A vibrational spectroscopic study of the silicate mineral ardennite-(As)

We have used a combination of scanning electron microscopy with EDX and vibrational spectroscopy to study the mineral ardennite-(As). The mineral ardennite-(As) of accepted formula Mn-4(2+)(Al,Mg)(6)(Si3O10) (SiO4)(2)(AsO4,VO4)(OH)(6) is a silicate mineral which may contain arsenate and/or vanadates anions. Because of the oxyanions present, the mineral lends itself to analysis by Raman and infrared spectroscopy. Qualitative chemical analysis shows a homogeneous phase, composed by Si, Mn, Al and As. Ca and V were also observed in partial substitution for Mn and As. Raman bands at 1197, 1225, 1287 and 1394 cm(-1) are assigned to SiO stretching vibrations. The strong Raman bands at 779 and 877 cm(-1) are assigned to the AsO43- antisymmetric and symmetric stretching vibrations. The Raman band at 352 cm(-1) is assigned to the nu(2) symmetric bending vibration. The series of Raman bands between 414 and 471 cm(-1) are assigned to the v(4) out of plane bending modes of the AsO43- units. Intense Raman bands observed at 301 and 314 cm(-1) are attributed to the MnO stretching and bending vibrations. Raman bands at 3041, 3149, 3211 and 3298 cm(-1) are attributed to the stretching vibrations of OH units. There is vibrational spectroscopic evidence for the presence of water adsorbed on the ardennite-(As) surfaces. (C) 2013 Elsevier B.V. All rights reserved.