A STEM/EELS method for mapping iron valence ratios in oxide minerals

Area of Science:

• Geosciences
• Materials Science
• Analytical Chemistry

Background:

• Iron valence state in minerals is crucial for understanding geological redox conditions.
• Traditional methods for determining iron valence lack high spatial resolution.
• Scanning Transmission Electron Microscopy/Electron Energy Loss Spectroscopy (STEM/EELS) offers potential for high-resolution analysis.

Purpose of the Study:

• To modify and validate a STEM/EELS method for precise iron valence ratio (Fe(3+)/ΣFe) measurements.
• To enable high-resolution line scans and valence mapping of iron in minerals.
• To demonstrate the method's application at mineral interfaces.

Main Methods:

• Calibrated STEM/EELS technique based on Fe L(3) and L(2) white line intensities.
• Developed a method to overcome energy drift in spectrum images by aligning energy-loss edges.
• Implemented an automated routine for batch processing of spectra and Fe valence calculations.

Main Results:

• Successfully generated line scans and maps of iron valence with high spatial resolution.
• Demonstrated the method on an interface between magnetite and ilmenite.
• Achieved an analytical error of +/-0.05 to +/-0.09 in Fe(3+)/ΣFe measurements.

Conclusions:

• The modified STEM/EELS method provides a rapid and accurate way to determine iron valence in minerals.
• This technique is valuable for estimating redox conditions and analyzing mineral microstructures.
• The automated processing allows for efficient analysis of multiple samples and creation of detailed valence maps.