Fjölrit RALA - 15.06.2004, Page 79

Proton pumping, electron pumping, and the activities of aluminosilicate
components in the formation of Andosols
Ward Chesworth1 and Felipe Macias2
1University ofGuelph, Guelph, Ontario, CANADA
2Universidad de Santiago, Santiago de Compostela, Galicia, SPAIN.
The chemical evolution of soil is essentially determined by fluxes of protons and electrons.
The proton flux is produced by the pumping of H+ ions from an organic or carbonic acid
source, to a sink comprised of the carbonate and aluminosilicate bases predominant on the
earth’s land surface. The electron flux takes place between organic matter as the major source,
and atmospheric oxygen as the pricipal sink. A convenient way to show these features of soil
genesis is by means of the Pourbaix, or Eh(pe)-pH diagram. The combined effect of proton
and electron pumping is to confine the field of most soils within an envelope with three
salients in Eh-pH space - acid, alkaline and hydromorphic, respectively. The acid salient is
the one of interest in any discussion of the formation of andosols. It represents the path of
evolution of three major types of soil genesis: podsolization, ferralitization and
andosolization. The first two are distinguished principally in terms of the proton source, the
last in terms of the parent materials - volcanic glass being characteristic. Volcanic glass, even
as an interstitial phase in basic rocks such as basalt, tends to be acidic in a petrological sense,
and normatively rich in felsic, rather than femic components. Si, Al, Na and K are the major
cationic constituents, and the typical products of low PT alteration are dominated by phases in
the system SÍO2-AI2O3-H2O. The commonly observed difference in andosols between
allophanic assemblages and those containing imogolite, are explainable in terms of initially
amorphous precipitates undergoing Ostwald ripening from different starting points in
logAl^VCH^j-logCHiSiO^) space.
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