64-7 Transmission Electron Microscopy Studies of Chemical Heterogeneity in Bedrock and Related Laterite Samples from Mayagüez, Western Puerto Rico

Session: Emerging Voices in Soil and Paleosol Science (Posters)

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The mineralogy of laterite soil systems is complex and largely dependent on parent composition and variation in the intensity of weathering to which it is subjected. Important resources such as nickel (Ni) and cobalt (Co) 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. Grain mounts were made from deionized water suspensions and transmission electron microscopy (TEM) was performed at the University of Cincinnati’s Advanced Materials Characterization Center using a Talos F200i (Thermofisher) instrument with a 200 kV field emission gun. The microscope was operated in scanning transmission electron microscopy (STEM) mode and high-angle annular dark-field (HAADF), and was equipped with energy-dispersive spectrometry (EDS). 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. The EDS revealed that Co is dispersed among both bedrock and goethite particles but is more heavily concentrated in goethite particles. Chromium (Cr), Ni, Co, and manganese (Mn) are evenly distributed within the goethite particles. Within the bedrock sample, it was found that Co and calcium (Ca) cluster together in higher concentrations than the rest of the silicon-rich minerals. 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(s) while exhibiting a minor amount of chlorite. These new mineralogical and geochemical constraints enable a better understanding of geogenic exposure issues and geochemical background associated with laterites in Mayagüez, Puerto Rico. Although mining is functionally banned in Puerto Rico, this study provides insight and a comparison to important economic deposits throughout the Caribbean region.

Geological Society of America Abstracts with Program. Vol. 57, No. 6, 2025

doi: 10.1130/abs/2025AM-7792

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