The effects of electron-beam damage on the Fe3+/ΣFe (total iron) ratio were measured by electron energy-loss spectroscopy (EELS) with a transmission electron microscope (TEM). Spectra were acquired from crushed and ion-beam-thinned cronstedtite. For fluences below 1 × 104 e/Å2, the Fe 3+/ΣFe values from crushed grains range between 0.43 and 0.49, consistent with undamaged material. These measurements were acquired from flakes 180 to 1000 Å thick. With increase in fluence, samples <400 Å thick become damaged and exhibit Fe 3+/ΣFe values >0.5. The critical fluence for radiation damage by 100 kV electrons as defined by Fe3+ /ΣFe <0.5 for cronstedtite at 300 K, is 1 × 104 e/Å2. The absorbed dose to the speciman during acquisition of a typical EELS spectrum is large, with values around 2.2 × 1010 Gy (J/kg), equivalent to the deposition of 620 eV/Å3. Cooling to liquid N2 temperature did not significantly slow the damage process. Ion-beam thinning produces an amorphous layer on crystal surfaces. Spectra from the thinnest regions, which are amorphous, exhibit Fe3+/ΣFe >0.7. With increase in sample thickness, the Fe3+/ΣFe values decrease to a minimum, consistent with data from the undamaged material. The increase of Fe3+/ΣFe with respect to electron-beam irradiation is likely caused by loss of H. At low fluences, the loss of H is negligible, thus allowing consistent Fe3+/ΣFe values to be measured. The cronstedtite study illustrates the care required when using EELS to measure Fe3+/ΣFe values. Similar damage effects occur for a range of high-valence and mixed-oxidation state metals in minerals. EELS is the only spectroscopic method that can be used routinely to determine mixed-valence ratios at the nanometer scale, but care is required when measuring these data. Consideration needs to be given to the incident beam current, fluence, fluence rate, and sample thickness.
ASJC Scopus subject areas
- Geochemistry and Petrology