Abstract
To determine factors controlling abundance and distribution pattern of clay minerals, four soil pedons derived from calcareous sedimentary parent rocks in Northern Iraq were investigated. Results revealed that the regional variation in annual rainfall plays a major role in determining amounts and distribution of clay minerals such as palygorskite,chlorite, vermiculite, smectite, illite and kaolinite. Palygorskite and smectite are available in three studied soils (Al-Khuder, Tel-Auskuf and Sumeel) with average annual precipitation of around 250 mm, 350 mm and 480 mm respectively, but these minerals are totally disappear in Batofa profile (Annual rainfall around 800mm).However vermiculite illite and kaolinite are the dominated clay minerals in Batofa soil.XRD analysis was detected a small amounts of random interstratified layer silicate minerals.Biotite-vermiculite mixed layers were found in Batofa soil,but in Al-Khuder soil an increasing amounts of (chlorite - smectite) mixed layer was detected while in Tel-Auskuf soils it appears that (chlorite-vermiculite) is the dominated mixed layer and (chlorite-smectite) is available at Sumeel soil. The general variations in clay minerals content and distributions in different soil locations can be attributed mainly to weathering conditions and mineral transformation. Palygorskite is considered to be inherited in soils of arid regions with gypsum content and limited annual rainfall, whereas illite, vermiculite and kaolinite occurs in soils with high annual rainfall.
