The Role of Fluid Chemistry in the Diagenetic Transformation of Detrital Clay Minerals: Experimental Insights from Modern Estuarine Sediments

Abstract

The diagenetic transformation of detrital clay minerals significantly influences sandstone reservoir quality, with fluid chemistry and temperature playing key roles in dictating transformation pathways during burial diagenesis. While these processes are well-documented in basinal settings, the diagenetic alterations of sediments in dynamic environments like estuaries remain underexplored. This study investigates the impact of fluid composition on the transformation of modern estuarine sediments through hydrothermal experiments using sediments from the Gironde estuary, SW France. A range of natural and synthetic solutions including seawater (SW), 0.1 M KCl (SF1), 0.1 M NaCl, KCl, CaCl₂·2H₂O, MgCl₂·6H₂O (SF2), estuarine water (EW), and 0.1 M Na₂CO₃ (SF3) were used under temperatures from 50 °C to 250 °C for 14 days, with a fixed fluid-to-sediment ratio of 10:1. The results revealed distinct mineralogical transformations driven by fluid composition. Dissolution of detrital feldspars and clay materials began at lower temperatures (<100 °C). The authigenic formation of smectite and its subsequent illitization in K-rich fluids (SW, SF1) occurred between 150 °C and 250 °C, replicating potassium-driven illitization processes observed in natural sandstones. Additionally, chlorite formation occurred through the conversion of smectite in SF2 and EW. Geochemical analysis showed that SF2 produced Mg-rich chlorites, while EW yielded Fe-rich chlorites. This aligns with diagenetic trends in coastal environments, where Fe-rich chlorites are typically associated with estuarine systems. The resulting authigenic illite and chlorite exhibited morphological and chemical characteristics similar to those found in natural sandstones, forming through dissolution-crystallization and solid-state transformation mechanisms. In contrast to illite and chlorite, SF3 produced entirely different mineral phases, including halite and analcime (zeolite), attributed to the high alkalinity and Na-rich composition of the solution. These findings provide valuable insights into the role of fluid chemistry in the diagenetic alteration of modern sediments and their implications for the evolution of sandstone reservoirs, which is critical for energy exploration and transition.

1. Introduction

The influence of clay minerals on sandstone reservoir quality has been widely documented [1,2,3,4,5,6,7,8,9]. Grain-coating clays, such as chlorite, illite, and mixed-layer clays, have been shown to inhibit authigenic quartz cementation in deeply buried reservoirs, thereby preserving reservoir quality [10,11,12,13]. Conversely, pore-filling clays, including smectite, kaolinite, and illite, occupy pore spaces and obstruct fluid flow, significantly reducing reservoir porosity and permeability [14,15,16,17]. Numerous studies have investigated the occurrence and distribution of grain-coating clays and their role in preserving reservoir quality [2,18,19,20]. These studies highlight the influence of depositional environments on the distribution of authigenic clay coats. For instance, chlorite commonly occurs in fluvial to marginal marine environments [18], whereas smectite is prevalent in nearshore marine sediments [21]. Diagenetic clay coatings develop through the recrystallization of detrital precursors or direct precipitation from pore fluids during burial diagenesis [22,23,24]. These clay coatings significantly influence fluid flow, mineral stability, and reservoir quality across sedimentary systems worldwide, including marginal marine environments [5,9,25].

Modern marginal marine sediments, such as those found in estuarine environments, often contain detrital clay coatings that vary in texture and composition based on depositional sub-environments [26]. These variations create heterogeneities that influence diagenesis and present significant challenges for reservoir quality prediction models [27]. Recent works have explored the controls influencing the occurrence and distribution of detrital grain-coating clays in modern marginal marine settings [28,29,30,31,32,33]. However, the physicochemical factors controlling the diagenetic transformation of these sediments remain understudied. Understanding these transformation pathways is particularly important as marginal marine reservoirs gain prominence for groundwater and energy exploration [34], carbon capture and storage (CCS), and hydrogen storage [9,27] due to their porosity, permeability, and mineralogical stability. The ability to predict how clay minerals will evolve under burial conditions is essential for evaluating the long-term storage capacity and injectivity of these reservoirs.

Variations in fluid chemistry, temperature, and pressure dictate the transformation pathways of detrital materials during burial diagenesis [34,35,36,37]. For example, the presence of Al³⁺, Mg²⁺, and Fe²⁺ in formation brines is crucial for chlorite formation during fluid–rock interactions [38,39], while K⁺ is essential for the precipitation of authigenic illite [14,30,40,41]. Changes in the fluid–rock ratio, fluid chemistry, and pH of formation water can result in the development of different authigenic clay minerals during burial, even when derived from the same detrital mineral precursors [42,43]. Consequently, understanding the mineralogical and geochemical pathways that govern clay diagenesis has become increasingly important, particularly in the context of sustainable practices such as geothermal energy production and carbon capture and storage (CCS) technologies, which are vital for the energy transition.

This study investigates the role of fluid chemistry in the diagenetic alteration of detrital clay minerals, using sediments from the Gironde estuary, SW France, as an analog for ancient marginal marine sandstones. These fresh sediments, free from extensive weathering or diagenetic alterations, provide a suitable framework for simulating burial diagenesis under controlled conditions and investigating the formation of clay minerals during the sand-to-stone transition. Hydrothermal reactor experiments were conducted to replicate diagenetic conditions, exposing sediments to fluids with varying pH and compositions under controlled temperatures. The specific objectives were to (1) simulate the diagenetic formation of clay minerals in modern estuary sediments as analogs for deep burial diagenesis; (2) determine the mechanisms of formation for these clay minerals; (3) assess the impact of fluid chemistry on clay mineral transformation pathways; and (4) evaluate the relevance of the hydrothermal experiments to natural sandstone diagenesis.

2. Study Area and Geologic Background of the Sample

This study utilized sediments from the modern Gironde estuary, located in the Nouvelle-Aquitaine region of southwestern France (Figure 1A). The estuary is one of Europe’s largest estuarine systems, covering an approximate surface area of 630 km² [32,44]. It extends about 160 km inland from its mouth to its upstream tidal limit [31]. The estuary is fed by two principal rivers, the Garonne and Dordogne, which drain waters from the Pyrenees and the Massif Central, respectively, forming an extensive watershed [45]. The Gironde estuary is macrotidal and characterized by semi-diurnal tides, with tidal ranges varying from 1.5 m during neap tides to 5.5 m during spring tides [32,46]. Its morphology and hydrodynamics are shaped by a complex interplay of fluvial and tidal processes, resulting in different patterns of sediment transport and deposition [31]. The estuary is typically divided into three main zones based on its morphology and hydrodynamics: the outer estuary funnel, the inner estuary funnel, and the Garonne and Dordogne estuarine channels [31,47,48]. A defining feature of the Gironde estuary is its well-developed Turbidity Maximum Zone (TMZ), a highly dynamic region where suspended sediment concentrations can reach up to 300 g/L during slack water [46,49]. The TMZ, which varies seasonally, facilitates hydrodynamic trapping of fine-grained sediments, including clay minerals, fine quartz grains, and organic matter [32].

The sediment distribution within the estuary reflects its heterolithic nature: sandy tidal bars, point bars, and fine-grained intertidal and subtidal deposits [31]. Specifically, the samples used in this study were collected from estuarine sands and muds within the inner estuary funnel (Figure 1A–C). The estuary’s sedimentary framework, with its extensive muddy deposits on estuarine banks and intertidal flats, serves as an analog to many ancient sedimentary systems. Understanding these systems is essential for interpreting sedimentary processes, including the authigenesis of clay minerals. Additionally, the origin and distribution of detrital clay coats within the Gironde estuary are well documented [31,32,50,51].

