Introduction
Glauconite is a widespread authigenic mineral in sedimentary successions. It forms predominantly in marine environments under low sedimentation rates and reducing conditions (Odin and Matter, Reference Odin and Matter1981; Amorosi, Reference Amorosi1997; Banerjee et al., Reference Banerjee, Bansal and Vilas Thorat2016). In sedimentary rocks, glauconite most commonly occurs as globular (granular) particles with typical diameters of 50–400 μm (Banerjee et al., Reference Banerjee, Bansal and Vilas Thorat2016). Natural glauconite granules (globules) are characterized by multi-scale porosity, an aggregated internal structure, and pronounced crystal-chemical heterogeneity (Drits et al., 2010; Baldermann et al., Reference Baldermann, Grathoff and Nickel2012; Rudmin et al., Reference Rudmin, Banerjee, Makarov, Mazurov, Ruban, Oskina, Tolkachev, Buyakov and Shaldybin2019a; López-Quirós et al., Reference López-Quirós, Sánchez-Navas, Nieto and Escutia2020). Glauconite belongs to the Fe-K-bearing dioctahedral phyllosilicates of the illite–glauconite series. These minerals are characterized by elevated Fe contents and high interlayer K, typically in the range of ~5–9 wt.% K2O. They commonly contain defect-rich and mixed-layer components (Rieder et al., Reference Rieder, Cavazzini, D’yakonov, Frank-Kamenetskii, Gottardi, Guggenheim, Koval’, Müller, Neiva, Radoslovich, Robert, Sassi, Takeda, Weiss and Wones1988; Drits et al., Reference Drits, Dainyak, Muller, Besson and Manceau1997). In geology, glauconite plays a specific role in absolute geochronology, most notably in K-Ar, 40Ar-39Ar, and Rb-Sr dating applications (Derkowski et al., Reference Derkowski, Środoń, Franus, Uhlík, Banaś, Zieliński, Čaplovičová and Franus2009; Solé, Reference Solé2021; Redaa et al., Reference Redaa, Farkaš, Gilbert, Collins, Löhr, Vasegh, Forster, Blades, Zack, Giuliani, Maas, Baldermann, Dietzel and Garbe-Schönberg2022; Zaitseva et al., Reference Zaitseva, Golubkova, Kuznetsov, Kuznetsov, Romanyuk, Dovzhikova and Kaurova2024).
Globular glauconite is prevalent in sandy-silty and Fe-rich sediments of different geological ages (McRae, Reference McRae1972; Van Houten and Purucker, 1984; Bansal et al., Reference Bansal, Banerjee and Nagendra2020), as well as in modern deposits (Logvinenko, Reference Logvinenko1982; Baldermann et al., Reference Baldermann, Warr, Letofsky-Papst and Mavromatis2015; López-Quirós et al., Reference López-Quirós, Lobo, Mendes and Nieto2023). This widespread occurrence enforces particular engineering-geological constraints (Westgate et al., Reference Westgate, DeGroot, McMullin, Zou, Guo, Van Haren, Beemer, Zeppilli, Miller and Browning2023). Along the Atlantic coast of the United States, glauconitic sands form laterally extensive horizons in coastal and shelf environments (Koschik and Liu, Reference Koschik and Liu2025). Engineering-geological studies indicate that glauconite globules are highly susceptible to crushing and deterioration under mechanical loads, such as those encountered during piling and drilling operations, particularly in wind turbine construction (Yuanjing et al., 2025). The destruction of globules results in a transition from sand-like to clay-like behavior, leading to abrupt changes in the ground’s engineering properties. Therefore, glauconite globules should be considered a mechanically sensitive component of glauconite-bearing sediments. Their influence on bulk sediment properties depends on glauconite abundance, grain-size distribution, packing, and the response of globules to mechanical loading.
Glauconite commonly occurs within quartz-rich or quartz-feldspathic sandstones and phosphorites (Jimenez-Millan et al., Reference Jimenez-Millan, Molina, Nieto, Nieto and Ruiz-Ortiz1998; Coles et al., Reference Coles, Wright and Sinclair2002; Laouar et al., Reference Laouar, Laouar, Bruguier, Bosch, Kechiched, Bouhlel and Tlili2024). This geological association simplifies beneficiation and processing because glauconite commonly occurs as discrete globules that can be separated from the host sandy or phosphatic matrix by size, density, or magnetic methods (Rudmin et al., Reference Rudmin, Banerjee, Mazurov, Makarov and Martemyanov2017; Rudmin et al., Reference Rudmin, Mazurov, Kapanov, Soktoev and Buyakov2018b). In phosphorite-bearing rocks and mining wastes, the association of glauconite with phosphate minerals can produce a combined K-P mineral concentrate during beneficiation, which may be used as an integrated fertilizer material (Kalinina et al., Reference Kalinina, Maximov, Makarov, Dasi and Rudmin2023). In recent years, glauconite globules have been studied increasingly as natural precursors for mineral materials with sorption, filtration, soil-amendment, and controlled-release functions (Hamed and Abdelhafez, Reference Hamed and Abdelhafez2020; Rudmin et al., Reference Rudmin, Banerjee, Yakich, Tabakaev, Ibraeva, Buyakov, Soktoev and Ruban2020; Turki et al., Reference Turki, Frini-Srasra and Srasra2022; Rudmin et al., Reference Rudmin, Makarov, López-Quirós, Maximov, Lokteva, Ibraeva, Kurovsky, Gummer and Ruban2023b; Dasi et al., Reference Dasi, Rudmin and Banerjee2024; Khitrin et al., Reference Khitrin, Maximov, Dasi, Ibraeva, Ponomarev, Maximova, Belousov, Ruban and Rudmin2024).
