Raman spectroscopic study of the selenite mineral mandarinoite Fe2Se3O9•6H2O

Selenites and tellurites may be subdivided according to formula and structure. There are five groups, based upon the formulae (a) A(XO3), (b) A(XO3 center dot) xH(2)O, (c) A(2)(XO3)(3)center dot xH(2)O, (d) A(2)(X2O5) and (e) A(X3O8). Of the selenites, molybdomenite is an example of type (a); chalcomenite, clinochalcomenite, cobaltomenite and ahlfeldite are minerals of type (b); mandarinoite Fe2Se3O9 center dot 6H(2)O is an example of type (c). Raman spectroscopy has been used to characterise the mineral mandarinoite. The intense, sharp band at 814 cm(-1) is assigned to the symmetric stretching (Se3O9)(6-) units. Three Raman bands observed at 695, 723 and 744 cm(-1) are attributed to the nu(3) (Se3O9)(6-) anti-symmetric stretching modes. Raman bands at 355, 398 and 474 cm(-1) are assigned to the nu(4) and nu(2) bending modes. Raman bands are observed at 2796, 2926, 3046, 3189 and 3507 cm(-1) and are assigned to OH stretching vibrations. The observation of multiple OH stretching vibrations suggests the non-equivalence of water in the mandarinoite structure. The use of the Libowitzky empirical function provides hydrogen bond distances of 2.633(9) angstrom (2926 cm(-1)), 2.660(0) angstrom (3046 cm(-1)), 2.700(0) angstrom (3189 cm(-1)) and 2.905(3) angstrom (3507 cm(-1)). The sharp, intense band at 3507 cm(-1) may be due to hydroxyl units. It is probable that some of the selenite units have been replaced by hydroxyl units. Copyright (c) 2008 John Wiley & Sons, Ltd.