3. Methods

This study integrated several laboratory experiments and analytical techniques, including hydrothermal reactor experiments, scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDS), X-ray fluorescence (XRF), X-ray diffraction (XRD; bulk and clay fraction), scanning transmission electron microscopy (STEM), inductively coupled plasma optical emission spectroscopy (ICP-OES), and focused ion beam scanning electron microscopy (FIB-SEM). All experiments and analyses were conducted at the laboratories of the Center for Integrative Petroleum Research, College of Petroleum Engineering and Geosciences, King Fahd University of Petroleum and Minerals, Saudi Arabia. The detailed methodologies for each analysis are outlined below.

3.1. Hydrothermal Experiments

The hydrothermal reactor experiments were conducted using an Ollital OLT-HP-500 (Ollital Tech., Fujian, China) multi-position parallel reactor autoclave system, connected to a nitrogen gas cylinder (Figure 2). This system comprises four stainless steel micro-stirred parallel reactors, each equipped with independent controls for temperature, pressure, and elapsed time. The reactor kettles have a maximum capacity of 500 mL and can be heated to temperatures of up to 350 °C and pressures of 160 bars. Temperature regulation is achieved through a Smart PID algorithm control system, ensuring an accuracy of ±0.1 °C. The sediment samples were divided into twelve portions before the experiments, each weighing 20 g. For each experiment, 20 g of sediment were placed into a 50 µm stainless steel mesh and positioned at the bottom of the reactor kettles. The kettles were then filled with 200 mL of the respective experimental solutions, as outlined in

Table 1. Conditions for the hydrothermal experiments.

Experiment	Sample ID	Solutions	Temperature (°C)	Duration (Days)	Final Pressure (Bar)
1	SW@50	Red Sea water	50	14	46.7
2	SW@100	Red Sea water	100	14	52.7
3	SW@150	Red Sea water	150	14	65.1
4	SW@200	Red Sea water	200	14	91.2
5	SW@250	Red Sea water	250	14	116.3
6	SF1@250	0.1 M KCl in Red Sea Water	250	14	117.6
7	SF2@100	0.1 M NaCl, KCl, CaCl2·2H2O, MgCl2·6H2O	100	14	56.1
8	SF2@150	0.1 M NaCl, KCl, CaCl2·2H2O, MgCl2·6H2O	150	14	67.5
9	SF2@200	0.1 M NaCl, KCl, CaCl2·2H2O, MgCl2·6H2O	200	14	79.8
10	SF2@250	0.1 M NaCl, KCl, CaCl2·2H2O, MgCl2·6H2O	250	14	108.3
11	EW@250	Estuarine water	250	14	112.5
12	SF3@250	0.1 M Na2CO3	250	14	128.7

SW = seawater; SF = synthetic fluid; EW = estuarine water.

. The experimental solutions included the following: Red Sea water for experiments 1–5; a 0.1 M KCl-enriched Red Sea water solution for experiment 6; a single 0.1 M mixed solution containing NaCl, KCl, CaCl₂·2H₂O, and MgCl₂·6H₂O for experiments 7–10; Gironde estuarine water for experiment 11; and a 0.1 M Na₂CO₃ solution for experiment 12. Each salt in the mixed solution for experiments 7–10 was prepared at an individual concentration of 0.1 M, ensuring that all major cations were included at equal molarity.

The reactors were then sealed and pressurized with nitrogen gas to an initial pressure of approximately 40 bar at room temperature. They were subsequently heated to the target experimental temperatures, which ranged from 50 °C to 250 °C (Table 1), at a heating rate of 15 °C per minute. While the initial pressure was consistent across all experiments (~40 bar at room temperature), it increased with temperature and the specific reaction fluid, stabilizing at the final pressure after 1–3 h (Table 1). The samples were maintained at the designated experimental temperatures for 14 days under strictly closed conditions, following the procedures described by [36,52]. After each experiment, the reactors were switched off and cooled to room temperature. The sediment samples were then recovered and air-dried. Post-experiment solutions were also retrieved, immediately analyzed for pH, and subjected to ICP-OES analysis at room temperature. The synthetic fluid compositions used in this study were simplified to include only the major cations. The fluid-to-rock ratio of 10:1 ensured sufficient interaction between the fluid and sediment to facilitate mineral transformations.

3.2. Analytical Techniques

3.2.1. SEM/SEM-EDS Analyses

The pre- and post-experiment samples were analyzed using a Zeiss Gemini 550 scanning electron microscope (SEM) (Zeiss, Oberkochen, Germany). The SEM was equipped with a backscattered electron (BSE) detector and an Aztec energy-dispersive spectrometer (EDS) system, developed by Oxford Instruments, UK. For pre-experiment analysis, loosely disaggregated sediments were bonded to an aluminum stub using carbon adhesive tape, while post-experiment consolidated samples were mounted on stubs. Both samples were coated with a 30 nm layer of gold using a Quorum Q150R sputter coater (Quorum Tech., Laughton, UK) to ensure optimal imaging and analysis [53,54]. The SEM analyses were conducted at accelerating voltages ranging between 10 and 15 kV, with probe currents of 1 to 2 nA, respectively. The SEM and SEM-EDS investigations were conducted to examine the morphology, chemical composition, and textural relationships of detrital minerals in the initial material, as well as the formation and characteristics of any authigenic minerals that developed during the experiments.

3.2.2. XRF Analysis

The oxide compositions of pre- and post-experiment samples were analyzed using a Panalytical Zetium (Malvern Panalytical, Almelo, Netherlands) wavelength-dispersive X-ray fluorescence (WD-XRF) spectrometer. Samples were prepared by fusion with lithium metaborate flux using a Classie The OX Advanced fusion instrument (Malvern Panalytical, Almelo, Netherlands). For each sample, 1.0 g of finely ground material was mixed with 10.0 g of flux (LiBO₂:Li₂B₄O₇ in a 49.75:49.75:0.50 ratio) in a platinum-gold crucible. The mixture underwent a controlled heating program: an initial hold at 500 °C for 2 min, a gradual ramp to 1125 °C over 27 min with intermittent rocking, and final vigorous swirling before being poured into a mold to form homogeneous glass disks. The cooled glass disks were analyzed on the Zetium spectrometer, equipped with a high-resolution Cu anode X-ray tube and specialized analyzing crystals for enhanced sensitivity and resolution across major, minor, and trace elements. The quantitative analysis utilized synthetic-certified reference materials (WROXI series) and was validated with periodic quality control standards. PANalytical SuperQ software (version 6) was used for spectral acquisition, matrix corrections, and data processing, ensuring accuracy and precision. This methodology, optimized for geological materials, supported the reliable detection of major oxides and trace oxides at ppm levels.

3.2.3. Bulk and Clay Fraction XRD Analysis

The mineralogical compositions of bulk and clay fractions (<2 µm) in the starting and post-experiment samples were determined via X-ray diffraction (XRD), using the Malvern Empyrean PANalytical (Malvern Panalytical, Almelo, Netherlands) diffractometer with Ni-filtered Cu Kα radiation. Bulk sediments were air-dried, powdered with a Retsch RM 200 mill, and analyzed under conditions of 45 kV and 40 mA across a 4.5–70° 2θ range with an 8.7 s scan time using the procedures outlined by [17].

Clay fractions (<2 µm) were isolated through gravity settling and centrifugation, as described by [9]. Five grams of bulk sample were dispersed in 100 mL of deionized water, ultrasonicated for 20 min with periodic stirring, and settled to remove sand and silt particles. The clay-rich supernatants were concentrated by centrifugation at 5000 rpm for 15 min. The separated clay fractions were prepared as oriented aggregates and were analyzed in three states: air-dried, ethylene glycol-solvated, and heated at 550 °C (1 h). Analyses were conducted over a 4–40° 2θ range with a 0.05°/s step size. Clay minerals were identified using HighScore Plus (v. 4.9), with the ICDD PDF-4-2022 mineralogical library and additional data from [55]. The baseline peak positions of air-dried clays were compared with those after ethylene glycol (EG) solvation and heating treatments to distinguish the clay minerals. For semi-quantitative analysis, Rietveld refinement was conducted in HighScore Plus using structure files from the Inorganic Crystal Structure Database (ICSD, 2021.1), carefully selected for compatibility with oriented clay samples. Profile-matching techniques ensured alignment between experimental spectra and reference patterns, while the March–Dollase function was applied to correct preferred orientation effects. Profex software (5.4.1) was also used for cross-validation to enhance result reliability. Despite following these protocols, the results remain semi-quantitative due to HighScore’s inherent limitations in accurately quantifying oriented clay minerals.

