A contrastive study of effects of different organic matter on the smectite illitization in hydrothermal experiments

Abstract

Although influence of organic matter (OM) on smectite illitization has received considerable attention in previous studies, the effect of different OM types on smectite illitization mechanism has rarely been examined. In the present study, hydrothermal experiments were conducted with N,N-dimethylhexadecylamine (16DMA)-smectite (M2) and lysine-smectite (M3) complexes to explore the effect of OM types on smectite illitization mechanisms. X-ray diffraction (XRD) analysis showed that mineral contents, illite percentage in mixed-layer illite-smectite (I% in ISm), the average number of layers (Nave), and stacking mode of ISm changed weakly at <300 °C while changed greatly at >300 °C. The changes varied obviously between M2 and M3 at >300 °C, indicating characteristics of solid-state transformation illitization mechanism in M2 and dissolution crystallization mechanism in M3. Moreover, the mid-infrared and thermo-XRD analyses indicated that the OM was mainly adsorbed in smectite interlayers at <300 °C, which delayed the exchange of K+ by interlayer cations and thus suppressed the smectite illitization. Above 300 °C, the OM was largely desorbed and the resulting changes of solution properties led to the acceleration of illitization and difference in illitization mechanisms in M2 and M3. The bulk mineralogy, element, and infrared analyses showed that illitization was accompanied by formation of new minerals and more minerals were formed in M2 than M3. The new minerals were further increased above 300 °C, especially the ankerite in M2. The binding mechanisms and strength varied in M2 and M3 because of differences in OM types, resulting in differences in bulk mineralogy evolution. This demonstrated the effect of organic-inorganic interactions on smectite illitization and mineral formation. The disparities in smectite illitization between M2 and M3, therefore, were linked to differences in mineral evolution of water-rock-OM systems with different OM types in natural environments. The insights obtained in the present study should be of high importance in understanding organic-inorganic interactions, hydrocarbon generation, and the carbon cycle.