The manganese oxide mineral, lithiophorite, in an oxisol From Hawaii

Abstract

The mineralogical composition of the surface soil horizon (0-15 cm) of Wahiawa soil (Tropeptic Eutrustox) was investigated by X-ray diffraction (XRD), high gradient magnetic separation (HGMS), transmission electron microscopy (TEM), and infrared methods. The concentration of lithiophorite decreased with particle size and none was present in the clay fraction as indicated by XRD. Lithiophorite was further concentrated from the crushed sand-sized fraction by HGMS. Hexagonal, electron-dense, often twinned lithiophorite particles were identified by electron diffraction. Differential infrared (DIR) spectra obtained by dissolving Mn oxides in acidified hydroxylamine hydrochloride (HAHC) indicated lithiophorite as the HAHC-soluble Mn-phase. Lithiophorite compositiion, as revealed by chemical analysis of the HAHC extracts, consisted of appreciable amounts of Mn, Al, Zn, Co and Mg, and less than stoichiometric amounts of Li. Sodium hydroxide treatment apparently altered the lithiophorite, as revealed by the DIR spectrum of the hydroxylamine-soluble fraction of the NaOH-treated sample compared with the untreated sample. The high crystallinity of the lithiophorite was suggested by its resistance to chemical dissolution and narrow X-ray diffraction lines. No evidence for the presence of todorokite or birnessite was found, contrary to earlier reports. Examination of sand-sized nodules by scanning electron microscopy indicated large (2-5 �m) platy lithiophorite crystals at the surface of these nodules. Electron microprobe analysis of these platy particles indicated iron enrichment near the surface. The freshly fractured nodule surface revealed numerous unaltered platy crystals of lithiophorite filling the veins of the nodule.