The sorption, ion-exchange, and buffering functions of glauconite globules are controlled by several structural and textural features. Globular-scaly morphology and a developed pore network, including micro-, meso-, and macropores, increase the available reactive surface (Ivanovskaya et al., Reference Ivanovskaya, Sakharov and Zaitseva2023; Rudmin et al., Reference Rudmin, Maximov, Dasi, Kurovsky, Gummer, Ibraeva, Kutugin, Soktoev, Ponomarev, Tararushkin, Makarov and Ruban2023a; Simakova, Reference Simakova2023). The layered 2:1 structure with interlayer K provides ion-exchange and nutrient-release capacity (Kossovskaya and Drits, Reference Kossovskaya and Drits1970; Meunier and El Albani, Reference Meunier and El Albani2007; Zviagina et al., 2017; Nieto et al., Reference Nieto, Abad, Bauluz and Reolid2021). This structure is associated with variable isomorphic substitutions and corresponding changes in residual charge on crystal planes and edges (Drits et al., Reference Drits, Dainyak, Muller, Besson and Manceau1997). Glauconitic materials commonly exhibit cation-exchange capacities of 15–50 meq 100 g–1 (Thompson and Hower, Reference Thompson and Hower1975; Franus et al., Reference Franus, Bandura and Madej2019) and specific surface areas of 24–70 m2 g–1 in natural unmodified samples (Fernandez-Bastero et al., Reference Fernandez-Bastero, Gil-Lozano, Briones and Gago-Duport2008). The specific surface area can increase to >130 m2 g–1 after acid activation (Franus et al., Reference Franus, Klinik and Franus2004). These properties make glauconites effective sorbents for a broad spectrum of contaminants. These include heavy metals (Franus and Bandura, Reference Franus and Bandura2014; Franus et al., Reference Franus, Bandura and Madej2019; Martemianov et al., Reference Martemianov, Plotnikov, Rudmin, Tyabayev, Artamonov and Kundu2020), radioactive elements (Semenishchev et al., Reference Semenishchev, Ryabukhina, Voronina, Mashkovtsev and Nikiforov2015; Semenkova et al., Reference Semenkova, Belousov, Rzhevskaia, Izosimova, Maslakov, Tolpeshta, Romanchuk and Krupskaya2020; Bruneel et al., Reference Bruneel, Van Laer, Brassinnes and Smolders2021), petroleum hydrocarbons (Khopyak et al., Reference Khopyak, Tkachenko and Manenko2016; Belousov et al., Reference Belousov, Gorbunova, Kim, Niftaliev, Krupskaya, Rudmin, Koroleva and Rumyantseva2024), and organic compounds (Hao et al., Reference Hao, Tsai and Huang1987; Abukhadra et al., Reference Abukhadra, Fadl Allah, Shaban, Alenazi, Alqhtani, Bin-Jumah, Allam and Bellucci2024).
In agrochemical applications, glauconite globules are considered a natural source of K, usually containing 6–10 wt.% K2O. They also provide essential micronutrients, including Fe, Mg, Mn, Zn, and Cu. In addition, glauconite globules are used as a mineral matrix for the development of slow-release or controlled-release fertilizers (Castro and Tourn, Reference Castro and Tourn2003; Santos et al., Reference Santos, Mattiello, da Costa, Abrahão, de Novais and Cantarutti2015; Rudmin et al., Reference Rudmin, Oskina, Banerjee, Mazurov, Soktoev and Shaldybin2018a; Rudmin et al., Reference Rudmin, Banerjee, Makarov, Belousov, Kurovsky, Ibraeva and Buyakov2022; Dasi et al., Reference Dasi, Rudmin and Banerjee2024). Experimental studies have shown that modified glauconite-based materials can provide a gradual release of nutrients over weeks to months. This behavior reduces nutrient losses due to leaching and lowers phytotoxic effects (Rudmin et al., Reference Rudmin, Banerjee, Yakich, Tabakaev, Ibraeva, Buyakov, Soktoev and Ruban2020; Rudmin et al., Reference Rudmin, Makarov, López-Quirós, Maximov, Lokteva, Ibraeva, Kurovsky, Gummer and Ruban2023b; Khitrin et al., Reference Khitrin, Maximov, Dasi, Ibraeva, Ponomarev, Maximova, Belousov, Ruban and Rudmin2024). For glauconite-based nanocomposites, increases in specific surface area and mesoporosity have been demonstrated. These materials also show the formation of internal and external coordination complexes of micronutrients (Tararushkin et al., Reference Tararushkin, Ruban, Bu, Yanfu and Rudmin2026). As a result, a positive agronomic effect has been reported (Rudmin et al., Reference Rudmin, Abdullayev, Ruban, Buyakov and Soktoev2019b, Reference Rudmin, Maximov, Dasi, Kurovsky, Gummer, Ibraeva, Kutugin, Soktoev, Ponomarev, Tararushkin, Makarov and Ruban2023a).
Environmental and engineering applications of glauconite globules include their use in water treatment systems (Hao et al., Reference Hao, Tsai and Huang1987; Ali et al., Reference Ali, Osman, Sayed and Shalabi2015), filtration of drainage and technogenic waters (Spoljaric and Crawford, Reference Spoljaric and Crawford1978), and as mineral components of barrier and stabilization materials (Bruneel et al., Reference Bruneel, Van Laer, Brassinnes and Smolders2020; Bruneel et al., Reference Bruneel, Van Laer, Brassinnes and Smolders2021). Despite the rapid development of applied research, the effects of aqueous pH variations on the structural and chemical state of glauconite globules remain insufficiently constrained (Fernandez-Bastero et al., Reference Fernandez-Bastero, Gil-Lozano, Briones and Gago-Duport2008; Zhang et al., Reference Zhang, Awolayo, Nightingale and Tutolo2022). This limitation is particularly relevant for long-term mineral-solution interactions, during which changes in structure, morphology, and functional properties may occur (Fernandez-Bastero et al., Reference Fernandez-Bastero, Gil-Lozano, Briones and Gago-Duport2008; Heřmanská et al., Reference Heřmanská, Voigt, Marieni, Declercq and Oelkers2023; Weibel et al., Reference Weibel, Rasmussen, Al-Masri, Holmslykke, Fogden, Hillier, Keulen, Olivarius, Didriksen, Stokkendal and Schovsbo2025). The results of a one-year natural experiment conducted in a peat bog environment showed that glauconite exhibits measurable changes even at early stages of contact with an organic-rich environment. These changes include potassium loss, increased structural disorder, and redistribution of elements (Rudmin et al., Reference Rudmin, Bu, Savichev, Khitrin and Ruban2026b). However, natural systems include multiple coupled factors, such as pH, organic matter, redox conditions, microbial activity, and variable solution chemistry (Rudmin et al., Reference Rudmin, Ruban, Deeva and Khitrin2026a). Flow-through experiments provide important constraints on glauconite dissolution, but they represent open systems with continuous solution renewal and enhanced removal of released components (Fernandez-Bastero et al., Reference Fernandez-Bastero, Gil-Lozano, Briones and Gago-Duport2008; Zhang et al., Reference Zhang, Awolayo, Nightingale and Tutolo2022). The response of intact glauconite globules under static aqueous conditions remains less constrained. In such systems, pH cannot be treated as an independent factor because it affects surface charge, interlayer exchange, cation release, and the stability of near-surface domains. Nevertheless, pH can be used as the primary controlled experimental parameter to compare mineral responses across a consistent set of static water-mineral systems.
