Raw materials

The lizardite and chlorite samples were obtained from the Beiwagou Cuiyu jade mine, Xiuyan County, Liaoning Province, China. The talc sample was obtained from Guilin City, Guangxi Province, China. The samples were crushed, ground, passed through a 200 mesh sieve and ultrasonically rinsed three times with ultrapure water to minimize the effects of the high-energy surface sites and fine particles produced, and they were analysed using X-ray diffraction (XRD).

Experimental procedure

At 25℃, 1 g each of lizardite, chlorite and talc was placed in a 150 mL conical flask. A total of 100 mL of a 1.0 mol L^–1 sulfuric acid solution was added to a volumetric flask at a solid-to-liquid ratio of 1:100. The acid concentration is designed for 1 mol L^–1. This can accelerate the process of mineral–water interfacial reaction and can help the mineral to fully react in the acid solution. It also provides sufficient H⁺ to prevent experimental errors due to the dramatically smaller H⁺ concentrations in the later stages of mineral–water interface reactions. The reaction times studied were 1, 2, 4, 12, 24, 72, 120, 168 and 216 h. The conical flask was placed in a constant-temperature shaker and the reaction was allowed to proceed fully. The reaction suspension was filtered to obtain the solid product and washed three times with distilled water until the pH of the filtrate was neutral. The solid product was dried in an oven at 60°C and set aside in a desiccator. The filtrate was collected in a 50 mL volumetric flask and left for subsequent inductively coupled plasma mass spectrometry testing. The samples were labelled as K-T, where K is the mineral sample number (i.e. Lz, Chl and Tlc, for lizardite, chlorite and talc, respectively) and T was the reaction time.

Characterization methods

The specific surface areas of lizardite, chlorite and talc were measured using the Brunauer–Emmett–Teller (BET) method with five-point N₂ adsorption isotherms after degassing the samples for 60 min at 120℃ using a McASAP 2460 (Norcross, GA, USA) instrument.

Chemical analysis of major elements was performed using X-ray fluorescence (XRF). The sample was ground to a particle size of <74 µm (200 mesh) and then dried at 105℃ in an oven for 2 h. Subsequently, the sample was precisely weighed to 0.8 g and incinerated at 1000℃ for 1 h until the sample attained a constant weight, from which the ash content of the sample could be calculated. The fused sample was blended with a uniform mixture of solvent: 8.00 g Li₂B₄O₇–LiBO₂ (m(Li₂B₄O₇):m(LiBO₂) = 67:33), transferred into a platinum crucible and supplemented with four drops of a LiBr solution at a concentration of 200 g L^–1 as a releasing agent. Subsequently, the sample was subjected to a melting process at 1050℃, followed by controlled cooling to facilitate the formation of a glassy slab. Elements were analysed using a S4 Pioneer (Bruker) wavelength-dispersive XRF spectrometer equipped with X-ray tube with a Rh anode and a maximum voltage/current of 60 kV/150 mA. The fundamental parameter method using SpectraPlus software was used to quantify the elements.

XRD was used to determine the mineralogical changes in the samples before and after the mineral–water interfacial reaction. XRD data were obtained using a D/max-IIIA diffractometer manufactured by Rigaku (Japan). A copper target with a sum-tube voltage of 40 kV and a sum-tube current of 40 mA was accompanied by a DS1/2(°) slit system. The powdered samples were analysed between 3 and 80°2θ. The step size was 0.02°2θ and the scan rate was 2 s step^–1.

A Fourier-transform infrared (FTIR) spectrometer was used to determine the changes in the crystal chemistry of the samples before and after the mineral–water interfacial reaction. The FTIR spectra were measured using a Frontier spectrometer from Parkin Elmer Instruments (China) following the KBr pressed pellet technique. For this measurement, a 0.9 mg aliquot of the dried subsample was mixed with 80.0 mg of oven-dried, spectroscopic-grade KBr salt (refractive index 1.559, particle diameter 5–20 μm), and then the mixture was ground for 1 min. The mixture was then pressed in a die with 10 t of pressure applied for 1 min to form a disc or pellet. The spectra were measured at room temperature in a test range of 4000–400 cm^–1 over 64 scans.

