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

Abstract

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 Mn42+(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 1394cm−1 are assigned to SiO stretching vibrations. The strong Raman bands at 779 and 877cm−1 are assigned to the AsO43- antisymmetric and symmetric stretching vibrations.The Raman band at 352cm−1 is assigned to the ν2 symmetric bending vibration. The series of Raman bands between 414 and 471cm−1are assigned to the ν4 out of plane bending modes of the AsO43- units. Intense Raman bands observed at 301 and 314cm−1 are attributed to the MnO stretching and bending vibrations. Raman bands at 3041, 3149, 3211 and 3298cm−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.