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February 7, 2019

Pulsed terahertz radiation offers a new tool
for quantifying the composition of carbonate minerals

1. Key Points

JAMSTEC scientists have developed a highly sensitive method for quantifying the compositions of carbonate mineral crystals using terahertz time-domain spectroscopy.
The method is capable of generating observations with even greater sensitivity than traditional x-ray-based analyses and is able to determine carbonate mineral composition (i.e., crystal polymorphs), such as low-Mg calcite.
The technique is promising for generating new methods of assessing the degree of mineral alteration of coral skeletons, which are used as important records of past marine environments.


Saburo Sakai and colleagues at the Department of Biogeochemistry of the Japan Agency for Marine-Earth Science and Technology (JAMSTEC), in collaboration with Purdue University, Hamamatsu Photonics K.K., Tokyo University, and Hiroshima University, discovered that it is possible to quantify the composition of carbonate mineral crystals with a high level of sensitivity using terahertz (THz) time-domain spectroscopy. The main, naturally occurring carbonate minerals are high-magnesium (Mg) calcite, low-Mg calcite, aragonite, and dolomite. These are primary mineral components of the Earth and of the skeletal structures of many living organisms (e.g., marble and corals, respectively). The identification and quantification of these minerals is important from various perspectives, including Earth and planetary sciences, medicine, and industry. To date, the identification and quantification of these carbonate minerals has been achieved using X-ray diffraction (XRD) (Figure 1a). In recent years, it has also become possible to observe the absorption features caused by vibrations in crystal lattices with a high level of sensitivity using absorption spectroscopy at THz frequencies (Figure 1a), and the use of THz-frequency radiation, which is safer than X-ray radiation, shows promise for potential application to crystallographic analyses. However, there have only been a few fragmentary examples where this method has been used for detecting the aforementioned major carbonate mineral components.

In the present study, the researchers used broadband THz time-domain spectroscopy (0.5–7 THz; Figure 2a) with attenuated total reflection (Figure 2b) to measure the absorption characteristics of carbonate minerals, and succeeded in acquiring characteristic absorption spectra for high-Mg calcite, low-Mg calcite, aragonite, and dolomite (Figure 1b). Based on these absorption spectra, they were able to determine the contents of each of the materials comprising the measured samples (Figure 1c), demonstrating that it is possible to detect trace levels (1% or less) of low-Mg calcite in aragonite with a high degree of sensitivity (Figure 1d). This technique is promising for the development of new methods utilizing higher sensitivities relative to the traditional XRD method of assessing, for example, the degree of alteration in the composition of coral skeletons, which is important as a historical record of the marine environment.

The light used in this spectroscopy method was of the safe THz frequency, making it superior to XRD in terms of handling. In the future, standard spectra of carbonate minerals obtained with THz frequencies could find applications not only in Earth and planetary sciences, but also in providing more-detailed analyses of the compositions of pharmaceuticals than have been possible up to present times. Additionally, this technique can be used for assessing the fillers used in the production of paper and other engineered products, and in assessing the carbonate composition of white pigments in historical paintings.

This research was supported by the Japan Society for the Promotion of Science (JSPS) Grands-in-Aid for Scientific Research [Grant No. JP15KK0179].

Title: Pulsed terahertz radiation for sensitive quantification of carbonate minerals
Authors: Saburo Sakai1,2, Danzhou Yang2, Takashi Yasuda3, Koichiro Akiyama3, Takayoshi Kuga3, Akihiro Kano4, Fumito Shiraishi5, Shota Amekawa4, Susumu Ohtsuka5, Kazumitsu Nakaguchi5, Shyuhei Yamaguchi5
Affiliations: 1. Department of Biogeochemistry, JAMSTEC 2. Department of Medical Chemistry and Molecular Pharmacology, College of Pharmacy, Purdue University 3. Hamamatsu Photonics K.K. 5. The University of Tokyo 5. Hiroshima University

Figure 1

Figure 1. (a) Schematic overview of crystal compositional analysis using THz time-domain spectroscopy; (b) characteristic spectra of carbonate minerals between 1 and 6 THz (high-Mg calcite, low-Mg calcite, aragonite, dolomite); (c) theoretical and measured ratios of carbonate minerals; (d) percentage of low-Mg calcite in aragonite samples.

Figure 2

Figure 2. (a) Broadband (0.5–7 THz) time-domain spectroscopy device; (b) schematic of the attenuated total reflection method. Because the path from the THz emitter to the sample to the THz receiver is entirely within the total reflection prism in the device used in this study, it can generate measurements with a high degree of sensitivity, avoiding the influence of absorption by water in the measurement environment.


(For this study)
Saburo Sakai, Department of Biogeochemistry
(For press release)
Tsuyoshi Noguchi, Manager, Press Division, Public Relations Department
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