3.2.4. STEM Analysis

The morphological and structural characterization of clay minerals was conducted using scanning transmission electron microscopy (STEM) on a Dual Beam Laser FIB-SEM system (Zeiss 550, Zeiss, Oberkochen, Germany). The integrated STEM detector, positioned beneath the specimen and aligned perpendicularly to the optical axis, collected transmitted electrons that passed through the samples. The separated clay fractions were prepared as aqueous suspensions in deionized water and aspirated onto carbon-coated copper 200-mesh TEM grids (QUANTFOIL, Thüringen, Germany). The grids were dried in a desiccator before being mounted on a specialized TEM grid holder adapted for the SEM stage. The analyses were conducted under operating conditions of 10 kV accelerating voltage and a probe current of 500 pA. The segmented design of the STEM detector enabled selective enhancement of image contrast by activating or deactivating specific detector segments (aSTEM4 B). Multiple micrographs were acquired under these conditions to ensure thorough characterization of the samples. Additionally, elemental compositions of specific minerals were determined using energy-dispersive X-ray spectroscopy (EDS) at several points of interest.

3.2.5. ICP-OES Analysis

Inductively Coupled Plasma-Optical Emission Spectroscopy (ICP-OES) was employed to determine the elemental composition of major cations in solutions before and after the hydrothermal experiments. Analyses were performed using a PerkinElmer Plasma Quant 9000 ICP-OES instrument, (PerkinElmer, Shelton, CT, USA) which utilizes plasma as an excitation source to detect optical emissions. Post-experiment fluids were collected, transferred to vials, and diluted to the appropriate ratios prior to analysis. This procedure allowed for the quantification of key cations, including Al, Ca, Fe, Mg, K, and Na, in both pre-and post-experiment solutions.

3.2.6. FIB-SEM Analysis

Focused Ion Beam Scanning Electron Microscopy (FIB-SEM) was employed to examine the internal structure of the samples with high-resolution imaging. The analysis was performed using a Zeiss Crossbeam 550 FIB-SEM (Zeiss, Oberkochen, Germany), equipped with a Gallium FIB column and automated using SmartFIB software (v. 7.05). For cross-sectioning, a trapezoidal trench with a 30 μm width and 20 μm depth was milled in three stages. Coarse milling (30 kV, 15 nA) rapidly excavated the trench, followed by intermediate milling (30 kV, 700 pA) to expose the target area. A final fine polishing step (30 kV, 300 pA) created a smooth surface and removed residual debris. Tilt correction was applied to optimize imaging accuracy, and micrographs were captured to document key features. Serial sectioning was then conducted with a 50 nm slice thickness over a 40 μm length, generating sequential layers for detailed analysis. Both secondary electron (SE) and backscattered electron (BSE) micrographs were automatically recorded for each section, enabling comprehensive characterization of the sample’s cross-sectional properties.

4. Results

4.1. Compositional Evaluation of the Starting Material

The starting material consists of fine to silt-sized grains that are moderately sorted and angular to subangular in shape. WD-XRF oxide analyses (

Table 2. WDX-XRF analysis of the starting material and post-reacted samples.

		Oxides Measured in Weight Percent (wt %)
Experiment	Sample ID	SiO2	Al2O3	CaO	Fe2O3	MgO	Na2O	K2O	ClO2	MnO	P2O5	SO3	SrO	TiO2
SM	SM@0	57.43	17.43	9.87	7.38	2.12	0.79	3.07	0.09	0.15	0.18	0.14	0.02	1.01
Exp. 1	SW@50	52.86	17.55	8.62	7.48	2.32	3.37	3.24	2.64	0.09	0.18	0.51	0.02	0.94
Exp. 2	SW@100	56.43	18.04	8.43	7.66	3.18	1.21	3.46	0.09	0.13	0.21	0.06	0.02	1.04
Exp. 3	SW@150	54.59	18.36	5.90	8.49	3.23	2.34	3.35	1.62	0.12	0.20	0.54	0.02	0.97
Exp. 4	SW@200	57.30	15.47	3.59	8.68	4.93	2.36	2.73	1.51	0.11	0.16	1.88	0.02	0.90
Exp. 5	SW@250	52.77	16.94	2.22	10.67	4.78	2.01	6.18	2.26	0.10	0.19	0.71	0.02	0.93
Exp. 6	SF1@250	54.59	16.12	6.29	7.19	2.03	0.73	9.14	2.39	0.12	0.18	0.04	0.01	0.98
Exp. 7	SF2@100	55.08	17.15	9.15	7.94	2.64	0.98	3.72	1.66	0.09	0.17	0.11	0.02	0.94
Exp. 8	SF2@150	58.00	16.34	4.04	9.09	5.25	0.78	3.44	1.33	0.08	0.19	0.06	0.01	0.99
Exp. 9	SF2@200	55.85	16.62	3.39	9.79	5.84	0.86	3.79	2.19	0.09	0.19	0.06	0.01	0.96
Exp. 10	SF2@250	58.24	18.08	1.39	9.58	5.06	1.94	3.16	0.77	0.09	0.22	0.16	0.01	1.06
Exp. 11	EW@250	57.06	16.59	3.26	8.23	7.62	0.60	3.59	1.33	0.12	0.19	0.05	0.01	1.35
Exp. 12	SF3@250	53.97	17.26	9.36	7.30	2.14	5.46	2.88	0.09	0.14	0.19	0.04	0.02	0.95

SM = Starting material; SW = seawater; SF = synthetic fluid; EW = estuarine water.

) indicate that the sediment is dominated by SiO₂ (57.43 wt%), Al₂O₃ (17.43 wt%), CaO (9.87 wt%), Fe₂O₃ (7.38 wt%), K₂O (3.07 wt%), and MgO (2.12 wt%) (Figure 3). Minor amounts of TiO₂ (1.01 wt%) and Na₂O (0.79 wt%) are also present, while ClO₂, MnO, P₂O₅, SO₃, and SrO occur in trace quantities (Table 2). Bulk XRD analysis (

Table 3. Bulk and clay fraction composition in pre-and post-reacted samples.

Sample ID	Bulk Mineralogy (%)										Clay Mineralogy (<2 µm) (%)
	Qtz	Orth	Pla	Cal	Mus	CM	Hal	Syl	Ana	Total	Kao	Sme	Ill	Chl.	Qtz
SM	40.7	5.3	14.7	9.7	9.3	20.3	-	-	-	100.0	52	5	41	-	2
Exp. 1	41.0	8.1	17.3	5.1	7.6	20.9	-	-	-	100.0	50	5	40	-	5
Exp. 2	39.3	7.7	15.2	3.7	6.9	27.2	-	-	-	100.0	48	6	42	-	4
Exp. 3	38.5	7.6	14.9	3.1	6.3	29.6	-	-	-	100.0	41	7	49	-	3
Exp. 4	35.4	4.8	16.5	1.8	5.1	34.7	1.7	-	-	100.0	33	3	58	-	6
Exp. 5	38.9	2.4	15.6	1.2	3.5	38.4	-	-	-	100.0	18	2	76	-	4
Exp. 6	29.4	6.3	14.5	5.8	3.3	37.1	-	3.6	-	100.0	20	TR	80	-	0
Exp. 7	38.2	9.1	18.7	3.0	8.1	22.9	-	-	-	100.0	44	2	48	-	6
Exp. 8	36.9	7.4	14.9	3.9	7.2	29.7	-	-	-	100.0	40	1	52	4	3
Exp. 9	37.6	4.0	15.5	0.5	3.5	38.9	-	-	-	100.0	34	0	55	8	3
Exp. 10	35.2	3.8	9.7	0.3	3.3	46.7	-	-	-	100.0	26	0	52	16	6
Exp. 11	35.1	3.5	14.4	1.9	5.8	36.9	-	-	-	100.0	25	1	48	24	3
Exp. 12	31.2	5.3	12.1	9.1	5.2	13.1	2.4	-	21.7	100.0	16	40	41	-	3

SM = starting material; Qtz = quartz; Orth = orthoclase; Pla = plagioclase; Cal = calcite; Mus = muscovite; CM = clay minerals; Hal = halite; Syl = sylvite; Ana = analcime; Kao = kaolinite; Sme = smectite; Ill = illite; Chl = chlorite.

