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The nature and properties of amorphous colloids formed from Mazama tephra
Soil samples from the Mazama tephra deposit were collected
from the AC and C2 horizons at five different locations along the axis
of the main depositional lobe. The clay fraction from the samples
was recovered and studied by means of chemical, x-ray, DTA and
electron microscopy methods in order to characterize the nature and
properties of amorphous clays forming in this relatively homogeneous
parent material of known age.
Dispersion of the amorphous clays from the pumice samples
was found to be difficult unless samples were first pretreated to
remove iron oxides and organic matter. The diothionite-citratebicarbonate
(DCB) iron removal treatment, however, was found to
significantly alter the morphology of a reference imogolite sample by
leaving individual strands of mineral coated with a gel. The comparison
of DTA data from the imogolite and synthetic allophanes before
and after defferation treatment also shows that significant changes in
the amorphous clays resulted from the clean up treatment. Alternate
methods for amorphous clay dispersion were tried on a variety of
ando soil samples. None of the methods were found to be effective
for dispersing all samples. Several washings in distilled water
adjusted to pH 5. 0 with HC1 resulted in the dispersion of most samples
and was used in this study to separate the amorphous clays from the
Mazama tephra.
The clays removed from the pumice samples contained from 47
to 84 percent amorphous colloids. The molar ratio of SiO₂:Al₂0₃
for all the samples was generally between 1 and 2.
The types of clays found at each site studied along the transect
were rather varied. Halloysite was found at all locations except one
and generally was more common near the base of the pumice profile.
The mineral dominated the clay fraction at a poorly drained site near
the source but was absent in another similar site about 400 km away.
The latter contained abundant imogolite and allophane. This difference
in secondary mineralogy from the same parent material under
similar weathering regimes presents an enigma. One possible
explanation is based on the observation that samples of undisturbed
tephra at far distances from the source contain fewer feldspars than
samples near the source. The halloysite is thought to be related to
feldspar weathering. Samples from drier sites along the transect had
clay fractions containing predominately allophane with some few crystals of halloysite and some imogolite All samples had appreciable
quantities of clay size glass shards. Masses of silica bodies and
oxides of iron in addition to halloysite and allophane were found in the
C2 horizon at the driest site.
The cation exchange capacities of the clay samples were highly
pH dependent. All samples had appreciable anion exchange capacities
at pH 4.0. The DCB treated samples had CEC values much higher
than non-treated samples. The magnitude of CEC for the synthetic
gels depended on their composition and was highest when the molar
SiO₂ :A1₂O₃ ratio was about 4.0.
The clays sorbed large amounts of phosphate. The samples
highest in the amorphous gels generally sorbed more phosphate than
samples containing halloysite.
All the clays had weak acid buffering curves. Most samples had
two distinct buffering ranges, one at low pH and one at high pH. The
curves for the samples highest in halloysite were buffered at much
higher pH values than samples containing mostly amorphous clays.
The nature and properties of the amorphous clays in the Mazama
tephra were found to be similar to those reported by Japanese and
New Zealand workers for other volcanic tephra deposits