Abstract:
This study investigated the relationship between clay minerals and different soil
moisture regimes in gypsiferous soils of Fars Province in southern Iran. The overall
climate of the Province is arid and semi-arid and, under this condition, parent material is
the most important factor affecting clay minerals distribution. Beside this factor, climate
conditions have determining role too. Palygorskite, smectite, chlorite, illite, and kaolinite
were identified as the main clay minerals in gypsiferous soils, using XRD, TEM and SEM
analyses. Chlorite and illite were inherited largely from parent rocks and their abundance
in soils with different moisture regimes was generally uniform. The presence of gypsum
and saline and alkaline ground water in some pedons has favoured the neoformation of
palygorskite from soil solution. Palygorskite shows an increasing trend with depth that
may be related to its authigenic formation in the presence of gypsum. The correlation
between palygorskite percentage and gypsum content was estimated (R2
= 0.56). The
highest amount of palygorskite was observed in soils with aridic moisture regimes, and its
lowest amount was estimated in soils with xeric moisture regimes. Notably, with
increasing moisture, the length of palygorskite fibber decreased. High soil moisture and
rainfall and low evaporation are reasons for instability of palygorskite relative to smectite
in xeric moisture regime. Large amounts of well-bundled and elongated palygorskite in
soils of piedmont plain are related to their authigenic formation; while presence of slight
amounts of short palygorskite fibres in lowlands suggest their transformation to smectite.
Results of soil and rock samples analyses showed that some palygorskite in all moisture
regimes originated from parent materials. Also, results indicated that the
smectite/(illite+chlorite) ratio increased with increase in moisture and the largest value
(equal to 2.12) was observed in soils with xeric moisture regime.
Keywords: Clay minerals, Land forms, Palygorskite, Smectite.
