Guide 1 of 6

A first look at Mössbauer spectroscopy

Mössbauer spectroscopy uses gamma rays to look at the iron atoms in a sample. It tells you what kinds of iron are present and how they are bonded, which helps identify minerals.

What happens in the instrument

A radioactive source (cobalt-57, which decays to iron-57) gives off gamma rays with an energy of 14.4 keV. Iron-57 nuclei in your sample can absorb these gamma rays, but only when the energy matches one of their nuclear transitions almost exactly.

The chemistry around each iron atom shifts and splits these transition energies by tiny amounts. To scan across them, the source is moved backwards and forwards which takes advantage of the Doppler effect to raise or lower the energy of the gamma rays very slightly. In transmission geometry, a detector (typically a proportional counter) behind the sample counts how many gamma rays get through at each velocity. If there is no sample, then all the gamma rays reach the detector, but if there is iron in the sample then some of them are absorbed and the count drops. The result is a spectrum showing how many gamma rays get through at each velocity.

Reading a spectrum

The horizontal axis is the velocity of the source in millimetres per second. It stands for energy, and 1 mm/s is about 48 neV, an extremely small change. Most spectrometers sweep from −12 to +12 mm/s, which is wide enough for the magnetic iron oxides.

The vertical axis shows how many gamma rays pass through the sample. Where the iron absorbs them the count drops, so the spectrum shows dips, often called lines or peaks. Their positions, spacing and shapes are described by the hyperfine parameters.

What it can tell you

  • Oxidation state. Fe(II) and Fe(III) give clearly different lines.
  • Bonding and coordination. The symmetry of the surroundings changes how the lines split.
  • Magnetic ordering. Magnetically ordered iron gives six lines instead of one or two.
  • Proportions. The area of each set of lines is roughly proportional to the amount of iron in that environment. It is only roughly proportional, because some environments absorb more strongly than others (the recoil-free fraction).

Together these let you identify iron minerals, follow how they change in experiments, and measure how much of the iron is Fe(II) or Fe(III).

Worth knowing early on

  • Only iron-57 takes part, which is about 2% of natural iron, so samples need enough iron in them.
  • A measurement often takes hours or days, so careful sample preparation pays off.
  • Temperature matters a great deal. Many minerals look alike at room temperature and only separate when cooled, as the guide on temperature explains.
  • Fitting a spectrum means choosing a model, and different choices can fit equally well. The guide on line shapes covers this.

Further reading

  • Dyar, M. D., Agresti, D. G., Schaefer, M. W., Grant, C. A. and Sklute, E. C. 2006. Mössbauer spectroscopy of earth and planetary materials. Annual Review of Earth and Planetary Sciences, 34, 83–125.
  • Gütlich, P., Bill, E. and Trautwein, A. X. 2011. Mössbauer Spectroscopy and Transition Metal Chemistry: Fundamentals and Applications. Springer, Berlin.
  • Murad, E. and Cashion, J. 2004. Mössbauer Spectroscopy of Environmental Materials and their Industrial Utilization. Kluwer Academic Publishers, Boston.
  • Thompson, A., Byrne, J. M., Dreher, C. L., Grigg, A. R. C., Joshi, P., Latta, D. E., Neumann, A., Notini, L., O’Neill, K. E. B. and Rothwell, K. A. 2025. Current practices for analyzing soils and sediments via Mössbauer spectroscopy. Journal of Plant Nutrition and Soil Science, 188, 742–773. doi:10.1002/jpln.12024