Fjölrit RALA - 15.06.2004, Page 103

Development and composition of surface coatings in volcanic soils
Herre A.1), Lang F.1), Siebe C.2) and M. Kaupenjohann1)
1) Institutfiir Ökologie, TU Berlin 2) Instituto de Geología, UNAM
Compared to crystalline minerals, volcanic glasses exhibit rather high dissolution rates. As a
consequence, the ion activities in soil solution of Andosols can be high enough to allow the
formation of amorphous secondary phases like allophane, ferrihydrite, silica or aluminium
hydroxide. Besides, Andosols in the vicinity of active volcanoes receive rather large amounts
of acid inputs, often in the form of H2SO4 or HF, which in tum accelerate mineral weathering.
Also, the added S and F can form precipitates with Al. In the case that precipitates form on
the surface of weathering minerals, they could exert a substantial influence on weathering
kinetics.
We hypothesize the formation of Al-S secondary mineral phases on the surface of primary
minerals in volcanic ash soils in the vicinity of active volcanoes. In order to test this
hypothesis we analysed samples of sand grains from selected soil horizons using scanning
electron microscopy with energy dispersive X-ray analysis (SEM-EDX).
Our results show the formation of amorphous silica and Al-S secondary mineral phases.
Both seem to form coatings on the surface of weathering primary minerals as well as separate
phases.
In order to investigate which solid phases are thermodynamically favoured, we will analyse
soil solution data using the geochemical model Minteqa2. Using this thermodynamic
calculations, we will also test possible explanations for the differences observed between the
analysed horizons on their plausibility. Results of these analysis will be shown.
In future research, this topic could be evaluated on a broader band of volcanic ash soils.
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