shows diffraction consistent with goethite-diaspore solid solution, and multiple types of pyroxene including ferroan enstatite, and probable clinopyroxene. The bedrock samples are dominated by serpentine while exhibiting a minor amount of chlorite. Although mining is banned in Puerto Rico, this study provides insight and a comparison to important economic deposits throughout the Caribbean region. These new mineralogical and geochemical constraints enable a better understanding of geogenic exposure issues and geochemical background associated with laterites in Mayagüez, Puerto Rico. Results provide context for ongoing environmental investigations.