INVESTIGATIONS OF PHYLLOSILLICATES IN CLAY-RICH MINE TAILINGS FROM A CLOSED PHOSPHATE MINE NEAR BARTOW, FLORIDA

Clay-rich mine tailings from phosphate mine operations in Florida are a major environmental and economic problem. Options for land management for the tailings are limited and are fundamentally controlled by the poor physical properties such as low mechanical strength and low hydraulic conductivity of the materials. In turn the major control on the bulk physical properties is the phyllosilicate mineralogy of the materials. Phyllosilicate clay mineralogy of samples from a tailings deposit near Bartow, Polk County Florida was investigated using powder X-ray diffraction (XRD) and transmission electron microscopy (TEM) techniques to asses the variability of the deposit. Phyllosilicates compose an estimated 70 to 90 percent of the tailings. The phyllosilicate mineralogy of the deposit is dominated by montmorillonite, with lesser amounts of palygorskite, illite and kaolinite. No systematic variation in the relative proportions of phyllosilicates was observed in the clay deposit. Energy dispersive spectroscopy EDS analysis indicates that chemical compositions of phyllosilicates and somewhat typical but overall are enriched with respect to Fe compared to theoretical end members. Montmorillonite particles have an average of approximately 11.00 wt % Fe2O3 and this composition is consistent with solid solution with nontronite. Approximately 20 percent of the montmorillonite may be R0 illite-montmorillonite with K2O content being approximately 1 to 2 wt %. Palygorskite fibers have Fe2O3 contents as up to 17.64 wt % which is comparatively high with respect to other Hawthorne Formation palygorskites which are typically 3-7 wt% Fe2O3. Kaolinite was also observed to have Fe with concentrations between 0.81 and 1.63 wt % Fe2O3. The high percentages of montmorillonite and palygorskite explain the high bulk water contents observed. The relative enrichment of Fe is interpreted to be primary sedimentary feature and not the result of recrystallization during mine processing. The results of this study provide baseline information for long-term land management strategies.