On the basis of data from analyses (particle-size, mineralogical, morphological and morphostructural) considerable differences are observed in the mineralogical and particle-size composition of the sands. Tervel sand is more coarse-grained and unique with the presence of feldspars - microcline, orthoclase, plagioclase (up to 40 %). Doulovo sand is more fine-grained, without feldspar, but anatase, rutile and muscovite are more abundant than in Tervel sand. Kaolinite is the essential clay mineral, but smectite is determined only in the Tervel clay. The elemental composition of the clay fractions <2 µm (bulk), 1–2 µm and <1 µm were analyzed. In the Doulovo clay the mean Al : Si ratio is higher. Na₂O, CaO and MgO are present only in Tervel clay.