12.2.3: Other Analytical Techniques

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Mineralogists and petrologists use many other analytical techniques besides those described above. All are destructive techniques, and most are more useful for analyzing rocks than individual minerals because they require large samples. A fairly comprehensive list of analytical techniques can be found at https://serc.carleton.edu/research_education/geochemsheets/browse.html. Some of the most important are:

X-ray fluorescence (XRF) is a technique used mainly for analyzing rocks. It requires relatively large samples melted with a flux to make a glass disk. The disc is placed in the machine, where a high-energy X-ray beam hits it. Atoms in the sample fluoresce, producing characteristic X-rays that are analyzed using EDS or WDS detectors. We compare unknowns with well-characterized standards to obtain quantitative results.

Atomic absorption spectroscopy (AAS) is based on the absorption of light by an atomized sample. We dissolve samples in an acid solution or flux that we then introduce to a hot (2,000 °C/3,600 °F) flame or graphite furnace. The flame atomizes the liquid, converting it to a gas phase. We use special light sources to pass light of different wavelengths, characteristic of different elements, through the gas. We compare light absorption of unknown samples and standards to determine the concentration of the element of interest. We can use AAS to analyze most metals but not nonmetals.

Inductively coupled plasma (ICP) is used mostly to analyze rocks, not individual minerals. After we dissolve it in acid, or in a flux, we heat the sample in a plasma (6,000 °C/11,000 °F), ionizing its atoms. At 6,000 °C, the atoms emit light at wavelengths that depend on the elements present. We measure the intensity of light emission at different wavelengths to calculate the concentrations of each element present. ICP analysis is very sensitive and can measure elements present at the parts-per-billion level.

Inductively coupled plasma mass spectrometry (ICP-MS) is a variant of ICP analysis. Instead of analyzing the emitted light, displaced ions are analyzed using a mass spectrometer to measure concentrations of individual isotopes. ICP-MS can be used with a laser that ionizes small spots on individual mineral grains. ICP-MS is very sensitive and can analyze many elements at concentrations as low as 1 part per trillion.

Ion microprobes, also called secondary ion mass spectrometers (SIMS), analyze small spots (on the order of a few microns) on a sample. A focused beam of ions bombards the sample, burning a pit as it creates a plasma and releases ions. The ions then travel to a mass spectrometer for analysis. Ion microprobes are much more sensitive than electron microprobes and we use them to measure the concentrations of very light elements that we cannot analyze with a microprobe. However, standardization is problematic and limits accuracy.

Mössbauer spectroscopy is a limited but very valuable analytical technique. We use it to determine the oxidation state and coordination of Fe in minerals. A sample is exposed to gamma radiation emitted by an ⁵⁷Fe source, and a detector measures how much of the radiation is absorbed by Fe as it passes through the sample. The gamma-ray source is accelerating back and forth, changing the energy slightly due to the Doppler effect. The absorption at different velocities (energies) reflects Fe valence and coordination number in the specimen. We could use Mössbauer spectroscopy to analyze a few other elements besides Fe, but none are generally of geological interest.

Visible and infrared spectroscopy is an analytical technique based on the absorption of visible light or infrared radiation (IR) by a sample. Samples are prepared in one of several ways to make them thin enough for light/ radiation to pass through. We shine a beam of light through the sample and measure the absorption of different wavelengths. Absorption of visible light can tell us the oxidation state and coordination number of important transition metals. Absorption of IR radiation helps identify minerals, and reveals the relative amounts of H₂O and (OH)^–, or of CO₂ and (CO₃)^2-, in a specimen.

Raman spectroscopy is based on inelastic (Raman) scattering of light from a sample. A monochromatic laser beam is focused on a small spot on the sample, and most light photons bounce off with the same wavelength as the original light (elastic scattering). The few that do not will have slightly increased or decreased wavelengths and energies. The difference in energy, called the Raman shift, reflects the presence of specific ions and atoms in the sample. It also tells us the coordination numbers of some elements, helps distinguish polymorphs from each other, and provides a measure of the crystallinity of a specimen.