IDENTIFICATION OF PYROMORPHITE IN MINE-WASTE CONTAMINATED SOILS BY ATEM AND EXAFS

Calcium-rich pyromorphite [Pb-5(PO4)(3)Cl] has been positively identified as the major lead-bearing phase in mine-waste soils from the South Pennine Orefield, UK, by a combination of EXAFS spectroscopy, ATEM and XRD. Evidence for a pyromorphite-like structure was provided by electron diffraction of individual Ca-rich, lead chlorophosphate grains in the soil using ATEM. Approximate unit-cell parameters derived from electron diffraction were used to identify peaks on the powder XRD trace obtained from the soil sample, allowing the cell parameters to be refined. The unit-cell parameters derived for the Ca-rich pyromorphite [a = 0.9789(7), c = 0.726(1) nm] are smaller than those reported for pure end-member pyromorphite (a=0.998, c=0.733 nm; Baker, 1966) and are sufficiently different to prevent positive identification by conventional XRD analysis. Lead absorption spectra and interatomic distances obtained from EXAFS performed on bulk soils confirmed the predominance of pyromorphite-like structure. EXAFS is shown to be a useful tool in the identification of soil minerals. It gives information about the local environment of one specific element in solids, be they crystalline or amorphous, and is unaffected by the limited chemical substitution of Ca for Pb. The identification of impure Pb minerals in soil by techniques other than XRD suggests that previous studies, based on XRD, may have underestimated the amount of soil Pb present as discrete Pb compounds.