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 (PEELS) conducted in a scanning transmission electron microscope (STEM) from sample areas of nanometer dimensions. The B K edges of the minerals exhibit electron energy-loss near-edge structure (ELNES) characteristic 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 sharp rise in intensity with a maximum at ca. 199 eV followed by several weaker structures. For B-[4], the ELNES is interpreted as transitions to 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 from 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 discussed in terms of both molecular orbital (MO) and multiple scattering (MS) theory. We also present the B K-edge spectra of selected nonminerals and show how differences in edge shapes and energy onsets allow these nonminerals to be readily distinguished from berates and borosilicates.