Mineralogy, micromorphology and chemical analysis of andosols on the Island of Sao Miguel (Azores)

A study concerning soil genesis was conducted on surface and buried volcanic soils of Sao Miguel island (Azores). Of eight soil profiles, three were selected for detailed study of pedogenesis. A series of techniques which include chemical, micromorphological, XRD and DXRD analysis, micropicking on impregnated soil blocks, TEM-EDS and IR spectroscopy analyses were used. The soils showed an incipient to moderate degree of weathering, as typically occurs in many young volcanic districts. Four different stages of soil development were clearly identified. The first stage of soil development was characterised by dominance of primary minerals and some allophane and ferrihydrite formation, with an Fe-o/Fe-d ratio of about 0.86 (LOM soil). The second stage of soil development was indicated by halloysite and ferrihydrite formation in the lower horizons along with an Fe-o/Fe-d ratio of about 0.62 (SAN surface soil). The third stage of soil development was characterized by halloysite and ferrihydrite formation in the lower horizon but also by clay illuviation and stronger pumice weathering, and an Fe-o/Fe-d ratio of about 0.51 (SAN buried soil). The fourth stage of soil development concerned the LIX soils (buried soils older than 5000 years BP) which had a much higher content in hydrated halloysite and considerable amounts of goethite and hematite. These soils exhibit the most expressed weathering features and an Fe-o/Fe-d ratio of about 0.25. The presence of young surface soils and well-developed buried soils can be explained by (i) the longer pedogenesis of buried soils or, possibly, by (ii) the presence of a climate more seasonal than that of the present day. (C) 1999 Elsevier Science B.V. All rights reserved.