This study aimed to investigate experimentally the transformation of glauconite globules in aqueous environments through various pH values. Attention was given to identifying structural, chemical, and morphological changes in the mineral. The study considers the evolution of glauconite under controlled laboratory conditions. The results are compared with data from natural systems and activation experiments. A specific feature of this study is the use of intact glauconite globules rather than powdered or strongly activated material. This approach preserves the natural granular morphology of glauconite and relies on time-resolved experimental series under reproducible conditions. The combination of intact globules, controlled pH values, and varying contact times allows us to distinguish bulk structural stability from limited near-surface modification during prolonged mineral-water interaction.
Materials and methods
Material and experimental design
The starting material was natural glauconite from the Karin deposit (Chelyabinsk region, Russia). A concentrate of globular glauconite with a grain-size fraction of 0.05–0.4 mm was used in the experiments. The purified globular glauconite fraction was obtained from glauconite-bearing rock by elutriation in distilled water to remove the clay-size fraction, followed by magnetic separation using an ECM 10/5 electromagnetic separator (Russia) at 1.8 A and a 2 mm gap between the feed table and electromagnetic roll. The separated fraction was inspected under an optical microscope and cleaned manually to remove residual non-glauconitic grains. This material was previously characterized in detail by Rudmin et al. (Reference Rudmin, Maximov, Dasi, Kurovsky, Gummer, Ibraeva, Kutugin, Soktoev, Ponomarev, Tararushkin, Makarov and Ruban2023a). The average crystal-chemical formula of the material is: K0.7(Al0.3Mg0.3Fe1.6)2(Si3.7Al0.3)4O10(OH)2nH2O. In the present study, the initial composition and its variability were also checked (Table S1, Supplementary material).
The experiments were designed to investigate glauconite alteration during interaction with aqueous solutions at various pH values (Fig. 1). The following target pH values were used: 2, 3, 4, 6 (distilled water), 7, 8, 9, and 10. Model solutions were prepared using distilled water. Acidic conditions were created by adding hydrochloric acid (HCl). Alkaline conditions were created by adding sodium hydroxide (NaOH). The use of NaOH was essential because K is a structural component of glauconite. The use of K-bearing reagents could interfere with the interpretation of the interlayer cation exchange processes.
Experimental scheme for the interaction of glauconite with aqueous solutions of varying acidity under static conditions, including a photograph of the globular glauconite concentrate.
Figure 1. Long description
At the top-left is a circular micrograph showing dark green, rounded globular glauconite grains with a 200 mu m scale bar. A line connects this photo to a large array of Erlenmeyer flasks below. The flasks are organized into eight distinct groups based on acidity, arranged horizontally from left to right.
Each group contains six flasks staggered in depth to represent time intervals of 1 d, 3 d, 7 d, 14 d, 30 d, and 60 d. The groups are labeled as follows from left to right.
* p H 2 with plus 300 mu l H Cl.
* p H 3 with plus 30 mu l H Cl.
* p H 4 with plus 1 mu l H Cl.
* p H 6 with H sub 2 O.
* p H 7 with plus 5 mu l N a O H.
* p H 8 with plus 7 mu l N a O H.
* p H 9 with plus 13 mu l N a O H.
* p H 10 with plus 75 mu l N a O H.
Each flask is filled with a light blue solution and has a dark green sediment layer at the bottom, representing the glauconite concentrate interacting with the aqueous solution.
The target pH values were adjusted by micro-additions of concentrated HCl or NaOH using a microdispenser. The following empirically calibrated scheme of micro-additions was applied to the base solution volume used in the laboratory procedure: 300 μL HCl for pH 2, 30 μL HCl for pH 3, 1 μL HCl for pH 4, distilled water for pH 6, 5 μL NaOH for pH 7, 7 μL NaOH for pH 8, 13 μL NaOH for pH 9, and 75 μL NaOH for pH 10. The pH values were controlled using a pH meter. Redox potential was neither controlled nor measured during the experiments. Because the solutions were prepared under ambient laboratory conditions without deoxygenation or reducing agents, the initial systems were expected to be oxic to mildly oxidizing, although their redox evolution during incubation was not monitored.
All experiments were conducted at a solid-to-liquid ratio of 1:3, corresponding to 100 g of glauconite and 300 mL of model solution. The reaction systems were placed in sealed flasks and maintained under static conditions at room temperature. The flasks were opened only during sampling of the solid phase. Continuous gas exchange with the atmosphere was not allowed during the experiments. The interaction times were 1, 3, 7, 14, 30, and 60 days. After the specified interaction time, the mineral fraction was removed from the solution. It was separated from the liquid phase, rinsed gently with distilled water to remove residual solution from the particle surfaces and dried at 40°C to constant weight. The rinsing step was kept short to minimize additional alteration after the experiment. The dried samples were then used for subsequent analytical investigations.
The following sample coding scheme was used to identify the experimental series: GltpH{a}d{b}, where a is the target pH value and b is the experiment duration in days.
Analytical methods
After incubation in aqueous solutions of various acidity, glauconite globules were examined using several analytical techniques. These included X-ray diffraction (XRD), scanning electron microscopy with energy-dispersive spectroscopy (SEM-EDS), and Fourier-transform infrared spectroscopy (FTIR).
The structural characteristics of the mineral concentrates collected after 1, 14, and 60 days of experiments were analyzed using a Shimadzu XRD-7000S diffractometer (Shimadzu, Kyoto, Japan) equipped with a vertical goniometer and Cu-Kα radiation. Dried glauconite samples were ground gently in an agate mortar to obtain analytical powders with a particle size of <10 μm. The XRD analyses were performed on randomly oriented powder mounts. This approach was used to preserve the natural aggregate texture of the concentrates and to avoid structural modifications associated with oriented sample preparation. Diffraction patterns were recorded over the angular range of 4 to 65°2θ. The step size was 0.02°2θ, and the counting time was 1.5 s per step. The divergence slit was set to 1 mm, the anti-scatter slit to 3 mm, and the receiving slit to 0.3 mm.
