Fjölrit RALA - 15.06.2004, Blaðsíða 46

Extractability and FTIR characteristics of poorly-ordered minerals in a
collection of volcanic ash soils
P. Buurman1, E.L. Meijer1, A. Fraser2 and E. Garcia Rodeja3
1 Wageningen University (NL); 2McAuley Institute (GB); 3Santiago University (E)
Volcanic ash soils are characterized by either Al-organic complexes, or by low-order
aluminium silicates such as allophane and imogolite. In practice, the presence of Al-
complexes or -silicates is assessed by combinations of more or less specific extractions (acid
oxalate, pyrophosphate pHlO, etc). Especially for aluminium silicates, the relation between
the character of the extracted component and its extraction behavior is hardly known.
Extractions are standardized and performed only once. At high contents of low-order Al-
silicates, such extractions are incomplete. In addition, they extract unknown amounts of other
minerals, such as halloysite and gibbsite, which obscures the interpretation.
We undertook the present study to investigate the relation between extractability of low-order
Al-silicates and factors such as total amount, Al/Si ratio, and admixed crystalline minerals.
Special attention was given to the hydration state of the material and to coordination of its Si
component. In general, there is a loss of oxygenated groups and a relative increase of aliphatics with
depth.
We measured acid-oxalate extractability and FTIR spectra of 34 samples from Costa Rica
and from nine COST-622 reference profiles in four European countries. Si0, Al0, and Fe0,
were measured following two procedures, i.e., standard extraction and sequential extraction in
5 to 7 steps. Solid phase FTIR spectra were recorded for original samples and for residues
after extraction. To obtain spectra of the extractable fraction, spectra of residues were
subtracted from those of the samples before extraction. Repeated oxalate extraction removes
all amorphous Al-silicates, but not necessarily all non-silicate iron phases. For this reason,
also dithionite-extractable Fe (Fed, single extraction) was measured.
The aim was to distinguish similarities and differences between the characteristics of
(aggregates of) poorly-ordered minerals in this collection of samples. This is important for
understanding the variety of physical and chemical properties of volcanic-ash soils.
Many samples show characteristic decreasing yield with each subsequent extraction, but a
number of anomalies were observed. Also FTIR spectra may show great similarity between
samples from different origin, but mostly they show a wide range of transitions between
typical mineral spectra from literature.
Especially for Fe, 5-7 sequential extractions sometimes extracted more than a single
dithionite extraction, probably indicating dissolution of crystalline Fe-oxides such as
magnetite.
Oxidation of organic matter by H202 to remove organic matter resulted in the formation of
ammonium dioxalato aluminate, an artifact that was encountered in most spectra of this
collection of volcanic-ash soil samples. We expect that this complex formed from organically-
bound aluminium. It has a typical combination of adsorption bands at 1721-1700-1419-1286-
805 or 1722-1699-1417-1288-808 cm"1. Another artefact is Ca-oxalate, which is visible in
some FTIR spectra of residues after acid-oxalate extraction, with typical absorption bands at
1619-1318-782 cm"1. Samples that have been dried insufficiently show a peak of adsorbed
water at about 1635 or 1653 cm'1. Top horizons frequently show peaks at 2924 and 2854 cm'1,
due to CH2 and CH3 in organic matter that was not removed by peroxide oxidation.
The allophane-type (Al/Si=2) that has a FTIR spectrum most similar to that of imogolite is
called proto-imogolite. The typical difference of imogolite in comparison to proto-imogolite
31