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Carbonate analysis through X-ray diffraction (XRD) techniques has been well documented, and research into both carbonates and clays dates back many decades. An abudance of research has been published since the turn of the millennium, and even now new carbonate and clay-based research is coming to fruition through XRD analysis methods.

One specific area that is not as well documented is the influence of clay layers in carbonate rocks.

X-Ray Diffraction Principles

XRD is one of the best technologies for determining the minerals present in clay-rich materials and for identifying the structural composition of carbonates. XRD is a technique that is used to determine the crystalline structure and atomic spacing of a material, and is represented as an X-ray spectral pattern. Generally, patterns are referenced against a database of known structures to deduce the crystalline structure.

XRD is based around the constructive interference of monochromatic X-rays and a crystalline sample. X-rays are generated through a cathode tube and are filtered to produce monochromatic X-rays.

The X-rays are then concentrated and directed towards the sample. When the X-rays hit the sample, it causes a constructive interference if the condition satisfies Bragg’s law. This is a law that relates the electromagnetic radiation to the diffraction angle and the lattice spacing, where nλ=2d sin θ.

When the X-rays hit the sample, the sample is scanned through an angle of 2θ and allows the diffraction pattern to be observed in all directions. This is due to the random orientation of the material and the X-rays undergoing elastic scattering, where the diffracted X-Rays have the same energy and wavelength as the incoming X-rays.

XRD is an efficient method for the determination of crystalline structures and is widely used across many scientific fields.

Examples of Research Surrounding Clay Layers in Carbonates

XRD can be used to determine clay layers in the presence of carbonate rocks for the determination of geological structures and processes.

Whilst much of the research on clays and carbonates was produced far back into the past, recent research has been shown to document the relationship between both clay layers and carbonate rocks and how the surrounding geological environment is affected.

One such example, is that of using XRD (CheMin) to determine the different clay layers during the Arctic Mars Analog Svalbard Expedition (AMASE). The main focus of the AMASE research was to determine how the carbonates formed in several Norwegian volcanoes, and how they resemble carbonates found in Martian meteorites.

Through XRD analysis, the researchers found different carbonate-based minerals in various concentrations and layers of smectite clay. The clay layers were found to be more volatile than the surrounding carbonates due to the dehydroxylation of Al-OH in smectite octahedral sites.

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The researchers were able to determine, detect and identify small amounts of clay in the samples, showing promise for identifying similar compositions of clay-based and carbonate materials in Martian rock samples.

The most recent example has been produced by a team of Italian scientists, where XRD was used to deduce that ultra-thin clay layers are responsible for seismic slips in carbonate fault boundaries.

The researchers employed a PANalytical θ-θ diffractometer with a long fine-focus Cu X-ray tube and an X’Celerator multi-strip detector. The researchers found that in the Apennines, the occurrence of micrometer thick clay-based phyllosilicate mineral layer caused a weakening of the surrounding carbonate-hosted seismogenic fault.

The clay-based microlayers were found to weaken quicker than the calcite gouges or mixed calcite-phyllosilicate gouges along the fault boundary, and their presence can actually increase the surface displacement during earthquakes.

This weakening was even found to occur when the concentration of clay layers was as low as 3 wt%, especially in shallow continental crusts. In such environments, the researchers deduced the slip rate of the faults to be 1 ms-1.

Sources and Further Reading

Smeraglia L, Billi A., Carminati E., Cavallo A., Di Toro G., Spagnuola E., Zorzi F., Ultra-thin clay layers facilitate seismic slip in carbonate faults, Scientific Reports, 2017, 7 , 664

, 664 McAdam A. C., Mahaffy A. C., Blake D. F., Ming D. W., Franz H. B., Eigenbrode J. L., Steele A., Evolved Gas Analysis and X-Ray Diffraction of Carbonate Samples, 41st Lunar and Planetary Science Conference (2010)

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