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A mineralogical study was carried out in
Coast Region, Tanzania, on soils developed
from different parent materials. These
materials are; unconsolidated sands and
gravels of Neogene age, Grey-green marls
of Cretaceous age, Calcareous silt stone and
sandstone of Jurassic age and migmatitic
gneisses of Precambrian age (Usagaran system).
The soil clay fractions exhibit pertinent
differences in mineralogical composition.
Soils developed from migmatitic
gneisses are predominantly kaolinitic with
accessory amount of goethite. Kaolinite in
these soils is pedogenic and is formed from
feldspars and mica. Soils formed from calcareous
siltstone and sandstone have
mixed clay mineralogy including smectite,
vermiculite and illite. Both smectite and
vermiculite in these soils are pedogenic
and are formed from mica, while illite is
inherited from the parent materials. Soils
developed from the grey-green marls are
dominated by smectite and have some
accessory amounts of kaolinite and illite;
while those developed from tertiary sediments
are characterized by mixed clay
mineralogy including smectite, kaolinite
and traces of gibbsite. The pedogenic environment
suggests that both kaolinite and
smectite in these soils are not formed insitu
but are inherited from transported parent
materials. The clay mineralogy is clearly
reflected on the cation exchange capacity,
moisture retention characteristics and carrying
capacity (n value) of these soils |
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