Clay minerals in the diagenetic/very low-grade metamorphic-sedimentary series from southeastern Anatolia in Turkey were analyzed to determine their mineralogical and chemical compositions. In the Amanos region, the lowermost unit is composed of metaclastics with primary clastic textures, as well as slaty cleavages and chlorite-mica stacks including volcanic rock intercalations. The Lower Cambrian is composed of mainly very low-grade metamorphic clastic rocks, while the Ordovician units have siliciclastic and carbonate rocks. In the Hazro region, the Late Silurian-Lower Triassic units are represented by highly diagenetic carbonate and clastic rocks. All of the rock units include illite. In addition, chlorite, mixed-layered illite-chlorite and chlorite-vermiculite are present in the Amanos region, while calcite, dolomite, kaolinite, mixed-layered illite-smectite (I-S) and glauconite occur in the Hazro region. The illites are characterized by the dominance of 2M(1) polytype in the Amanos samples; and 1M(d) + 2M(1) in the Hazro samples. The I-S, glauconite and kaolin have R1 and R3, 1M and kaolinite polytypes, respectively. The illites have greater tetrahedral and lower octahedral substitutions than the I-S. Total trace element contents, elemental substitutions and chondrite-normalized trace element and REE values decrease toward illite-I-S-kaolinite. There are obvious fractionations for some major - trace and rare earth elements with respect to each other and clear enrichment with respect to the chondrite, with strong anomalies of positive for Gd and negative for P, K and Eu in the clay minerals. The textural, morphological and geochemical data indicate that kaolinite and I-S in the Hazro area occur in supergene conditions with due to a full neoformation mechanism, whereas illites in the Amanos region represent the hypogene origin. In brief, the K2O contents, ratios of Eu/Eu* and La-N/Lu-N and delta O-18 and delta D values of I-S and illite exhibit notable relationships with increasing diagenetic/metamorphic grade. (C) 2013 Elsevier Ltd. All rights reserved.