, Figure 4) reveals that the sediment is predominantly composed of quartz (40.7%), plagioclase (14.7%), and orthoclase (5.3%) (Table 3). Additionally, significant amounts of calcite (9.7%) and muscovite (9.3%) are present. Clay minerals, including kaolinite, smectite, and illite, constitute 20.3% of the total bulk mineralogy (Table 3).

These findings are further supported by SEM and EDS analyses, which demonstrate that the dominant detrital grains are monocrystalline quartz (Figure 5A,B), K-feldspar (Figure 5A), albite, and muscovite (Figure 5B). Carbonate materials are predominantly calcite (Figure 5A). The results also reveal that the framework grains are partially coated with detrital clay and silt-sized materials, which occur as drapes (Figure 5A) and aggregates (Figure 5B). These grain coats are primarily composed of clay minerals, with minor amounts of silt-sized quartz grains and diatoms (Figure 5C,D). However, even under higher-resolution SEM imaging (Figure 5E), it was challenging to identify specific clay minerals based on morphology due to poor crystallization and poorly defined internal structures. EDS analysis confirms that the grain coats are composed of Si, O, Al, K, Fe, Mg, and Ca (Figure 5F). XRD analyses of the clay fraction reveal that the detrital clay mineral assemblage is dominated by kaolinite (51.5%) and illite (41.1%), with minor contributions from smectite (5.3%) and no detectable chlorite (Figure 6A, Table 3). Smectite peaks were identified after ethylene glycol solvation. They disappeared, along with kaolinite, following heating to 550 °C for 1 h (Figure 6A). The d-spacing values of the identified clay minerals are annotated on the overlaid diffraction peaks (Figure 6A).

4.2. Geochemical and Mineralogical Evaluation of Post-Experiment Samples

A detailed compositional assessment of the reactive fluids and solids retrieved after the hydrothermal experiments was conducted using ICP-OES, WD-XRF, SEM, SEM-EDS, STEM, and XRD analyses. The results demonstrate distinct geochemical and mineralogical changes influenced by the experimental fluids’ chemistry and increasing temperatures (Figure 3, Figure 4, Figure 5, Figure 6, Figure 7, Figure 8, Figure 9, Figure 10, Figure 11, Figure 12 and Figure 13; Table 2, Table 3 and

Table 4. ICP-OES analysis showing the concentrations of major cations in solutions before and after the hydrothermal experiments.

Identification		pH	Major Cations (mg/L)						TDS (ppt)
			Al	Ca	Fe	Mg	Na	K
SW	SW@0	8.12	57.7	396	638	1820	18,650	798	36.71
Exp. 1	SW@50	7.21	420	7730	770	1740	15,680	6460	56.19
Exp. 2	SW@100	6.88	542	14,260	1080	1400	14,900	7100	56.73
Exp. 3	SW@150	6.42	648	22,500	1690	939	12,060	6060	58.01
Exp. 4	SW@200	6.41	782	44,800	2310	631	5950	2470	58.94
Exp. 5	SW@250	6.34	1080	67,000	2350	431	5510	2000	105.2
SF1	SF1@0	7.78	70.1	410	650	1690	16,430	17,200	43.95
Exp. 6	SF1@250	6.86	477	58,000	919	818	9600	9060	104.5
SF2	SF2@0	5.71	ND	8730	ND	15,500	12,800	7650	45.33
Exp. 7	SF2@100	6.22	47.8	21,400	13.7	3690	12,500	9280	56.15
Exp. 8	SF2@150	6.45	126	29,900	16.6	3520	10,610	10,300	62.08
Exp. 9	SF2@200	6.28	520	42,500	60.1	600	6820	2990	60.69
Exp. 10	SF2@250	6.22	787	48,400	132	167	2590	2620	62.29
EW	EW@0	7.26	5.2	12.1	112	381	10,600	70.5	13.02
Exp. 11	EW@250	6.91	120	14,200	5.1	120	8700	1250	26.39
SF3	SF3@0	11.32	ND	ND	ND	ND	8250	ND	13.79
Exp. 12	SF3@250	8.31	860	2910	1820	38.4	390	1140	7.79

TDS = total dissolved solids; SW = seawater; SF = synthetic fluid; EW = estuarine water.

). Below, the findings are presented based on the observed changes in experiments conducted with different reactive fluids.

4.3. Seawater-Based Experiments (Experiments 1–6)

Experiments 1–5 were performed using natural Red Sea water under increasing temperatures of 50, 100, 150, 200, and 250 °C, respectively, with a constant duration of 14 days per experiment, while Exp. 6 was conducted with 0.1 M KCl dissolved in Red Sea water (SF1) at 250 °C for 14 days (Table 1). XRF analyses of the sediments from these experiments reveal variations in major and minor oxides with increasing experimental temperature (Table 2, Figure 3). The concentration of SiO₂ fluctuates, increasing from 52.86 wt% in Exp. 1 to 56.43 wt% in Exp. 2, before decreasing to 54.59 wt% in Exp. 6 (Table 2). During Experiments 1–3, the Al₂O₃ content in the reacting solids increases slightly, from 17.55 wt% in Exp. 1 to 18.36 wt% in Exp. 3, but then decreases significantly to 15.47 wt% and 16.94 wt% in Experiments 4 and 5, respectively. Calcium oxide (CaO) consistently decreases with rising experimental temperature, from 8.62 wt% in Exp. 1 to 2.22 wt% in Exp. 5, and increasing to 6.29 wt% in Exp. 6 (Table 2). Similarly, Na₂O shows a slight decrease as temperature increases. In contrast, Fe₂O₃, MgO, and K₂O concentrations generally increase with temperature, except in Exp. 4, where K₂O decreases slightly to 2.73 wt%. Notably, K₂O increases significantly, from 3.07 wt% in the starting material to 9.14 wt% in Exp. 6. Trace oxides such as ClO₂, MnO, P₂O₅, SO₃, and SrO remain relatively stable across all experiments, except for ClO₂, which shows irregular fluctuations (Table 2).

The hydrochemical composition of the Red Sea water and SF1 pre- and post-experiments (Experiments 1–6) is summarized in Table 4 and Figure 10. The pH of the fluids decreases consistently with increasing temperatures, dropping from 8.12 in the initial Red Sea water to 6.34 in Exp. 5. Similarly, in Exp. 6, the pH decreases from 7.78 in the pre-reacted solution to 6.86 post-experiment (Table 4). Total dissolved solids (TDS) increase significantly during the experiments, with TDS rising from 36.71 ppt in the starting solution of Experiments 1–5 to 58.95 ppt and 105.2 ppt in Experiments 4 and 5, respectively. For Exp. 6, TDS increase from 43.95 ppt pre-experiment to 104.5 ppt post-experiment (Table 4). ICP-OES analysis highlights variations in six major cations (Al, Ca, Fe, Mg, K, Na) during the experiments. Aluminum (Al) concentration rises steadily from 57.7 mg/L in the starting solution to 420 mg/L in Exp. 1 and 1080 mg/L in Exp. 5, corresponding to increasing temperatures. Calcium (Ca) shows a marked increase in the fluid phase, from 396 mg/L in the starting solution to 67,000 mg/L in Exp. 5, contrasting with its rapid depletion in the solids (Table 2). Potassium (K) concentrations increase during Experiments 1 and 2 but decline in Exp. 3, before being significantly consumed in Experiments 4–6 (Table 4). Iron (Fe) concentrations increase slightly with temperature, while magnesium (Mg) and sodium (Na) concentrations decrease (Table 4).

