Variations and geological significance of solid acidity during smectite illitization

Abstract Solid acidity is an inherent property of clay minerals and intimately related to the crystal chemistry and structure. Smectite illitization is a common phenomenon in mudstones, during which the variations of solid acidity are our concern. In present study, we prepared samples with different illitization extent and conducted a series of measurements, aiming to figure out the evolution of solid acidity during mineral transformation and draw some inspirations for the impacts of solid acidity on hydrocarbon generation approach and mechanism during illitization. In the early stage of illitization from T-25 to T-250, the morphology is irregular and clay minerals are composed of randomly interstratified mixed-layered mineral (MLM) with total illite layer less than 54%. Bronsted (B) acid sites increase slightly even if smectite layer reduces. The slight reduction of interlayer water and increase of interlayer charge are considered to improve the proton-donating ability of interlayer water. Meanwhile, H3O+ adsorbed by the net structural negative charge also contributes to the increase of B acid sites. For Lewis (L) acid sites, the reduction of octahedral Al, which is caused by incomplete crystallization of illite layers as proved by amorphous particles consisting of Si and Al, controls the decline of L acid sites in early stage. In the late stage from T-300 to T-350, R1 and R2 ordered MLMs are predominant with around 80% total illite layers and regular morphology. B acid sites decrease progressively while L acid sites increase at first and then decrease. The match of reductions of interlayer water and B acid sites indicates the significant reduction of smectite layer turns into the main factor controlling the reduction of B acid sites. Meanwhile, the improvement of the ordering, dramatic increase of total illite layer, and absence of amorphous particles render L acid sites increase. However, further increased crystallinity makes Al bond with hydroxyl groups and thus the decrease of L acid sites. T-450 represents another scenario in which decrease of clay minerals and formation of quartz dilute both B and L acid sites. In summary, the amount, strength, and type of solid acidity are varying constantly in the process of illitization. Random MLMs are favorable for B acid sites while ordered MLMs prefer L acid sites. The acid strength peaks when random MLMs are dominated. B and L acid sites could promote clay-organic matter (OM) interactions via different mechanisms, in this way, variation of solid acidity determines priority of two reaction mechanisms of clay-OM interactions and the composition of the hydrocarbons. In conclusion, the variation of solid acidity during illitization connects the mineral transformation and hydrocarbon generation, affecting the mechanisms of organic-inorganic interactions during hydrocarbon generation and providing implications for further investigation of the organic-inorganic interactions in nature