Manganese Oxide Analysis in Soils by Combined X‐ray Diffraction and Selective Dissolution Methods

Abstract

AbstractIt is difficult to identify the Mn oxides in many soils due to their poor crystallinity, low concentration, the diffuse nature of the x‐ray diffraction patterns, and the coincidence of diagnostic peaks with those of associated minerals. A selective dissolution procedure is proposed to characterize Mn oxides and the associated minerals by using successive sodium hydroxide, hydroxylamine hydrochloride, and dithionite‐citrate‐bicarbonate (DCB) treatments. Mineral composition was determined by x‐ray diffraction analysis before and after each treatment. Three samples of soil Mn nodules from Okinawa Island, Japan, and synthetic sodium birnessite and lithiophorite ore were used as test materials. The NaOH treatment destroyed kaolinite and gibbsite but concentrated birnessite, lithiophorite, and goethite. Heat treatment followed by x‐ray diffraction analyses indicated that birnessite, lithiophorite, and goethite were destroyed at 200, 400, and 300°C, respectively. Birnessite was dissolved by hydroxylamine hydrochloride leaving lithiophorite and goethite unchanged. A DCB treatment following hydroxylamine hydrochloride dissolved lithiophorite and goethite leaving a layer silicate residue. The chemical separation and dissolution procedures were effective in characterizing the Mn oxides in soils.