Mineral Components of an Ultisol in Southern Chile Determined with Polarizing Microscope, SEM, TEM and Mössbauer Spectroscopy Analysis.

Most of agricultural activities in Chile are carried out in soils formed from volcanic material. These soils comprise about 65 % of the area of Chilean agricultural land. The mineralogy of the sand, silt and clay fractions from the B horizon of an Ultisol derived from volcanic material in southern Chile (36º 58` S 72º 09` W) was determined in this study. All particle-size fractions had relatively complex mineral assemblages. Kaolinized feldspar, weathered biotite (vermiculite), volcanic glass, biogenic opal, and magnetic materials such as (Mg, Fe)- and (Ti, Fe)-oxides were identified in the fine sand fraction by polarized light microscopy and scanning electron microscopy. Magnetite was identified in the magnetic portion of the sand fraction using Mössbauer spectroscopy. In silt and clay fractions, goethite, hematite, and maghemite were identified from Mössbauer spectral analysis at 6 K. Some poorly crystalline iron oxyhydroxides, easily removed by ammonium oxalate treatment, were identified in both the silt and clay fractions. Halloysite, kaolinite, gibbsite, and Fe-oxides coating aluminosilicates were identified on the clay sample, too.