Illite 1Md polytype is dominant in the clay fractions in the clayey sediments of Latvia. However separate individual illite unbasal reflexes in 1Md structure not always is simple, because admixture of another mineral phases in the clay fractions is inconvenient, especially in the glacigenic sediments. Therefore was made mineral and illite structure systematical determination by sediments of different genesis and mineral composition in various clays fractions. Study of illite structures was carried out on glacigenic sediments of last glaciation and clayey sediments of Upper Devonian Katlešu formation. Obtained analytical results indicate, that with hight XRD measurement parameters is possible determinate individual 2M1 and 1M reflexes, which correspond to 1Md polytype. Analytical results shown that individual illite polytype reflexes are different in the bedrock clays and they covered till and after illite polytypes analyse is possible divide different origin of illites.