The morphology and local chemical composition were determined by SEM-EDS using a Tescan VEGA 3 SBU microscope equipped with an Oxford X-Max 50 detector. Samples collected at 1, 14, and 60 days into the experiment were analyzed. Glauconite globules were mounted on aluminum stubs using conductive carbon tape and coated with a thin conductive carbon layer before analysis. SEM imaging was performed in secondary electron mode. EDS point analyses were collected from glauconite globule surfaces and near-surface microaggregates, avoiding visible non-glauconitic inclusions where possible. The accelerating voltage was 20 kV. The probe current ranged from 2.0 to 12.2 nA, and the working distance varied between 4 and 15 mm. The EDS measurements were performed at a probe current of 12.2 nA and a working distance of 15 mm. Calibration was carried out using Co standards, as well as pyrite, quartz, and microcline reference materials.
Crystal-chemical calculations were performed for the 1-, 14-, and 60-day experimental products based on SEM-EDS data normalized to O10(OH)2. Fe was treated as predominantly Fe3+ for structural formula calculations, following the Fe-dominant dioctahedral character of the studied glauconite and previous characterization of the starting material. This assumption was applied consistently to all samples. Therefore, the calculated formulae are mainly used for comparing relative trends within the experimental series, rather than for independent determination of the Fe redox state.
The FTIR spectra were recorded using a Shimadzu FTIR-8400S spectrometer in the range 400–4000 cm–1, with a spectral resolution of 4 cm–1. For each measurement, 0.5 mg of powdered glauconite sample was dispersed in 200 mg of KBr. The mixture was homogenized in an agate mortar, loaded into a 3 mm pellet die, and pressed to form a transparent KBr pellet. The sample:KBr ratio was 1:400, corresponding to 0.25 wt.% sample in the pellet.
Results
Morphological and structural characteristics of glauconite
In all experimental series, glauconite retained a globular morphology. The globules showed elongation sizes of approximately 50–400 μm (Fig. 2). At the same time, the surface microstructure of the globules varied depending on pH and experiment duration. In acidic series (pH 2–4), especially after 14–60 days, the surface was defined by pronounced micro-scaly and micro-porous textures. The microscale consists of disoriented platy particles with lateral dimensions of ~0.5–3 μm. These particles form loose aggregates, increasing surface roughness. Under neutral conditions (pH 7), the micro-scales were more densely packed. The degree of micro-porosity and surface roughness is reduced. In the alkaline series (pH 8–10), the micro-scaly structure was preserved. However, the surface becomes smoother, and the sizes of micro-particles are more commonly in the 1–2 μm range. In all series, no destruction of the globular morphology was observed. The morphological changes were mainly confined to near-surface zones.
SEM images (secondary electron detector) of glauconite globules after interaction with aqueous solutions of different pH values. The main images show standardized SE images acquired at the same magnification and view field (100 μm) to compare near-surface textures across the experimental series. Insets in the lower-right corners show low-magnification (2 mm of view field) BSE images. In all experiments, the globular morphology of glauconite (500–400 μm) was preserved. Under acidic conditions (pH 2–4), the surface exhibits well-developed microflaky and microporous textures formed by loosely packed platy particles with sizes of 0.5–3 μm. Under neutral and alkaline conditions, the surface is more compact and relatively smooth.
Figure 2. Long description
A grid of nine S E M micrographs labeled A through I. Each panel features a main high-magnification S E image with a 20 micrometer scale bar and a smaller B S E inset in the bottom right with a 500 micrometer scale bar.
* Panel A (Glt-origin): Shows the original glauconite surface with a dense, overlapping platy texture.
* Panel B (p H 2): Exhibits a highly porous and fragmented surface with deep crevices between rounded mineral masses.
* Panel C (p H 3): Displays a rough, irregular surface with significant micro-pitting and loose platy particles.
* Panel D (p H 4): Shows a well-developed microflaky texture with visible 0.5 to 3 micrometer platy particles and high porosity.
* Panel E (p H 6): Transitions to a more rounded, smoother globular surface with less visible porosity.
* Panel F (p H 7): Features a compact surface with a relatively uniform texture and fewer open pores compared to acidic samples.
* Panel G (p H 8): Shows a smooth, dense surface with large, flat mineral faces and minimal micro-porosity.
* Panel H (p H 9): Displays a compact surface with some linear cracking but otherwise smooth, non-porous textures.
* Panel I (p H 10): Shows a very dense and relatively smooth surface texture under alkaline conditions.
The B S E insets in all panels show a consistent distribution of individual glauconite globules, confirming that the overall globular morphology remains intact despite the surface texture changes.
In all experimental series, glauconite (Fig. 3) preserved the characteristic set of reflections of the illite–glauconite series. This includes a well-defined basal (001) reflection with interplanar spacing (d) in the range of 9.92–10.25 Å (d 001). The smallest d 001 values were observed for samples GltpH2d60, GltpH3d60, and GltpH4d60. In experiments conducted at pH 2–4, a slight but regular shift of d 001 was observed. The basal spacing decreased from ~10.22 Å in the initial glauconite to 9.92 Å after experimental treatment. In acidic series (pH 2–3), an increase in contact time was accompanied by a decrease in the normalized intensity of the (001) reflection relative to the initial mineral. Peak broadening was also observed. These changes indicate an increase in structural disorder.
XRD patterns of glauconite obtained under various experimental conditions compared with the diffraction pattern of the initial glauconite from the Karin deposit (Glt-Karino).
Figure 3. Long description
A multi-plot X R D pattern showing 25 stacked patterns. The X-axis is labeled 2 theta in degrees, ranging from 5 to 65. The Y-axis represents relative intensity.
At the bottom is the reference pattern Glt-Karino in black. Above it, samples are grouped by p H levels (p H 2, 3, 4, 6, 7, 8, 9, and 10), with each p H group containing three time intervals: 1 day (lightest shade), 14 days (medium shade), and 60 days (darkest shade).
Vertical dashed lines align peaks across all samples. Key peaks are labeled at the top with Angstrom values and mineral indices:
* 10.25 and 9.92 Angstrom Glt (001) near 9 degrees 2 theta.
* 5.0 Angstrom Glt (002) near 17.5 degrees.
* 4.52, 4.48, and 4.37 Angstrom Glt (020)/(110) near 20 degrees.
