Article

How Can X-ray Photoelectron Spectroscopy Be Used to Determine Iron Oxidation State in Metamorphic Fe-Ti-oxides in the Adirondack Mountains, New York?

Surface Analysis Spotlight: XPS

by Jennifer Mann     Staff Scientist

Key Takeaways 

• XPS is used to analyze geological samples, focusing on a rock sample from the Adirondack Mountains, N.Y.
• Determining the Fe2+/Fe3+ ratio in ilmenite and magnetite is crucial for inferring peak-metamorphic temperatures
• XPS offers a promising complementary technique to traditional methods.
• The PHI Genesis unique scanning X-ray microprobe enables XPS analysis exclusively from ilmenite or magnetite sections of the rock.

This upcoming presentation explores the application of X-ray Photoelectron Spectroscopy (XPS) for analyzing geological samples, specifically a rock sample from the Adirondack Mountains, N.Y. [1] The Adirondacks are notable due to their complex history and high-temperature metamorphic mineral compositions.  These rocks represent the roots of an ancient mountain belt that have been exposed by uplift and erosion. The central Adirondack Highlands were metamorphosed, 1090 to 1020 million years ago, at pressures of ~0.8 GPa (depths of ~25 km) and temperatures up to 850 ºC, transforming the mineral-chemistry of many of the rocks. [2-3] The unique geochemistry provides an interesting test case for applying XPS analysis to this metamorphic transformation.

Of particular interest to geochemists is the ability to determine the Fe²⁺/Fe³⁺ ratio in ilmenite and magnetite that can be used to infer peak-metamorphic temperatures.  Traditional techniques like electron microprobe analysis have limitations in accurately differentiating these oxidation states. XPS, with its capability for detailed chemical state analysis, offers a promising complementary technique.  However, Fe oxides are notoriously difficult to separate when multiple species are present, due to peak overlaps and changes in relative intensities of the satellite structure.[4] A library of Fe²⁺/Fe³⁺ results for quantitative analysis is important for successful identification. In addition to a polished Adirondack rock sample, multiple hematite, magnetite, and ilmenite standards were measured.  This library will be used to determine the oxidation states of iron within the rock's mineral phases.

The PHI Genesis has unique XPS capabilities in that it scans a focused (< 5 mm) X-ray beam across the sample surface.  Using a combination of an optical image and PHI’s unique scanning X-ray imaging capability, areas of interest on the petrographic thin section can be found quickly. The PHI scanning microprobe enables XPS analysis exclusively from ilmenite or magnetite sections of the rock. The Fe 2p_3/2, Fe 3p, O 1s and valence band spectra from each of these two areas will be analyzed, by comparing relative intensities and binding energies of the peaks and satellite structures when present.

[1] Bohlen, S.R. and Essene, E.J., Contributions to  Mineralogy and  Petrology , 62, 153-169 (1977).

[2] McLelland, J.M., Selleck, B.W., and Bickford, M.E., Geological Society of America Memoir 206, p. 1–29 (2010).

[3] Darling, R.S. and Peck, W. H,  Adirondack Journal of Environmental Studies, 21, 61-80 (2016).

[4] Biesinger, M.C, Payne, B.P., Grosvenor, A.P., et al., Appl. Surf. Sci., 257, 2717 (2011).

To learn more about using XPS to determine oxidation states of iron in geologic samples, please join us for Dr. Jennifer Mann’s presentation “Using X-ray Photoelectron Spectroscopy to Determine Iron Oxidation State in Metamorphic Fe-Ti-oxides, Adirondack Mts, New York” at AVS 71 on Wednesday, September 24, 2025 at 8:00 AM.