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
