Raman spectroscopic analysis of minerals of the arsenbrackebuschite group

The arsenbrackebuschite group belongs to the brackebuschite supergroup of minerals that currently has about twenty members. These minerals are monoclinic with space group P21/m, and they have general formula A2M3+(TO4)2(OH) with A = Pb, Ba, Sr, Ca, M = Al, Fe3+, Mn3+, Zn, Cu, T = V, As, P, S. Members of the arsenbrackebuschite group are characterized by the presence of As in the tetrahedral sites. This group includes aldomarinoite [Sr2Mn3+(AsO4)2(OH)], arsenbrackebuschite [Pb2Fe3+(AsO4)2(OH,H2O)], canosioite [Ba2Fe3+(AsO4)2(OH)], feinglosite [Pb2Zn(AsO4)2(OH,H2O)], grandaite Sr2Al(AsO4)2(OH)] and lombardoite [Ba2Mn3+(AsO4)2(OH)]. A crystal chemical study has been conducted on minerals of this group collected in Valletta mine, a small Fe-Mn deposit located in Valletta Valley (Canosio municipality, Maira valley, Piedmont, Italy). Raman spectroscopy analyses show that chemical composition variations produce a variation in the spectra peak intensity and some shifts of the peaks that allow distinguishing between the Ba-rich and Sr-rich end members and the V-rich crystals. Raman spectra are characterised by two main peaks in the region of 750-950 cm-1 that can be assigned to the stretching modes of (AsO4)3- (810-880 cm-1) and (VO4)3- (780-875 cm-1) groups. We have observed a clear relation between Sr/Ba content and the position of these peaks that allows, at first, to distinguish Sr-rich from Ba-rich samples and that could be used to calibrate the Ba-Sr content. In fact, intermediate compositions show also intermediate values for some of the most important peaks. A broad peak is always observed in the 2500-4000 cm-1 region, due to the stretching modes of the O-H bonds.