Systematic comparison of Mg K-edge XANES spectra of magnesium-bearing clay minerals and magnesium silicate hydrates: A promising tool for identifying magnesium silicate hydrate in natural samples

Magnesium silicate hydrate (MSH) is a low-crystalline material of recent interest in the fields of geoengineering, low-temperature geochemistry, and planetary science. Despite the potential importance of the phase in a wide range of research areas, methods for identifying MSH in natural samples are limited because of its low crystallinity and variable chemical composition. The use of Mg-XANES spectroscopy for identifying MSH and Mg-bearing clay minerals were examined in this study. The spectra of various MSHs synthesized under different conditions were systematically compared against the spectra of various other natural and synthetic clay mineral species (montmorillonite, Fe-montmorillonite, saponite, Fe-saponite, stevensite, talc, antigorite, chrysotile, and chlorite). The spectra for all the mineral species were similar, regardless of composition and crystallinity. Quantitative comparison of the spectral similarities revealed that species with the same structure had similar spectra, such that 2:1 dioctahedral smectites (montmorillonite), 2:1 trioctahedral smectites (saponite and stevensite), 2:1 trioctahedral talcs, and 1:1 trioctahedral serpentines all have unique Mg-XANES spectra. The spectra of six freshly prepared MSHs with Mg/Si ratios of 0.7–1.3 and synthesized under different pH conditions (pH = 10 or 12) were comparable. Two freshly prepared Fe-containing MSHs showed very similar Mg-XANES spectra to those of the MSHs. Spectral analysis revealed that the spectra of the freshly prepared MSHs could not be reconstructed by combining the spectra of the other Mg-bearing clay minerals, indicating that the coordination environment of Mg in freshly prepared MSH is unique and common regardless of the material's composition and reaction pH. The spectra of two MSHs (Mg/Si ratio, 0.8–1.3) aged for 12 months were slightly different from those of the freshly prepared MSHs due to partial transformation of the MSH to trioctahedral clay minerals. In summary, Mg-XANES spectroscopy can be used to identify MSHs and certain other clay minerals due to their unique spectra.