Bulk mineralogical results from XRD analyses indicate that quartz remains the dominant mineral throughout the experiments (Table 3, Figure 4), though its concentration decreases slightly from 41.0% in Exp. 1 to 38.9% in Exp. 5 and 29.4% in Exp. 6. Orthoclase decreases significantly, from 8.1% in Exp. 1 to 2.4% in Exp. 5, while plagioclase undergoes a moderate decrease (Table 3). Both muscovite and calcite dissolve rapidly with rising temperatures, with calcite reducing from 5.1% in Exp. 1 to 1.2% in Exp. 5. Conversely, clay mineral content increases significantly, from 20.9% in Exp. 1 to 38.4% in Exp. 5 and 37.1% in Exp. 6. Halite is observed in minor amounts (1.7%) in Exp. 4, while sylvite appears at 3.6% in Exp. 6 (Table 3). Oriented XRD analyses of the clay fraction (<2 µm) identify kaolinite, smectite, and illite (Table 3, Figure 6B,C). In Exp. 5, the primary smectite peak (001) is detected at a d-spacing of 14.9 Å in the air-dried analysis, shifting to 16.99 Å after ethylene glycol treatment, with a 002 peak at 8.5 Å. The 001 peak disappears after heating to 550 °C for 1 h (Figure 6B). Smectite is absent in Exp. 6 (Figure 6C). Kaolinite’s 001 and 002 peaks at 7.15 Å and 3.57 Å, respectively, are observed in both air-dried and ethylene glycol-treated samples but disappear after heating in Exp. 5. In Exp. 6, kaolinite peaks appear weaker compared to Exp. 5 (Figure 6C). Illite’s 001, 002, and 003 peaks are identified at 10.01 Å, 4.99 Å, and 3.07 Å, respectively, and are present in both untreated and treated analyses (Figure 6B,C). Semi-quantitative analyses of the clay fraction reveal that kaolinite decreases with increasing temperatures, while smectite fluctuates irregularly (Table 3). Kaolinite declines from 50% in Exp. 1 to 18% in Exp. 5 and 20% in Exp. 6. Smectite decreases slightly in the low-temperature experiments, from 5% in Exp. 1 to 4.9% in Exp. 2, before increasing to 7.2% in Exp. 3. It subsequently declines to 3%, 2%, and trace amounts in Experiments 4, 5, and 6, respectively. In contrast, illite increases significantly, from 40% in Exp. 1 to 76% and 80% in Experiments 5 and 6, respectively (Table 3). Minor silt-sized quartz is also observed in the clay fraction, and sylvite peaks appear in Exp. 6 (Figure 6C).

SEM and EDS analyses of low-temperature experiments (Experiments 1 and 2) show minimal changes in detrital grain distribution, with no significant mineralogical transformations observed (Figure 7A). However, SEM reveals the disaggregation and partial dissolution of aggregated detrital grain coats in Exp. 2 (Figure 7A). During Exp. 3 (150 °C), smectite precipitates adjacent to kaolinite platelets, forming incipient crenulated to honeycomb morphologies (Figure 7B). These newly formed smectite coatings develop further in Exp. 4 (200 °C), displaying improved internal structures and crystallinity (Figure 7C). Illitization of smectite begins in Exp. 4 and progresses into Exp. 5 (250 °C). SEM shows the replacement of smectite by mixed-layer illite/smectite and eventually illite, which retains the honeycomb morphologies of smectite but develops spiny terminations characteristic of illite (Figure 7D,E). EDS confirms that the newly formed illite is dominated by Si, O, Al, and K, with minor Fe (Figure 7F). STEM analyses further identify pseudohexagonal kaolinite, smectite, and illite in Exp. 5 (Figure 8).

In Exp. 6, SEM and EDS analyses reveal similar transformations to those observed in Exp. 5. SEM observations show the dissolution of kaolinite and the formation of sylvite adjacent to mixed-layer illite/smectite (Figure 9A,B). Newly formed illite from smectite displays rugged honeycomb morphologies, while illitized kaolinite exhibits flaky to radial morphologies with hairy spikes (Figure 9C). EDS analysis confirms that illite is dominated by Si, Al, O, K, and minor amounts of Fe and Ca (Figure 9D).

4.4. Mg-Rich Synthetic Fluid-Based Experiments (Experiments 7–10)

Experiments 7, 8, 9, and 10 were conducted at temperatures of 100, 150, 200, and 250 °C, respectively, using a 0.1 M NaCl, KCl, CaCl₂·2H₂O, and MgCl₂·6H₂O synthetic solution (SF2) for 14 days each. Results from these experiments highlight some differences in mineralogical and chemical transformations, compared to seawater-based experiments. XRF analysis revealed that SiO₂ content increased from 55.08 wt% in Exp. 7 to 58.00 wt% in Exp. 8, slightly decreased in Exp. 9, and rose again in Exp. 10 (Table 2, Figure 3). Al₂O₃ concentrations exhibited a minor decrease in Exp. 8, remained relatively stable in Exp. 9, and increased in Exp. 10 (Table 2). CaO concentrations significantly decreased with increasing experimental temperatures, from 9.15 wt% in Exp. 7 to 1.39 wt% in Exp. 10. Notably, both Fe₂O₃ and MgO concentrations increased with rising experimental temperatures, with MgO rising from 2.64 wt% in Exp. 7 to 5.06 wt% in Exp. 10 (Table 2). The concentrations of Na₂O and K₂O remained relatively unchanged across the experiments (Table 2).

XRD analysis of the bulk mineralogy identified quartz and feldspars (orthoclase and plagioclase) as the dominant minerals in all experiments. Quartz concentrations fluctuated slightly, decreasing in Exp. 10 after an increase in Exp. 9 (Table 3). Orthoclase and plagioclase feldspars decreased with increasing temperature (Table 3). Calcite and muscovite were also present but decreased in concentration with rising temperatures from Exp. 7 to 10 (Table 3). The total clay mineral content in the bulk mineralogy increased progressively, rising from 22.9% in Exp. 7 to 46.7% in Exp. 10 (Table 3). Kaolinite, illite, and chlorite were detected in oriented clay fraction XRD analyses (Figure 6D). In Exp. 10, kaolinite exhibited characteristic peaks at 7.15 Å (001) and 3.57 Å (002) in air-dried and ethylene glycol-treated samples, which disappeared upon heating (Figure 6D). Illite peaks were consistent across all untreated and treated analyses (Figure 6D). Smectite was present in minor to trace amounts and decreases with increasing temperature, with only a weak peak observed at 3.24 Å in Exp. 10 (Figure 6D). A distinctive feature of these experiments was the authigenic formation of chlorite, which began in Exp. 8 and increased with temperature. Chlorite exhibited a primary (001) peak at 13.99 Å (Figure 6D), with low-intensity peaks identified in both air-dried and treated samples. Semi-quantitative analysis showed that kaolinite concentration in the clay fraction decreased with increasing temperature, while smectite decreased from 2% in Exp. 7 to trace amounts in Exp. 10. Illite concentration increased in Experiments 8 and 9 before slightly declining in Exp. 10 (Table 3). Chlorite concentrations consistently increased, rising from 4% in Exp. 7 to 16% in Exp. 10 (Table 3).

The pH and TDS of the pre-experiment solution (SF2) were measured at 5.71 and 45.33 ppt, respectively (Table 4). Post-experiment, the pH increased to 6.22 in Exp. 7 and remained stable throughout the experiments, while TDS rose from 56.15 ppt in Exp. 7 to 62.29 ppt in Exp. 10 (Table 4). ICP-OES analysis of the reacting fluid showed that Al and Fe, initially absent in the pre-reacted solution, were detected post-experiment, with concentrations increasing with temperature (Table 4). Ca concentration increased substantially, reaching 48,400 mg/L in Exp. 10 from an initial 8730 mg/L. Conversely, Mg was consumed, decreasing from 15,500 mg/L in the starting solution to 167 mg/L in Exp. 10 (Table 4, Figure 10). Na concentrations decreased during the experiments, while K showed irregular fluctuations, increasing in Experiments 7 and 8 but decreasing in Experiments 9 and 10 (Table 4, Figure 10).

