Raman Microscopy Study of Cafarsite

Abstract

Normal, polarized, and cross-polarized Raman spectra of a single crystal of cafarsite, Ca₈(Ti, Fe, Mn)_6–7(AsO₃)₁₂·4H₂O, from Binntal (Switzerland) are reported. Comparison with known metal oxides was used to achieve assignments of the observed bands. Differences between the polarized and cross-polarized spectra helped to identify the symmetric and asymmetric modes. The bands at 258, 319, 721, and 763 cm⁻¹ are assigned to the ν₄(E), ν₂(A₁), ν₃(E), and ν₁ arsenite modes after comparison to AsF₃. The bands at 204, 437, and 620 cm⁻¹ are assigned to the asymmetric Fe(2)–O modes, while the bands at 232, 288, 464, and 506 cm⁻¹ are the symmetric ones. The two bands at 837 and 880 cm⁻¹ are ascribed to the symmetric and asymmetric Fe(1)–O modes, respectively. The Ti–O modes are assigned to the bands at 204 and 648 cm⁻¹ (asymmetric) and to 319, 390, and 802 cm⁻¹ (symmetric). Bands at 288 and 587 cm⁻¹ represent the symmetric and asymmetric modes, respectively, of the Ca–O bonds. On the basis of the Raman spectra, no distinction can be made between the two crystallographic Ca sites. Also no difference can be made between the two crystallographic Mn sites in cafarsite. Mn–O bands are observed at 335, 437 (asymmetric), 464, 506, and 601 cm⁻¹ (symmetric).