The mineralogy of coarse and fine fractions of fersiallitic soils developed on volcanic-phonolitic ash, is studied. In the coarse fraction of the younger soils without argillic horizon, pyroxenes (augite) and amphiboles (hornblende) predominate. Olivines and micas are also present in lower quantities. In the evolutionated soils with argillic horizon, the coarse fraction show a high weathering degree. Iron oxides and hydroxides predominate. At the phonolitic levels, micas are present and traces of pyroxenes, amphiboles, titanite, etc. In the clay fraction, there is a mixture of 1:1 (hallosyte-metahallosyte, sometimes kaolinite) and 2:1 (open illite and montmorillonite) clay minerals. The ecological conditions and nature of parent material determine the prevailing kind of clay minerals.

Peer reviewed