* 3.645 Angstrom Glt (112) near 24.5 degrees.
* 3.324 Angstrom Qz/Glt (003) near 26.5 degrees.
* 3.077 Angstrom Glt (112) near 29 degrees.
* 2.590 and 2.576 Angstrom Glt (131) near 35 degrees.
* 2.415 Angstrom Glt (004) and 2.390 Angstrom Glt (221) near 37 degrees.
* 2.251 Angstrom Glt (221) near 40 degrees.
* 2.000 and 1.992 Angstrom Glt (202)/(005) near 45 degrees.
* 1.815 Angstrom Glt near 50 degrees.
* 1.514 and 1.508 Angstrom Glt (060) near 61 degrees.
The patterns remain largely consistent across all p H and time conditions, maintaining the characteristic peak positions of the initial glauconite.
The second basal reflection (002) was also present in all experimental series. Its interplanar spacing varied from 5.17 to 5.00 Å, compared with 5.08 Å in the initial glauconite (Fig. 3). However, in nearly all series, the intensity and sharpness of the (002) reflection were lower than in the initial material. Reflections in the mid-angle region were preserved in all samples. These include reflections of the (020)/(110) type with d-values of 4.37–4.52 Å. Under acidic conditions, these reflections showed reduced relative intensity and a less well-defined shape. This effect was most pronounced in the 60-day experiments.
The high-angle diagnostic (060) reflection was observed in all series. Its d values ranged from 1.508 to 1.514 Å, compared with 1.513 Å in the initial state (Fig. 3). These variations indicate minor contractions or slight expansions of the octahedral sheet. In the acidic series, a weak contraction of the (060) spacing was observed, with d values of 1.508–1.509 Å at pH 2–3. This behavior is consistent with changes in octahedral-site occupancy, such as Al3+ substituting for Fe3+ and possible variations in the Fe valence state.
Crystal-chemical characteristics of glauconite
All experimental series showed variations from the initial composition (Table S1). The magnitude and direction of these deviations were controlled by solution pH and contact time. The most pronounced changes were observed under acidic conditions (pH 2–3). In these series, the interlayer potassium content decreased relative to the initial glauconite even at early stages of the experiment (1–7 days). The reduction reached ~25–30%. Maximum deviations were observed after 7–14 days, when the total decrease reached ~40–45% (Fig. 4A,B). For example, in sample GltpH2d14, the average K2O content decreased to 4.5 wt.%, whereas the initial glauconite contained an average of 8.3 wt.% K2O. After 60 days, the interlayer K values either stabilized at a reduced level of ~30–35% relative to the initial state or, in some series, showed a weak tendency towards partial recovery (Fig. 5A). Under neutral as well as weakly alkaline conditions (pH 6–8), interlayer K remained within ±5% of the initial values throughout the experiment. No systematic time-dependent trend was observed in these series.
Crystallochemical relationships illustrating the evolution of glauconite composition during interaction with aqueous solutions of different acidity and contact duration: (A) K2O (wt.%) versus Al2O3 (wt.%); (B) K2O (wt.%) versus Fe2O3(total) (wt.%); (C) relationship between the M +/4Si ratio (Meunier and El Albani, Reference Meunier and El Albani2007) and the sum of octahedral cations (∑VI cations). Each data point represents an individual SEM-EDS spot analysis of a glauconite globule surface or near-surface microaggregate.
Figure 4. Long description
The figure consists of nine scatter plots arranged in three rows labeled A, B, and C, and three columns labeled Day 1, Day 14, and Day 30. A legend on the top right identifies data points by p H levels ranging from 2 to 10, plus an original glauconite sample.
Row A: The y-axis is K sub 2 O weight percent from 0 to 10. The x-axis is Al sub 2 O sub 3 weight percent from 0 to 20. The data shows a negative correlation where K sub 2 O decreases as Al sub 2 O sub 3 increases. Over time from Day 1 to Day 30, the data points for lower p H values (yellow and orange) shift further toward the bottom right, indicating loss of potassium and gain of aluminum.
Row B: The y-axis is K sub 2 O weight percent from 0 to 10. The x-axis is Fe sub 2 O sub 3 total weight percent from 0 to 40. The data shows a positive correlation. As time progresses, the cluster of points becomes more dispersed, with acidic p H samples moving toward the lower left origin, indicating a simultaneous loss of potassium and iron.
Row C: The y-axis is the M super plus forward slash 4 S i ratio from 0.0 to 1.0. The x-axis is the ratio of Fe super V I all over the summation of V I cations from 0.0 to 1.0. The data follows a linear upward trend from the bottom-left to the top-right. By Day 30, the points are more tightly clustered along this linear path compared to Day 1, though low p H outliers remain lower on both scales.
Temporal evolution of key crystallochemical parameters of glauconite during interaction with aqueous solutions of various acidity under static conditions: (A) interlayer potassium, (B) Al in the tetrahedral framework, and (C) Fe in the octahedral framework as a function of experimental duration. Colored symbols correspond to different pH values. SEM-EDS-based crystal-chemical calculations were performed for samples collected after 1, 14, and 60 days of interaction.
Figure 5. Long description
A multi-panel set of three line graphs labeled A, B, and C. All graphs share an x-axis representing Time in days with markers at 0, 10, 20, 30, 40, 50, and 60. Data points are recorded at 1, 14, and 60 days. A common legend in panel B identifies nine p H values: p H 2 (yellow), p H 3 (dark orange), p H 4 (gold), p H 6 (bright green), p H 7 (dark green), p H 8 (cyan), p H 9 (pink), and p H 10 (purple).
* Panel A: The y-axis is K super plus sub interlayer in a dot p dot f dot u dot ranging from 0.3 to 0.8. Most p H values show fluctuations between 0.4 and 0.6. Notably, p H 4 peaks sharply at 0.7 on day 14 before declining, while p H 10 shows a steady increase from day 14 to day 60.
* Panel B: The y-axis is Al super 3 plus sub tetr in a dot p dot f dot u dot ranging from 0.0 to 0.4. Most data points cluster between 0.2 and 0.3. p H 3 shows a distinct peak at nearly 0.4 on day 14, followed by a decline to 0.2 by day 60.
* Panel C: The y-axis is Fe sub oct in a dot p dot f dot u dot ranging from 0.8 to 1.3. Trends are highly variable. p H 10 (purple) shows a significant upward trajectory from 0.9 at day 14 to nearly 1.3 at day 60. p H 3 (dark orange) shows a continuous downward trend from 1.25 at day 1 to approximately 1.05 at day 60.
