Raman spectroscopic study of clinopyroxenes in the join CaScAlSiO6-CaTiAl2O6.

Abstract

Four clinopyroxenes in the join CaScAISiO6-CaTiA12O6 are synthesized at 1atm. Deconvolution of Raman spectra of the clinopyroxenes indicates that the chains in the CaScAISiO6 pyroxene are composed of Si-O-Si, Si-O-Al , and Al-O-Al bonds and the alu minum avoidance rule is violated. The weakness of the Si-O-Si stretching vibration band in CaScA1SiO6 (70)-CaTiAl2O6 (30) pyroxene indicates that Al and Si are not randomly distributed throughout the tetrahedral sites but distributed in Si/Al ordering to some extent.

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“…The combination of high‐frequency Si−O and low‐frequency Al−O vibrations generated broad Raman scattering signals (Figure 2a) [16] . The coexistence of Al−O−Al, Al−O−Si, and Si−O−Si stretching vibrations at approximiately 625 cm −1 , 650 cm −1 , and 670 cm −1 , respectively, further demonstrated the disordered nature of Al 3+ and Si 4+ ions (inset of Figure 2a) [17] . The ordered nature of [SiO 4 ] in CaMgSi 2 O 6 generated narrow Raman signals (Figure 2b), and only Si−O−Si stretching vibration was observed at ≈670 cm −1 (inset of Figure 2b).…”

“…Irrespective of on-site Coulomb interactions (DFT + U; Page S2), Cr 3 + ions tended to substitute 6-CN Mg 2 + sites, and Al 3 + entered 4-CN Si 4 + sites, due to their lower relative total energy. [17] We also calculated the relative total energies of Ca 4 (Sc 3 Cr)-(Si 4 Al 4 )O 24 and Ca 4 (Sc 3 Al)(Si 4 Al 3 Cr)O 24 (Table 1) to investigate the Cr 3 + site occupancy in CaScAlSiO 6 , and Cr 3 + preferred to occupy octahedral B sites. Because of the coexistence of multiple conformations (Type I-IV) for CaScAlSiO 6 , the energies of Cr 3 + in the four conformations were investigated by DFT calculation (Figure S5).…”

“…Hence, the total energies of Ca 4 (Mg 3 Cr)(Si 7 Al)O 24 and Ca 4 (Mg 3 Al)(Si 7 Cr)O 24 were calculated to analyze the site preference of Cr 3+ ions (Table 1 and Figure S4). Irrespective of on‐site Coulomb interactions (DFT+U; Page S2), Cr 3+ ions tended to substitute 6‐CN Mg 2+ sites, and Al 3+ entered 4‐CN Si 4+ sites, due to their lower relative total energy [17] . We also calculated the relative total energies of Ca 4 (Sc 3 Cr)(Si 4 Al 4 )O 24 and Ca 4 (Sc 3 Al)(Si 4 Al 3 Cr)O 24 (Table 1) to investigate the Cr 3+ site occupancy in CaScAlSiO 6 , and Cr 3+ preferred to occupy octahedral B sites.…”

“…[16] The coexistence of AlÀ OÀ Al, AlÀ OÀ Si, and SiÀ OÀ Si stretching vibrations at approximiately 625 cm À 1 , 650 cm À 1 , and 670 cm À 1 , respectively, further demonstrated the disordered nature of Al 3 + and Si 4 + ions (inset of Figure 2a). [17] The ordered nature of [SiO 4 ] in CaMgSi 2 O 6 generated narrow Raman signals (Figure 2b), and only SiÀ OÀ Si stretching vibration was observed at � 670 cm À 1 (inset of Figure 2b). Similarly, the DFT-calculated Raman spectrum of CaScAlSiO 6 was broader than that of CaMgSi 2 O 6 (Figures 2c and d), implying the validity of our multiple conformations, due to the disordered CaScAl-SiO 6 structure.…”