MINERALOGICAL INVESTIGATIONS OF COASTAL SANDS FROM MAYAGÜEZ, PUERTO RICO PROVIDE GEOGENIC ENVIRONMENTAL CONSTRAINTS

Coastal sands in Mayagüez, Puerto Rico are comparatively understudied in the context of environmental geochemistry and mineralogy. These sands may be derived from local serpentinites and therefore there is an interest to investigate the potential environmental properties of these sands. Samples (n=15) from Mayagüez were acquired to investigate the mineralogy and geochemistry of the sediment to better constrain the geogenic background of the region for pollution studies. Sands were characterized using grain size analysis, polarized light microscopy (PLM), scanning electron microscopy with energy dispersive spectroscopy (SEM-EDS), X-ray diffraction (XRD), and initial inductively coupled plasma mass spectrometry (ICP-MS) to determine the geochemical composition of the samples, including elements of interest (e.g., Ni and Cr). XRD analysis confirmed that amphibole, chlorite, clinopyroxene, magnetite, olivine, orthopyroxene, plagioclase feldspar, quartz, and serpentine are present in the sands. PLM investigations of thin sections of representative shore, shelf, and river delta sands indicated chlorite, clinopyroxene, olivine, orthopyroxene, quartz, and serpentine were common. The observed mineralogy is consistent with that of serpentinites, which are common in the Mayagüez region. PLM and SEM indicate sand grain textures are diverse among observed samples. Both lithic fragments and individual mineral grains are observed. Lithic grains and some minerals (e.g., olivine) are extensively altered with chlorite and serpentine being pervasive. Sand from the shore environment is more altered than sands from the river delta and berm. Examples from SEM detected Ni and Cr in sand grains. ICP-MS has been conducted on 4 out of the 15 samples with ranges of concentrations for environmentally relevant metals being: V = 237 to 456 ppm; Cr = 2553 to 4996 ppm; Co = 44 to 47 ppm; Ni = 470 to 553 ppm; Cu = 32 to 36 ppm; Zn = 120 to 159 ppm; As = ~3 ppm; Pb = 6 to 10 ppm. This investigation opens questions regarding the specific concentrations of Ni, Cr, and other elements in the environment of Mayagüez, Puerto Rico. Results provide new mineralogical and geochemical constraints for future environmental pollution studies in the region.