PARALLEL ELECTRON-ENERGY-LOSS SPECTROSCOPY (PEELS) STUDY OF B IN MINERALS - THE ELECTRON-ENERGY-LOSS NEAR-EDGE STRUCTURE (ELNES) OF THE B-K-EDGE

The B K-edge spectra of a wide variety of minerals have been measured using the technique of parallel electron energy-loss spectroscopy (PEE LS) conducted in a scanning transmission electron microscope (STEM) fr om sample areas of nanometer dimensions. The B K edges of the minerals exhibit electron energy-loss near-edge structure (ELNES) characterist ic of B coordination. For threefold-coordinated B (B-[3]), th, spectra are dominated by a sharp peak at ca. 194 eV because of transitions to unoccupied states of pi character, followed by a broader peak at ca. 203 eV attributed to states of sigma character. The ELNES on the B K edge (B K ELNES) of fourfold-coordinated B (B-[4]) consists of a shar p rise in intensity with a maximum at ca. 199 eV followed by several w eaker structures. For B-[4], the ELNES is interpreted as transitions t o states of antibonding sigma character. Minerals that possess both B -[3] and B-[4] exhibit an edge shape that is composed of B K edges fro m the respective BO3 and BO4 units, and we demonstrate the feasibility of quantification of relative site occupancies in minerals containing a mixture of B coordinations. The origins of the B K ELNES are discus sed in terms of both molecular orbital (MO) and multiple scattering (M S) theory. We also present the B K-edge spectra of selected nonmineral s and show how differences in edge shapes and energy onsets allow thes e nonminerals to be readily distinguished from berates and borosilicat es.