Changes in the tetrahedral sheet also showed a weak pH dependence. In acidic solutions (pH 2–3), the Al/Si ratio increased progressively relative to the initial state. The magnitude of this variation increased with the duration of the experiment. This trend was mainly driven by an increase in Al in tetrahedral positions, with values ranging from +0.05 to +0.32 a.p.f.u. (Fig. 5B; Table S1). This increase occurred synchronously with greater Fe contents in octahedral positions (Fig. 5B). In alkaline solutions with pH ≥6, the Al/Si ratio showed an opposite trend. On average, it decreased with increasing experiment duration. For example, in the pH 10 series, Al/Si decreased from 0.23 to 0.14 a.p.f.u.
Parameters of the octahedral sheet showed a similar pattern. Under acidic conditions (pH 2–3), the Mg/Fe ratio was systematically greater than in the initial sample. The magnitude of this difference increased with the duration of the experiment. At early stages, the increase in Mg/Fe was of the order of 5–25% relative to the initial material. After 60 days, Mg/Fe showed variable behavior, ranging from –7% to +18% across experimental series. The Fe/(Fe+Al)oct parameter in the acidic series showed greater variability. In several samples, it showed decreases relative to the initial glauconite of ~0.05–0.14 (Table S1). Under neutral as well as weakly alkaline conditions (pH 6–8), both Mg/Fe and Fe/(Fe+Al)oct exhibited only small fluctuations. No systematic time-dependent trends were observed in these series. Overall, the quantitative data indicate that the most intensive crystal-chemical reorganization of glauconite globules occurred under acidic conditions and became more pronounced with increased experiment duration. In contrast, at neutral and alkaline pH values, the mineral structure remained more stable.
All samples retained the main FTIR absorption bands that reflect the crystalline framework of glauconite (Fig. 6). The band assigned to Si-O-Si stretching shifted from 444 to 428 cm–1. In contrast, the initial glauconite is located at 440 cm–1. The most pronounced changes in this band position are observed under acidic conditions (pH 2, 3, and 4). The maximum shift occurred in samples with contact durations of 14–30 days. After 60 days, the band position stabilized. This shift probably reflected changes in the geometry of silicate tetrahedra relative to the basal planes. It was consistent with variations in the Si-O-Si angle, which tended to increase under acidic conditions. These changes may be associated with additional isomorphic substitution of Al for Si. Samples with short interaction times of 1–3 days showed little to no shift of this band relative to the initial glauconite. Under neutral conditions, the changes were minimal. In the alkaline series, some samples showed band positions close to the initial state, whereas others displayed slightly higher frequencies up to 444 cm–1. These variations are considered minor. Bands assigned to Si-O-Al and Si-O stretching did not show significant frequency shifts with pH or contact time. However, their intensities increased markedly in samples with interaction times of 14–30 days. This effect was most evident in samples at pH 2 and 6 (Fig. 6A) and at pH 9 (Fig. 6B). In some samples, peak splitting was observed, reflecting an increase in band intensity. A similar trend was observed for the intensity and position of the ν(Si-O) band. Its frequency variations were small and did not appear to depend on contact time or pH. In contrast, its intensity increased progressively with increasing interaction time in most samples. An exception was the GltpH7d1–60 series (Fig. 6B). In several samples, the band split and reached its highest sharpness after 60 days of interaction. This splitting probably reflected the coexistence of structurally non-equivalent Si-O environments associated with tetrahedral reorganization and increasing local disorder in defect-rich domains.
FTIR spectra of glauconite obtained under various experimental conditions and in the initial state (Glt-Kar): (A) pH range 4–6; (B) pH range 7–10.
Figure 6. Long description
A two-panel attribute space diagram showing F T I R spectra. The x-axis represents Wavenumber in inverse centimeters from 4000 to 400. The y-axis represents intensity, with multiple stacked spectral lines.
Panel A (Left) displays p H range 4 to 6. The top black line is the initial state Glt-Kar. Below it, groups of spectra are color-coded by p H: green for p H 6, brown for p H 4, orange for p H 3, and light orange for p H 2. Each group contains six lines representing time intervals from 1 day (d 1) to 60 days (d 60).
Panel B (Right) displays p H range 7 to 10. Below the black Glt-Kar line, spectra are grouped by color: purple for p H 10, pink for p H 9, blue for p H 8, and teal for p H 7.
Key inflection points marked by vertical dotted lines across both panels include:
* High-frequency region: Peaks at 3603 (Al O H Mg), 3563 (Mg O H Fe super 3 plus), 3536 (Fe super 3 plus O H Fe super 3 plus), and 3443 (H sub 2 O).
* Mid-frequency region: Peaks at 1636 and 1628 (H sub 2 O).
* Low-frequency region: Major peaks at 1007 and 995 (nu Si O), 818 (delta O H I n), 679 and 664 (delta Si O), and a cluster between 503 and 428 (delta Si O, delta Si O Al, and delta Si O Si).
The spectra show consistent absorption bands across all samples, with variations in peak intensity and sharpness depending on the p H and duration of the experiment.
Schematic illustration of the experimental set-up and the key transformations observed in glauconite during interaction with aqueous solutions of various pH values.
Figure 7. Long description
The diagram proceeds from left to right in three main stages.
1. Initial State. A teal circle represents Globular glauconite with a size of 50 to 400 micrometers and a 10 Angstrom T O T structure. Three curved horizontal lines inside the circle indicate the mineral layers.
2. Experimental Setup. An arrow points to a beaker containing blue liquid labeled Static conditions 1 to 60 days. To the left of the beaker is a vertical p H scale. At the top is p H 2 in orange. In the middle is p H 2 to 10 in black. At the bottom is p H 10 in blue. The glauconite pellet is submerged in the liquid.
3. Observed Transformations. An arrow points to two resulting glauconite states.
* The top path shows a lime green circle. Text to the right indicates Interlayer K decrease and Tetrahedral A l / S i increase (acidic p H), accompanied by an upward-pointing orange arrow.
* The bottom path shows the original teal circle. Text to the right states 10 Angstrom structure preserved.
In the high-frequency region associated with OH vibrations, only minor changes in band intensity were observed in all samples. As contact duration increased, peak splitting became apparent. The longer the interaction time, the more clearly the splitting was resolved.
