USE OF ELECTRON-ENERGY-LOSS NEAR-EDGE FINE-STRUCTURE IN THE STUDY OF MINERALS

High-resolution electron-energy loss near-edge fine structure (ELNES) recorded in a scanning transmission electron microscope (STEM) is show n to provide information on the local structure and bonding of specifi c types of atoms in minerals. The L2.3 ELNES from Fe (Fe2+ and Fe3+), Mn (Mn2+, Mn3+, and Mn4+), and Cr (Cr3+ and Cr6+) show valence-specifi c multiplet structures that can be used as valence fingerprints. In ge neral, the L3 edge for a specific 3d transition metal exhibits a chemi cal shift toward higher energy losses with an increase in oxidation st ate. Examples of mixed valence Fe- and Mn-bearing minerals are present ed where the presence of multiple valence states is distinguished by a splitting of the L3 edge. The high spatial resolution that can be obt ained using the STEM allows variations in the relative proportions of the oxidation states to be detected on a scale down to 1 nm2. This res olution is illustrated from a sample of hausmannite that shows differe nt L3-edge shapes consistent with variations in the Mn2+-Mn3+ ratio ov er distances of ca. 100 nm. Furthermore, spectra of many elements exhi bit ELNES shapes characteristic of the nearest neighbor coordination, as is demonstrated for C in the carbonate anion and Si in the SiO4 tet rahedral unit. The C K edge from CO32- is compared with that for eleme ntal forms of C that exhibit very different ELNES. Similarly the Si L2 .3 ELNES from a range of SiO44- -containing minerals all show the same near-edge shape that is very different from Si and SiC. ELNES allows for its semiquantitative analysis, which is illustrated by two theoret ical techniques. Finally, the effects of electron beam damage are disc ussed in relation to the experimental changes observed in the ELNES.