Weathering of Silicate Minerals by Humic Acids: IV. Change in Surface Characteristics

This study investigated the alterations of surfaces of primary silicate minerals after reaction with humic acid (HA). The silicates studied were olivine, epidote, tourmaline, hornblende, biotite and microcline; HA was extracted from an Alfisol. The weathered residues were studied with petrographic microscope and scanning electron microscope (SEM). Petrographic microscopy revealed the deposition of HA on the mineral surface. This deposition appeared to be highest with tourmaline followed by olivine, epidote and then hornblende, microcline and finally least with biotite. SE micrographs showed etching and pitting of the surface of almost all the minerals, including tourmaline and microcline. There was deposition of fine material on the surfaces of epidote, hornblende and biotite. Deposition of humic material on mineral surfaces reaffirms the dissolution-precipation mechanism proposed earlier from kinetic studies. Only biotite remained almost intact with slight fraying of edges. It appears that in contrast to weathering by water or small molecular weight organic acids, weathering by humic acid does cause signifcant degradation of tourmaline and microcline.