Smectite is an important component of siliciclastic reservoirs in many petroliferous basins around the world and has numerous industrial and environmental applications. Beyond its ability to preserve or degrade porosity and permeability in sandstones, smectite also acts as a precursor for other diagenetic cements, such as illite and zeolites. While numerous studies have explored the role of smectite as a precursor for the formation of illite and chlorite, the controls and mechanisms governing the formation of authigenic smectite coatings from feldspar precursors are rarely investigated. In this study, the authigenic formation of smectite coats and their subsequent illitization were studied using a series of hydrothermal experiments. The starting material was modern quartzo-feldspathic sediments from the sandy intertidal flat of the Anllóns estuary, NW Spain. The experimental solutions were 0.1 M NaCl, KCl, CaCl2·2H2O, MgCl2·6H2O (SF1, pH 5.69), natural estuarine water (EW, pH 7.85), and 0.1 M Na2CO3 (SF2, pH 11.20). The experiments were conducted at temperatures ranging from 50°C to 250°C, over the durations ranging from 168 to 336 hours, with a fixed fluid/sediment ratio of 10:1. The results revealed distinct variations in the synthesized mineral phases, depending on the experimental fluid composition. Detrital K-feldspar dissolution commenced at lower temperatures (< 100°C), and the subsequent formation of authigenic smectite coats occurred between 150°C and 250°C in both SF1 and EW. Geochemical evaluation revealed that SF1 produced more Mg-rich smectite with intermediate compositions, while EW produced mostly Al-rich smectite due to the relatively lower Mg content in the estuarine water. Smectite formation occurred through dissolution–crystallization and was subsequently illitized via a mixed-layer illite–smectite intermediate phase with increasing temperatures. Illitization began at 200°C, with required K+ supplied by the dissolution of K-feldspar. The processes of smectite formation and its illitization released abundant silica, which precipitated as quartz overgrowths. The resulting authigenic grain coatings were chemically and morphologically similar to those found in natural sandstone reservoirs. As temperature increased (from 150 to 250°C), the thickness of these coatings grew from 1.9 µm to 49.2 µm, and surface coverage expanded from 24.3% to 96.2%. In contrast to SF1 and EW, SF2 yielded entirely different mineral phases of halite and chabazite (zeolite), attributed to the high alkalinity and Na content of the solution. These findings highlight the potential of hydrothermal experiments to simulate burial diagenesis in marginal marine settings, which can aid in reservoir quality prediction, critical for energy exploration and transition.

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