The aim of the study is to determine mineral composition of clayey deposits of the Jurassic Papile Formation of Latvia and also characterize their grain-size composition. The main attention during the research is paid to analysis of clay minerals comprasing estimation of smectite quantity and its changes within the clayey section of the Papile Formation for the greatest part. In the scientific literature this section is known as including the only kaolinite clay resources on the earth surface of Latvia. The mineral and grain-size composition of the deposits is characterized in three geological sections in the Jurassic clay fields Pulvernieki and Strēļi, where grey clayey deposits with sharp transition are covered by black clayey deposits incorporating brown-coal inclusions and interlayers. Till now this section of grey clayey deposits is the least investigated part of the Jurassic geological section. For that reason exactly these grey clayey deposits from six metres long drill-hole at Strēļi site are characterized the most detailed in this paper analysing their mineral and grain-size features. Research methods include grain-size analysis and XRD. Mineral qualitative and semi-quantitative analysis to various fractions of the deposits is done. Semi-quantitative analysis mostly on evaluation of smectite content within the samples is based. As a result of the study it is established that in the lower part of the section of gray clayey deposits of the Papile Formation, which is sandier in general, the deposits contain considerable amount of smectite as well as admixture of chlorite and also calcite and dolomite are found. Towards the upper part of the section deposits become more clayey and very fine-grained, as well as content of smectite decreases and also indications of presence of chlorite disappears, at the same time these minerals by kaolinite are replaced. In the covering section of black clayey deposits among clay minerals illite and kaolinite predominates, but smectite only as admixture is found. The conclusion is drawn that mineral composition and its changes within the section of grey clayey deposits of the Papile Formation are indicative to their relation with the beneath lying Triassics clay mineral composition where smectite dominates and also chlorite and carbonates are common. These distinctive changes of composition of the Papile Formation clayey deposits also points to probability to mine naturally with smectite and kaolinite refineded clays on a selective basis.