Mid- and far-infrared spectra of synthetic CaMg2(Al4-xGax)(Si1-yGey)O10(OH,OD)2-clintonite: Characterization and assignment of the Ca-Oinner and Ca-Oouter stretching bands

Abstract

X-ray diffraction Rietveld structure refinement and mid- and far-infrared spectral studies have been done on Ga-for-Al and Ge-for-Si substituted synthetic clintonites (Cln), CaMg2(Al4−xGax)(Si1−yGey)O10(OH,OD)2 (x = 0.0–4.0, y = 0.0–1.0). Rietveld analyses indicate that there is complete solid-solution in both the (Al4−xGax)Si-, (Al4−xGax)Ge-clintonite series. With increasing Ga-for-Al, the mean tetrahedral bond-length, , and mean tetrahedral basal oxygen separation, increase, whereas the octahedral and interlayer parameters, the tetrahedral rotation angles (α), and the dimensional misfits (Δ) vary little. With increasing Ga-for-Al substitution in the Si-series the broad OH and OD stretching bands shift downward from 3607 to 3529 cm−1 and from 2671 to 2620 cm−1, respectively. For the Ge-series samples, the OH and OD stretching bands shift downward from 3610 to 3523 cm−1 and from 2667 to 2611 cm−1, respectively. According to the data of the curve fitting analysis, both the OH- and OD-stretching bands are modeled by at least three broad [wider than 37 cm−1 of full-width at half height (FWHH)] I bands (M1M2M2 = MgMgAl3+/MgMgGa3+) and a narrow (FWHH = 23–33 cm−1) and weak N band (M1M2M2 = MgMgMg) on the highest frequency side. In some samples, a very weak and broad V (one vacant M site) band at the lowest frequency region is observed. Tetrahedral Si-O, Al-O, and Ga-O stretching bands lie in fairly discrete regions, which are at 1030–830, 880–760, and 790–640 cm−1, respectively. However, the band regions for both the tetrahedral Al-O and Ge-O stretching as well as the Ge-O and Ga-O stretching bands closely overlap each other. In the 720–500 cm−1 region, Si-O-Al, Al-O-Al, Si-O-Ga, Al-O-Ge, Al-O-Ga, Ge-O-Ga, and Ga-O-Ga deformational bands (tetrahedral-chain bending and “breathing” modes) are observed to overlap strongly. Around 300 and 200 cm−1 one observes the Ca-Oinner and Ca-Oouter stretching bands, respectively, which show a slight downward frequency shift (~7 cm−1 per Ga apfu) with Ga-for-Al substitution.