SEM analysis revealed the onset of detrital aggregate dissolution without new mineral formation in Exp. 7 (Figure 11A). However, evidence of smectite transformation was observed in Exp. 8 (Figure 11B). Mixed-layer chlorite/smectite with a rugged, honeycomb morphology appeared in Exp. 9, adjacent to kaolinite platelets (Figure 11C). Additionally, abundant microcrystalline quartz aggregates developed alongside the transforming mixed-layer chlorite/smectite in Exp. 9 (Figure 11D). Fully developed chlorite with a mixed honeycomb-to-rosette morphology was observed in Exp. 10 (Figure 11E), suggesting partial replacement of smectite while retaining its morphology. EDS analysis identified Si, O, Al, Mg, and Fe as the major elements in the newly formed chlorite, with minor amounts of Na, K, and Ca (Figure 11F).

4.5. Estuarine Water-Based Experiment (Experiment 11)

This experiment was conducted at 250 °C for 14 days using natural estuarine water collected from the Gironde estuary, SW France. Comprehensive geochemical and mineralogical analyses were performed on both the solids and solutions before and after the hydrothermal reaction. XRF, XRD, and SEM analyses revealed that all authigenic clay minerals formed in Exp. 10 were also present in Exp. 11, albeit with minor compositional differences (Figure 3, Figure 6, and Figure 12). The oxides identified in the XRF analysis exhibited similar trends to those observed in Exp. 10. The key distinction lies in the amounts of Fe₂O₃ and MgO (Table 2). While MgO decreased slightly in Exp. 11 (8.23 wt%) compared to Exp. 10 (9.58 wt%), Fe₂O₃ increased significantly to 7.62 wt% from 5.06 wt% (Table 2). Additionally, concentrations of other minor and trace elements remained largely unchanged (Table 2, Figure 3). All minerals identified in the bulk and clay fraction XRD analyses of Exp. 10 were also detected in Exp. 11, with some variation in concentration (Table 3, Figure 4 and Figure 6). Notably, the concentration of authigenic chlorite increased from 16% in Exp. 10 to 24% in Exp. 11, as quantified from the clay fraction XRD analyses (Table 3, Figure 6E).

The pH of the pre-experimental estuarine water was 7.26 but slightly decreased to 6.91 after the hydrothermal experiment (Table 4). In contrast, the TDS increased from 13.02 ppt to 26.39 ppt post experiment (Table 4). ICP-OES analysis indicated an increase in Al, Ca, and K concentrations in the post-experimental reacting fluid, while Fe was significantly depleted (Table 4). Mg and Na concentrations decreased after the hydrothermal reaction (Table 4, Figure 10).

SEM and STEM analyses provided evidence of authigenic chlorite formation and mixed-layer clays. Small acicular crystals of Fe-oxides (Figure 12A) were observed adjacent to kaolinite (Figure 12B). Moreover, SEM analysis revealed that chlorite in Exp. 11 formed by replacing smectite through mixed-layer stages (Figure 12C–E). The mixed-layer chlorite/smectite exhibited honeycomb to web-like morphologies and was found adjacent to newly formed small aggregates of microcrystalline quartz (Figure 12C). Fully developed authigenic chlorite exhibited rosette morphologies (Figure 12D,E). Elemental analysis of the authigenic chlorite showed Si, O, Al, and Fe as major constituents, with minor amounts of Mg, K, and Ca. This composition differed slightly from the chlorite observed in Exp. 10, which was richer in Mg than Fe (Figure 11F). Furthermore, STEM analysis revealed elongated chlorite crystals (Figure 8E,F) closely associated with mixed-layer chlorite/smectite (Figure 8E).

4.6. Na-Rich Synthetic Fluid-Based Experiment (Experiment 12)

This experiment utilized a highly alkaline 0.1 M synthetic Na₂CO₃ solution at 250 °C for 14 days. Post-reaction XRF analysis of the sediment indicated a substantial increase in Na₂O to 5.46 wt% from 0.79 wt% in the starting material (Table 2). This concentration was significantly higher than in all previous experiments (Table 2, Figure 3). While K₂O concentrations increased in most prior experiments, they decreased to 2.88 wt% in Exp. 12 from 3.07 wt% in the starting material. Concentrations of CaO, MgO, Fe₂O₃, and other trace oxides remained relatively stable compared to the starting material (Table 2, Figure 3).

The bulk mineralogical analysis of Exp. 12 showed key differences compared to the starting material, including a decrease in total clay minerals and the presence of analcime (zeolite) unique to this experiment (Table 3, Figure 4). The concentration of clay minerals decreased to 13.1% from 20.3% in the starting material, contrasting with prior experiments, where clay mineral concentrations increased with rising temperature (Table 3). A high-intensity broad peak of authigenic analcime was identified (Figure 4), accounting for 21.7% of the total post-experimental bulk mineralogy (Table 3). This mineral was absent in all previous experiments (Figure 4). Additionally, halite was identified, with a measured concentration of 2.4% (Table 3, Figure 4). Clay fraction XRD analysis revealed kaolinite, illite, and smectite as the primary clay minerals (Figure 6F). Kaolinite concentration decreased to 16%, smectite increased to 40%, and illite remained relatively unchanged at 41% compared to the starting material (Table 3).

The pre-experimental solution had a high pH of 11.32 and a TDS of 13.79 ppt. After the experiment, the pH and TDS decreased to 8.31 and 7.79 ppt, respectively (Table 4). Sodium (Na), the dominant cation in the starting solution, was significantly depleted from 8250 mg/L to 390 mg/L after the reaction (Table 4, Figure 10). Furthermore, ICP-OES analysis revealed that Mg and Fe dissolved during the experiment and were detected in the reactive fluid post experiment (Table 4).

SEM analysis showed no evidence of structurally developed illite or chlorite; however, detrital clay aggregates in the starting material exhibited signs of disaggregation, marking the initial stages of dissolution and/or transformation (Figure 13A, B). Interestingly, SEM analysis confirmed the formation of halite and analcime (zeolite) (Figure 13C–E). Fully developed halite occurred as clear cubic crystals (Figure 13C), while analcime exhibited massive aggregates (Figure 13D) and acicular to botryoidal morphologies (Figure 13E). The massive analcime aggregates formed within dissolving kaolinite (Figure 13D) and transitioned to acicular or botryoidal morphologies when fully developed (Figure 13E). Elemental analysis (EDS) of analcime identified Si, O, Na, and Al as its primary constituents (Figure 13F).

4.7. FIB-SEM Observations

The morphology, composition, distribution, and spatial relationships between framework grains and authigenic clay minerals were further investigated using FIB-SEM and EDS analyses (Figure 14A–F). The 2D internal structure analysis of the post-reacted samples revealed that authigenic clay minerals occurred both as pore-filling phases (Figure 14A–C) and as grain coatings (Figure 14E). Highly microporous pore-filling clay minerals were observed cementing detrital grains in Exp. 3 (Figure 14A,B) but appeared to become less microporous in Exp. 5 (Figure 14C). Additionally, quartz overgrowths were observed surrounding detrital quartz grains where authigenic grain-coating clays were absent (Figure 14C). EDS analysis confirmed that the quartz overgrowth consists of Si and O (Figure 14D). Grain-coating chlorite was identified surrounding detrital K-feldspar grains in Exp. 11 (Figure 14E). Elemental analysis via EDS indicated that chlorite was composed predominantly of O, Si, Al, and Fe, with minor amounts of Mg, K, and Ni (Figure 14F).