The band at 1628–1636 cm–1, assigned to bound water, shows intensity changes as a function of solution pH and contact duration. All samples with a 1-day interaction time displayed band intensities comparable to those of the initial glauconite. With increasing contact time, the intensity increased gradually and reached a maximum in samples with interaction durations of 14–30 days. After 60 days, a decrease in intensity was observed in most series. The sharpest expression of this band was recorded in samples GltpH2d14 and GltpH2d30 (Fig. 6A). In contrast, the band near 3443 cm–1, attributed to H2O stretching vibrations, showed a tendency toward progressive broadening and a shift to lower frequencies.
This pattern was most clearly expressed in samples exposed to acidic conditions (pH 2 and 3). The Fe3+-OH-Fe3+ band at 3536 cm–1 and the Mg-OH-Fe3+ band at 3563 cm–1 were present in nearly all samples, showing only minor variations in position and intensity (Fig. 6A). In samples GltpH9d7, GltpH8d7, GltpH7d7, and GltpH2d14, these two bands merged into a single feature (Fig. 6B). The Al-OH-Mg band at 3603 cm–1 was observed in all samples with only slight variations in position and intensity. In most cases, as in the initial glauconite, it exhibited a weak shoulder. Samples under the extreme experimental conditions exhibit the most pronounced deviations relative to the initial glauconite.
Discussion
Structural stability of glauconite globules and pH-controlled disordering under static conditions
In contrast to studies focused on dissolution kinetics or component leaching (Fernandez-Bastero et al., Reference Fernandez-Bastero, Gil-Lozano, Briones and Gago-Duport2008; Zhang et al., Reference Zhang, Awolayo, Nightingale and Tutolo2022), the present study considers glauconite globules as a structurally stable yet adaptive layered mineral. Their evolution was controlled by pH and the duration of contact under static aqueous conditions. The results show that within the investigated range (pH 2–10; 1–60 days), the 10 Å layered structure was preserved. No destruction of the basal framework was observed. Instead, a gradual development of structural disorder and crystal-chemical reorganization occurred while the basal layered framework remained intact.
Structural and chemical changes were mainly confined to the near-surface zones of glauconite globules measuring ~50–150 μm. Under acidic conditions, the globule surfaces developed a pronounced microflaky and microporous texture. This texture was formed by thin platy particles with typical lateral dimensions of ~0.5–3 μm, while the overall globular shape was preserved. In neutral and alkaline series, the microflakes were aggregated in more compact fashion and the surface remained relatively smooth (Fig. 2). This morphology reflected a spatially heterogeneous pattern of near-surface transformation of glauconite globules (Tang et al., Reference Tang, Shi, Jiang, Zhou, Shi and Sho2017; Bansal et al., Reference Bansal, Banerjee and Nagendra2020). Within this transformation framework, the inner parts of the globules retained the original layered structure, whereas the outer zones had undergone more intense structural and crystal-chemical reorganization.
A similar near-surface transformation has previously been described for glauconite and related Fe-rich phyllosilicates across various geochemical settings, ranging from marine shelf sediments to peat bogs and cold-seep environments (Baldermann et al., Reference Baldermann, Warr, Letofsky-Papst and Mavromatis2015; Nieto et al., Reference Nieto, Abad, Bauluz and Reolid2021; Rudmin et al., Reference Rudmin, Ruban, Deeva and Khitrin2026a). In most cases, a thin, defect-enriched, and often partially amorphized, layer forms. This layer co-exists with a relatively well-ordered inner domain that preserves the original 10 Å structural order (Drits et al., Reference Drits, Dainyak, Muller, Besson and Manceau1997; López-Quirós et al., Reference López-Quirós, Sánchez-Navas, Nieto and Escutia2020). The microflaky and microporous textures observed in the static experiments were consistent with the concept of reactive near-surface zones developing without destroying the globular morphotype or the layered mineral framework.
Glauconite globules preserved a layered TOT structure of the illite-glauconite type in all experimental series. This is confirmed by the persistent presence of the basal (001) reflection with interplanar spacings of d 001 = 9.92–10.25 Å (Fig. 3). In acidic series (pH 2–4), a slight but systematic decrease in d 001 was observed, from ~10.22 Å in the initial glauconite to ~9.92 Å after 60 days of interaction. These changes were accompanied by a decrease in intensity and broadening of the basal (001) and (002) reflections. They indicated a reduction in the coherent scattering length along the c-axis and a decrease in interlayer ordering. This behavior may be related to partial modification of interlayer sites and re-distribution of residual charge on the basal surfaces.
The higher mean interlayer K values observed in some acidic series after 60 days may reflect partial buffering of the initially acidic solutions and subsequent re-equilibration of K between the solid and liquid phases. This interpretation remains tentative because final pH and post-experimental solution chemistry were not measured.
In neutral and alkaline series (pH 6–10), interlayer K remained within ±5% of the initial values, and the parameters of the tetrahedral and octahedral layers showed no systematic temporal evolution. This was consistent with minimal variations in d 001, stable FTIR spectra, and a relatively smooth surface morphology. Glauconite globules were more stable in neutral and alkaline solutions because proton-promoted reactions at edge and interlayer sites were limited, reducing interlayer K loss, charge re-distribution, and basal disordering observed under acidic conditions.
Adaptive crystallochemical reorganization of interlayer and tetrahedral domains
Crystallochemical data indicated that the structural changes observed in glauconite were adaptive rather than destructive. In acidic series (pH 2–3), a systematic decrease in interlayer K of ~25–45% relative to the initial composition was observed (Figs 5 and 6), with maximum deviations at a contact duration of 14 days. This trend was consistent with the decrease in d 001 and the broadening of basal reflections, and indicates partial degradation of intrastructural K+ fixation without destruction of the TOT framework (Tararushkin et al., Reference Tararushkin, Ruban, Bu, Yanfu and Rudmin2026). At the same time, the Al/Si ratio in the tetrahedral layer increased. This reflected the re-distribution of residual charge and local reorganization of the tetrahedral framework. These changes correlated with a shift of the Si-O-Si band in the FTIR spectra to 428 cm–1 and an increase in the intensity of the Si-O and Si-O-Al bands (Fig. 6). The partial oxidation of Fe2+ to Fe3+ may also contribute to charge re-distribution and weakening of interlayer K fixation. This mechanism was plausible for Fe-rich glauconite, but direct verification would require Fe-speciation data for all experimental products.
