There is a number of factors having an influence on technological quality of clay materials – mineral composition, proportion of clayey substance, grain-size composition, admixture of carbonates and iron compounds etc. Increased amount of smectite in interlayer of mixed-layer clay mineral illite-smectite affects sorption properties of clays and consequently influences physico-chemical and mechanical properties of clay materials. The six X-ray powder diffraction based methods (determination of swelling smectice interlayer amount in illite structures, detection of illite politype, turbostratic index (TSI), crystallinity degree of illite (KP), illite crystallinity index (KI), illite phases modeling) are applied for characterization of illite and illite-smectite phases. Mutual correlation of data obtained is performed; correlation between amounts of swelling smectite interlayer amount in modeled illite phases – well and poor crystallized illite, illite-smectite and TSI is observed. Illite phase modeling is performed by software Fitik. Good correlation between KI and other calculation methods of smectite amount in illite interlayers was not found. Approaches are approbated using eight samples of Latvian clays. Three phase illite system – well crystallized illite, poor crystallized illite, illite-smectice and two phase illite system – well and poor crystallized illite are recognized in the samples examined. Illite phases and content of swelling smectite in illite interlayer are variable both in the samples from different clay deposits and also within the same deposit. The aforementioned is supposed to be related to different grain-size composition caused by different conditions during sedimentation and post-sedimentation process.