5. Discussion

5.1. Controls and Mechanisms of Authigenic Illite Formation

Illite is one of the primary clay minerals synthesized in the present study (Figure 6, Figure 7, Figure 8 and Figure 9), predominantly from seawater-based experiments (Experiments 1–6). These experiments resulted in smectite’s transformation into illite, a common diagenetic pathway observed in many natural sedimentary environments [16,56,57,58]. The formation of authigenic illite in siliciclastic rocks occurs due to the progressive transformation of precursor minerals such as K-feldspar, kaolinite, or smectite, beginning at low temperatures during burial and continuing under elevated temperatures and pressures [22,59]. This study has demonstrated the illitization of authigenic smectite to varying extents through mixed-layer illite/smectite stages, as evidenced by XRD (Table 3, Figure 6), SEM (Figure 7 and Figure 9), and STEM (Figure 8A–D) analyses. The authigenic smectite developed from the dissolution of unstable detrital materials, including orthoclase, calcite, and detrital clay aggregates. This process began at lower-temperature experiments (Experiments 1–2), which provided the necessary silica and cations for smectite neoformation (Table 3, Figure 7A). Consequently, well-crystallized authigenic smectite developed at 150 °C (Exp. 3, Figure 7B). With increasing temperature during progressive burial in natural sedimentary systems, smectite becomes unstable and transforms into illite through mixed-layer stages [14,60].

The smectite-to-illite transformation is influenced by temperature, fluid composition, and the availability of K⁺ [61,62,63,64,65]. In the present study, this transformation began at 150 °C through the formation of mixed-layer illite/smectite (Figure 7B,C) and reached a plateau at 250 °C (Figure 7E and Figure 8C). The required K⁺ was supplied through multiple sources, including the dissolution of K-feldspar and the K⁺ present in the experimental solutions. This conclusion is supported by the observed increase in K concentration in the reaction fluid during Experiments 1–2 (Table 4), which corresponds to the dissolution of detrital orthoclase (Table 3) and the subsequent rapid depletion of K⁺ during Experiments 4–6, coinciding with the onset of the illitization process (Table 4; Figure 7, Figure 8 and Figure 9). Additionally, the illitization of smectite has been experimentally documented under varied pH conditions, ranging from acidic [66,67] to neutral [43,64,68] and low alkaline [62,69] conditions. Experimental studies by [62,69] have shown that near neutral to high alkaline pH (7–11) increases Si and Al mobility, thereby enhancing illite development. The illite formed in the KCl-rich seawater experiment (Exp. 6) is more developed (Figure 9C) and constitutes over 80% of the total clay fraction (Table 3) compared to the illite formed in Exp. 5, which was conducted using seawater under the same temperature (250 °C). One plausible explanation for this variation in transformation rate is the higher K⁺ availability in Exp. 6 compared to Exp. 5 (Table 4). Experimental studies by [63] have shown that the rate of smectite-to-illite transformation strongly depends on the concentration of K⁺ in the reacting fluid. Experiments conducted under high K⁺ concentrations yield more structurally developed illite from smectite [70], consistent with the observations in this study. Furthermore, the honeycomb and lath-like morphologies of the synthesized smectite and illite closely resemble those observed in natural sandstones [16,56,71].

The mechanisms responsible for the smectite-to-illite transformation have been extensively debated over the past decades and are reviewed by [64,72]. The two most widely accepted mechanisms are dissolution-crystallization (DC) [64,73,74] and solid-state transformation (SST) [75,76]. In the seawater experiments (Experiments 4–5), the illitized smectite mostly retained the honeycomb morphology of smectite, characterized by curling illite flakes tangential to detrital grains (Figure 7C). This characteristic is commonly attributed to SST [64]. However, spikes of illite flakes were observed adjacent to poorly organized illite/smectite with a ragged honeycomb morphology (Figure 7D,E), suggesting the early dissolution of honeycomb smectite and its subsequent illitization. Additionally, studies by [64,77] have shown that precursor minerals can retain their morphology under DC transformation. Furthermore, the illite developed in KCl-rich seawater (Exp. 6) is characterized by lath-like morphology, indicating the dissolution of smectite and the precipitation of illite. This observation aligns with experimental studies by [65,78], which reported the formation of lath-like illite from smectite dissolution under similar temperature and fluid–rock ratios. Moreover, DC has been identified as the primary mechanism of smectite-to-illite transformation in natural fluid-dominated hydrothermal systems [79,80,81], based on observations of morphological changes and chemical variations, consistent with the findings of this study. Therefore, the smectite-to-illite transformation observed in this study (Experiments 4–6) is most likely due to DC rather than SST.

5.2. Authigenic Formation of Chlorite

As demonstrated in this study, the authigenic formation of chlorite is a key feature observed in experiments conducted with Mg-rich synthetic fluid (SF2; Experiments 7–10) and natural estuarine water (Experiment 11) (Figure 11 and Figure 12). The findings indicate that chlorite formed through the transformation of smectite, proceeding via intermediate mixed-layer chlorite/smectite (C/S) phases, as evidenced by SEM (Figure 11 and Figure 12), STEM (Figure 8E,F), and XRD (Figure 6D,E) analyses. In natural sedimentary systems, the chloritization of smectite is predominantly reported to occur through dissolution-crystallization (DC; [22,82,83], with occasional occurrences of solid-state transformation (SST; [84]. Additionally, temperature, fluid composition, and fluid–rock ratios are critical in facilitating the smectite-to-chlorite transformation [40,84,85].

During the DC mechanism, chlorite forms from the chemical components released during the dissolution of smectite, including silica, aluminum, and interlayer cations, while incorporating Fe²⁺ and Mg²⁺ supplied either by detrital minerals (e.g., biotite or volcanic fragments) or pore fluids [39]. In this study, the chlorite synthesized in Exp. 11 exhibits a well-developed rosette morphology (Figure 12E) that is closely associated with dissolved, honeycomb-like smectite through interstratified C/S phases (Figure 8E and Figure 12C), characteristic of the DC transformation. In addition, although the chlorite synthesized in Exp. 10 retains the honeycomb morphology of its precursor smectite (Figure 11E) and exhibits interstratified C/S phases (Figure 11C), the C/S is assumed to be newly formed through DC. The required Fe and Mg for the synthesis of chlorite in this study are provided by the experimental solutions, in addition to the dissolution of newly formed Fe-oxide in Exp. 11 (Figure 12A). Thus, our results suggest that chlorite formation predominantly occurs via the DC mechanism.

The chlorites formed in this study exhibit morphology comparable to both experimentally synthesized chlorites and natural authigenic chlorites found in ancient sandstone reservoirs. For instance, honeycomb and rosette morphologies have been reported in experimental studies by [52] and in the Triassic Yanchang Formation of the Ordos Basin, China [86], as well as in numerous natural sandstone reservoirs [18,59,71]. Furthermore, the release of Si and Al into the experimental solution during smectite transformation appears to have contributed significantly to the formation of by-products such as microcrystalline quartz (Figure 11D and Figure 12C–E) and quartz cement (Figure 14C).

5.3. Role of Fluid Chemistry in the Transformation Pathways of Modern Estuarine Sediments

The chemistry of pore fluids during fluid–rock interactions is critical in determining the pathways and extent of diagenetic alteration of detrital clay minerals in modern sediments [34]. Variations in pH, redox potential, salinity, and dissolved ion concentrations influence the mechanisms of mineral transformation and the resulting mineral phases [87,88,89]. These variations are particularly significant in dynamic environments such as estuaries, where fluid composition fluctuates due to freshwater–seawater interactions and organic matter degradation [34]. Additionally, the fluid composition used in hydrothermal experiments is crucial in shaping the mineralogy and chemistry of the synthesized minerals [42,83]. This study utilized five distinct fluids of variable composition, resulting in the formation of distinct mineral phases.

