ArticleA 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