Raman microscopy of haidingerite Ca(AsO3OH)•H2O and brassite Mg(AsO3OH)• 4H2O

Raman spectroscopy has been used to study the arsenate minerals haidingerite Ca(AsO3OH)center dot H2O and brassite Mg(AsO3OH)center dot 4H(2)O. Intense Raman bands in the haidingerite spectrum observed at 745 and 855 cm(-1) are assigned to the (AsO3OH)(2-) (nu 3) antisymmetric stretching and (nu 1) symmetric stretching vibrational modes. For brassite, two similarly assigned intense bands are found at 809 and 862 cm(-1). The observation of multiple Raman bands in the (AsO3OH)(2-) stretching and bending regions suggests that the arsenate tetrahedrons in the crystal structures of both minerals studied are strongly distorted. Broad Raman bands observed at 2842 cm(-1) for haidingerite and 3035 cm(-1) for brassite indicate strong hydrogen bonding of water molecules in the structure of these minerals. OH center dot center dot center dot O hydrogen-bond lengths were calculated from the Raman spectra based on empirical relations. Copyright (C) 2009 John Wiley & Sons, Ltd.