Both SW and SF1 facilitated the authigenic formation of smectite from detrital clay precursors and its subsequent transformation into illite. The illitization of smectite was more pronounced in Experiments 5 and 6, conducted at 250 °C. Nonetheless, slight differences were observed in the smectite illitization rate and the resulting illite’s chemistry. The most significant change in the experimental alteration was an increase in potassium (K) in the interlayer sites, accompanied by a decrease in sodium (Na) and calcium (Ca) (Figure 8D and Figure 9D). The XRD peaks of the authigenic illite were broader in Exp. 6 (Figure 6C) than in Exp. 5 (Figure 6B). Additionally, XRD patterns indicated the presence of smectite alongside illite in Exp. 5 (Figure 6B). In contrast, smectite was undetectable in Exp. 6 (Figure 6C). Sylvite was also formed in Exp. 6 (Table 3, Figure 6C). A plausible explanation for these variations is the higher concentration of K in Exp. 6 compared to Exp. 5, which accelerated the rate of smectite illitization. Kinetic modeling of smectite illitization by [70] demonstrated that elevated K concentrations in pore fluids enhance illitization and accelerate the reaction rate. The slight decrease in pH observed in the experiments is attributed to the release of silica and Fe (Table 4) during the dissolution of detrital clay minerals and the transformation of smectite into illite.

Distinctively, the Mg-rich synthetic fluid (SF2) and the estuarine water (EW) resulted in the formation of chlorite from smectite (Figure 11 and Figure 12). However, slight differences were observed in the chemistries of the authigenic chlorites. Chlorites formed from SF2, enriched in Mg, Ca, K, and Na, were dominantly Mg-rich (Figure 11F). In contrast, chlorites formed from the natural Gironde estuarine water, which contains higher Fe and lower Mg concentrations than SF2, were Fe-rich chlorites (Figure 12F). This observation can be attributed to the relative scarcity of Mg in estuarine water, which forces the chlorites to incorporate more Fe into their structures to maintain charge balance and structural integrity. This is supported by the rapid depletion of Mg in SF2 (Experiments 7–10), which was relatively higher than in Exp. 11 (EW), and the consumption of Fe in Exp. 11 (Table 4). These findings align with the preferential formation of authigenic chlorite in ancient sedimentary environments. In natural sandstone reservoirs, Mg-rich chlorites are commonly associated with continental sandstones, whereas Fe-rich chlorites are prevalent in marginal marine environments such as deltas and estuaries [18,39].

In contrast to the formation of illite and chlorite in Experiments 1–11, Exp. 12, conducted using highly alkaline Na-rich synthetic solutions (SF3), resulted in the formation of halite and analcime (zeolite) that were not observed in the previous experiments (Figure 13). Zeolite minerals are generally rare in sandstones [59,71]. However, where present, they exert significant control on reservoir quality [90,91]. Analcime is one of the most common zeolites and is an important component of many petroliferous reservoirs worldwide [71,92,93]. Although authigenic analcime has been reported to occur from clay mineral precursors [94], its formation in sandstones is commonly associated with the transformation of clinoptilolite from volcanic glass during burial [90,95]. The formation of analcime typically occurs under highly alkaline conditions [96] and is facilitated by the availability of silica, as well as Na [91]. In this study, analcime was formed from the dissolution of detrital clay minerals, which provided the required silica, whereas Na was supplied from the dissolution of calcite and the experimental fluid, respectively (Table 4). The availability of Na and its concentration in pore fluids is essential for analcime stability and its preferential formation over other zeolites [97]. In this study, analcime developed concurrently with halite (Figure 13C).

Despite the use of the same starting material in this study, different minerals were synthesized under varying experimental conditions. These variations are directly associated with the differences in fluid chemistry employed in the experiments. These findings underscore the complex interplay between fluid chemistry and diagenetic processes in modern estuarine sediments.

5.4. Relevance of Hydrothermal Experiments to Natural Sandstone Diagenesis

Although the experimental conditions in this study were designed to simulate natural diagenetic processes, they inevitably represent simplified models of natural sandstone diagenesis. One key limitation is the time scale: natural diagenesis occurs over geologic timescale (millions of years), allowing for gradual reactions influenced by fluctuating temperature, pressure, and fluid composition. In contrast, laboratory experiments accelerate these processes by using elevated temperatures to condense reactions into hours or days. Furthermore, the use of uniform starting materials and static fluid compositions restricts the ability to replicate the inherent heterogeneity of natural systems, where pore-fluid chemistry evolves with burial depth, sediment provenance, and organic matter degradation.

Despite these limitations, hydrothermal experiments remain valuable proxies for investigating diagenetic processes in natural sandstone reservoirs, as they enable the controlled simulation of mineralogical and geochemical transformations. The findings of this study demonstrate strong alignment with key processes observed in natural systems. For instance, the illitization of smectite in potassium-rich fluids (e.g., SF1) closely mirrors potassium-driven transformations documented in sandstone reservoirs [7,22,98]. Similarly, the formation of Mg-rich and Fe-rich chlorites in experiments using SF2 and EW aligns with chlorite development in continental and marginal marine environments, respectively. In deeply buried sandstone reservoirs, Fe-rich chlorites are commonly associated with marginal marine settings, such as estuaries and deltas, where Fe-rich flocculated materials are present [10,18,39]. This observation agrees with the Fe-rich chlorites formed from the natural Gironde estuarine water used in this study.

Another significant finding is the synthesis of analcime from detrital clays under highly alkaline, Na-rich conditions, which broadens the understanding of analcime formation beyond its conventional association with volcanic glass [90]. By employing diverse fluid compositions, including natural seawater, estuarine water, and synthetic solutions with varied ionic concentrations and pH levels, this study provides a comprehensive perspective on how fluid chemistry governs diagenetic pathways. These results emphasize the relevance of hydrothermal experiments in mimicking the geochemical processes that drive the diagenetic transformation of detrital clay minerals during burial. Furthermore, they highlight the potential of modern estuarine sediments as analogs for diagenetic processes, underscoring the critical role of dynamic fluid compositions in shaping mineralogical transformations and reservoir quality.

To deepen the understanding of the impact of fluid chemistry on the diagenetic pathways of modern estuarine sediments, future research should focus on longer experimental durations and the incorporation of dynamic fluid flow to reflect the evolving nature of natural sandstone diagenesis. In addition, conducting geochemical modeling using data from experimental studies would enhance the extrapolation of laboratory findings to geological time scales. While these areas of improvement are necessary, the findings of this study provide a solid foundation for understanding diagenetic alteration in modern estuarine sediments and their implications for sandstone reservoir evolution.

6. Conclusions

• During the hydrothermal experiments, the dissolution of detrital feldspars and clay aggregates occurred at temperatures below 100 °C, followed by the authigenic formation of smectite at 150 °C.
• At higher temperatures (150 °C to 250 °C), K-rich fluids (SW, SF1) facilitated the illitization of smectite, replicating potassium-driven processes observed in natural reservoirs. This transformation predominantly occurred via dissolution-crystallization, with the required K⁺ supplied by the dissolution of K-feldspar and the experimental solutions.
• The chloritization of smectite occurred in both Mg-rich synthetic fluids (SF2) and natural Gironde estuary water (EW) at 250 °C, producing chlorites with distinct chemistries. SF2 yielded Mg-rich chlorites, while EW produced Fe-rich chlorites, consistent with diagenetic trends in marginal marine environments, where Fe-rich chlorites are associated with estuarine and deltaic systems.
• The formation of authigenic chlorites involved a combination of dissolution-crystallization and solid-state transformation. The transformation of smectite into chlorite released abundant silica into the experimental solutions, resulting in the precipitation of microcrystalline quartz and quartz overgrowths.
• Analcime (zeolite) was synthesized under highly alkaline (pH 11.32), Na-rich conditions (SF3), advancing the understanding of zeolite formation beyond its traditional association with volcanic glass precursors.
• The synthesized mineral phases exhibited chemical and morphological similarities to those found in natural sedimentary systems, highlighting the critical role of fluid chemistry in influencing diagenetic pathways and improving our understanding of detrital clay transformations during burial diagenesis