Tlapallite H 6(Ca,Pb) 2(Cu,Zn) 3SO 4(TeO 3) 4TeO 6, a multi-anion mineral: A Raman spectroscopic study
Tellurates may be subdivided according to formula and structure. There are three types of tellurate minerals: type (a) (AB) m(TeO 4) pZ q, type (b) (AB) m(TeO 6)· xH 2O and type (c), compound tellurates in which a second anion is involved. Tlapallite, a multi-anion mineral containing both tellurate and tellurite units, as well as sulphate, is an example of type (a). Tellurates are rare minerals as the tellurate ion is easily reduced to the tellurite ion. Raman bands at 691, 708, 764 and 796 cm -1 are attributed to (TeO 6) 2- and (TeO 3) 2- stretching bands. The intense sharp Raman band at 973 cm -1 is assigned to the ν1 (SO 4) 2- symmetric stretching mode, whilst the two bands at 1062 and 1104 cm -1 are assigned to the ν3 (SO 4) 2- antisymmetric stretching mode. The spectral region 100 to 600 cm -1 displays the bands which are attributable to the (SO 4) 2, (TeO 3) 2- and (TeO 6) 4- bending modes. Some evidence from very low intensity Raman bands in the 2800-3600 cm -1 region provides evidence of proton-tellurate/tellurite anion interactions.