The increase in the intensity of the Si-O and Si-O-Al bands at the same time intervals was consistent with the weakening of reflections in the mid-angle region (d 020/110 ≈ 4.37–4.52 Å) in the XRD patterns (Fig. 3). This behavior indicated an increasing proportion of defect-rich and structurally active domains. Previous studies have shown that even within a single glauconite grain, domains with different degrees of glauconitization, Fe contents in octahedral sites, and levels of interlayer filling may coexist (Drits et al., Reference Drits, Dainyak, Muller, Besson and Manceau1997; Zviagina et al., 2017; Nieto et al., Reference Nieto, Abad, Bauluz and Reolid2021). In the present experiments, acidic conditions appeared to affect the most reactive domains preferentially. These domains are characterized by reduced K+ fixation and a higher proportion of Fe-enriched octahedral sites. This results in partial reorganization and charge re-distribution without compromising the integrity of more stable structural domains.
The octahedral framework of glauconite was very stable. The diagnostic (060) reflection was preserved in all series, with only minor variations in d 060 = 1.508–1.514 Å (Fig. 3), confirming the dioctahedral character of the structure. The parameters of the octahedral layer remain relatively stable. Variations in Mg/Fe and Fe/(Fe+Al)oct typically did not exceed 5–15% and indicated local redistribution of Fe and Mg cations (Table S1).
Similar near-surface textural changes have been reported in experiments on glauconite dissolution in flow-through systems, where they were interpreted as the development of passivating or amorphized surface layers that limit further mineral–solution interaction (Fernandez-Bastero et al., Reference Fernandez-Bastero, Gil-Lozano, Briones and Gago-Duport2008; Zhang et al., Reference Zhang, Awolayo, Nightingale and Tutolo2022). In contrast, under the static conditions of the present experiments, morphological transformations were not accompanied by intensive cation release or by an increase in the proportion of amorphous material. The observed stability of the globular morphology was consistent with data from natural glauconites in continental-shelf environments, where globular forms were preserved even during prolonged diagenetic evolution, despite the development of microfractured and flaky surface textures (Koschik and Liu, Reference Koschik and Liu2025).
In more complex geochemical scenarios, such as the interaction of glauconite-bearing sandstones with CO2-saturated brines under elevated temperature and pressure, the stability of the 10 Å structure has been shown to coexist with pronounced modification of near-surface zones and precipitation of secondary iron phases (Fernandez-Bastero et al., Reference Fernandez-Bastero, Gil-Lozano, Briones and Gago-Duport2008). Together with the new data presented here, these observations support the view of glauconite as a mineral phase with a rigid layered framework and more flexible interlayer and edge zones. These zones respond first to changes in pH, and solution composition, and control the sorption and buffering properties of the mineral. From a practical perspective, this implies that the performance of materials based on glauconite globules is governed primarily by the state of near-surface microzones (Rudmin et al., Reference Rudmin, Maximov, Dasi, Kurovsky, Gummer, Ibraeva, Kutugin, Soktoev, Ponomarev, Tararushkin, Makarov and Ruban2023a) and the degree of their textural disorder, rather than by bulk composition or the evolutionary stage of glauconitization only (Dasi et al., Reference Dasi, Rudmin and Banerjee2024).
Conclusions
The experimental results led to the following main conclusions.
(1) The interaction of glauconite globules with aqueous solutions over the pH range 2–10 and contact durations of 1–60 days resulted in the preservation of the 10 Å layered TOT structure in all experimental series. No destruction of the layered framework or phase transformations was observed.
(2) Under acidic conditions (pH 2–4), a slight yet systematic decrease in the basal spacing d 001 was observed, from ~10.22 Å in the initial glauconite to ~9.92 Å after 60 days. This was accompanied by broadening and a decrease in intensity of the basal (001) and (002) reflections. These changes reflected increasing structural disorder and reduced interlayer ordering, without disrupting the TOT framework.
(3) Crystallochemical characteristics showed a distinct dependence on pH and contact duration, reflecting adaptive reorganization of interlayer and tetrahedral domains. In the acidic series, interlayer K decreased relative to its initial composition, with the greatest deviation at a contact duration of 14 days. In neutral and alkaline conditions, interlayer K remained within ±5% of its initial value.
(4) An increase in the Al/Si ratio in the tetrahedral layer and a shift of the Si-O-Si band in the FTIR spectra, together with enhanced intensity of the Si-O and Si-O-Al bands, indicate local reorganization of the tetrahedral framework and an increasing proportion of structurally active and defect-rich domains. These changes correlate with the weakening of mid-angle reflections in the XRD patterns.
(5) The octahedral framework of glauconite remained highly stable throughout the experiments, as evidenced by preservation of the (060) reflection with d 060 = 1.508–1.514 Å and the absence of systematic degradation of octahedral layer parameters. Variations in Mg/Fe and Fe/(Fe+Al)oct reflected only local re-distribution of cations.
(6) Morphological changes in glauconite globules were predominantly confined to near-surface zones and appear as the development of microflaky and microporous textures whilst preserving the globular morphology. These observations were consistent with spatially heterogeneous transformation of glauconite under static aqueous conditions, without intensive cation release or formation of a significant amorphous fraction.
Supplementary material
The supplementary material for this article can be found at http://doi.org/10.1017/cmn.2026.10053.
Acknowledgments
The authors gratefully acknowledge Editor-in-Chief Dr Joseph W. Stucki, the Associate Editor, and the two anonymous reviewers for their careful reading, valuable comments, and constructive suggestions, which improved the clarity and quality of the manuscript.
Author contributions
Conceptualization: M.R.; Data curation: M.R., I.Kh., A.A.; Formal analysis: M.R., A.E.; Funding acquisition: M.R.; Investigation: I.Kh., I.V., A.A.; Methodology: M.S., D.V.; Project administration: M.R.; Resources: M.R.; Software: M.R., I.Kh., D.V., A.R.; Supervision: M.R.; Validation: M.R., A.R.; Visualization: M.R., I.Kh., D.V., A.A., A.R.; Writing—original draft preparation: M.R., I.Kh.; Writing—review and editing: M.R., A.R. All authors have read and agreed to the published version of the manuscript.
Financial support
This research was supported financially by the Russian Science Foundation (grant 22-77-10002-P).
Competing interests
The authors declare none.
