X-RAY PHOTOELECTRON SPECTROSCOPIC CHARACTERIZATION OF SILICA-SPRINGS ALLOPHANE

A range of allophane samples (atomic Al/Si bulk ratios 1.1-1.9) from S ilica Springs, New Zealand, has been characterized by X-ray photoelect ron spectroscopy (XPS). Binding energies of Si 2s, Si 2p, Al 2p, O 1s, C Is, and N Is electrons, together with the kinetic energies of Al KL 23L23 Auger electrons, at or near the surface of allophane aggregates, have been derived. The values for Al, Si and O electrons are similar to those for kaolinite but also to those for some framework silicates (feldspars) having 4-coordinate Al. Values for N electrons suggest tha t N occurs in organic structures. Comparison of XPS and bulk Al/Si rat ios shows an enrichment of Al at or near the surface of allophane aggr egates. The same is true for C and N. Extraction with citrate-dithioni te-bicarbonate (CDB) reagent leaves the surfaces depleted in Al. The C DB extracts have higher Al/Si ratios than the bulk allophanes. Similar ly, CDB treatment reduces the degree of surface enrichment of C and N. Small increases in the binding energies of Si electrons following CDB treatment suggest partial dissolution of the bulk structure though a concomitant removal of a separate phase or species cannot be ruled out . The results may be accounted for in terms of the structure previousl y suggested for the primary spherules of Silica Springs allophane (Chi lds et al., 1990) though the composition of the spherules at or near t he surface of the allophane aggregates is different from those of the bulk.