The cation (Si, Mg) content in solution was measured using an inductively coupled plasma emission spectrometer (ThermoiCAP 6500, Waltham, MA, USA). Prior to each analysis, the ICP was calibrated (r ² > 0.999) via serial dilution of certified standards.

Scanning electron microscopy (SEM) was conducted to study the micromorphology of the samples using a ZEISS Supra 55 Sapphire (Jena, Germany) field emission scanning electron microscope. The samples were spread on conductive carbon tape and the surface was carbon-sprayed before being placed in the sample chamber, vacuumed and observed at an accelerating voltage of 30 kV.

Transmission electron microscopy (TEM) was conducted using an American-FEI-Talos F200S (OR, USA) microscope accelerated at 200 kV. The lizardite, chlorite and talc samples were sonicated and dispersed in ethanol for 10 min. Drops of the sample suspensions were placed on a 200 mesh carbon-coated Cu grid for at least 10 min before being transferred to the microscope for measurement.

The average rate of dissolution of each ion in the mineral was calculated as Equation 1:

(1)\begin{equation}{r_i}\left( {\bar t} \right) = \frac{{{C_i}{ } \times { }{m_1}}}{{t{ } \times { }{m_0}{ } \times {\text{ SSA }} \times { }{{\eta }_i}}}\end{equation}

where C_i is the concentration of solute i (Si, Mg, Fe; mmol kg^–1) in the sample recovered at time t; m ₁ is the mass of the sample solution before sampling (in kg); t is the sample reaction time; m ₀ is the initial mass of the mineral (in g); SSA (m² g^–1) is the specific surface area of the mineral; and η_i is the stoichiometric coefficient of element i in the mineral.

Dissolution processes of Si and Mg at the lizardite–water, chlorite–water and talc–water interfaces in sulfuric acid solutions

Mineral–water interfacial reactions are related to the formation and transfer of charges, which occur when substances at the mineral surface break bonds with the lattice, forming ions and moving into the solution (Knauss & Wolery, Reference Knauss and Wolery1988). The concentrations of Mg, Si and Fe in the collected filtrate were analysed and calculated (Table 2), and the dissolution concentrations of Mg, Si and Fe in Lz, Chl and Tlc were obtained. When chlorite and talc react with the sulfuric acid solution, the Si concentration in the filtrate increases and the Si concentration in chlorite and talc slightly increases (Fig. 2a). However, the Si concentration in the filtrate reaches a peak and then slowly decreases after the reaction of lizardite for 120 h (Fig. 2a). This can be attributed to the reaction of H⁺ with (OH)^– and [SO₄]^2– with Mg²⁺ in the solution after the reaction of lizardite in the sulfuric acid medium for 120 h. This results in the reduction of the acid concentration in the solution, the formation of amorphous silica and the precipitation of metasilicic acid in the solution.

Table 2. Elemental analysis of the liquid phase during the mineral–water interfacial reaction of lizardite (Lz), chlorite (Chl) and talc (Tlc; mg L^–1).

Figure 2. The relationships between (a) Si, (b) Mg and (c) Fe dissolution concentrations of lizardite (Lz), chlorite (Chl) and talc (Tlc) in sulfuric acid solution with reaction time.

The Si dissolution concentration decreases in the following order: chlorite > lizardite > talc (Fig. 2a). The Si dissolution concentration in chlorite is the greatest, which can be attributed to the Al substitution of Si in the silica tetrahedral sheet of chlorite and preferential dissolution of Al from the silica tetrahedral sheet compared to Si (Table 3; Hamer et al., Reference Hamer, Graham, Amrhein and Bozhilov2003). This leads to more structural defects, resulting in Si being connected to the mineral surface by fewer than three bridging oxygenates. The Si can detach from the octahedral sheet faster (Saldi et al., Reference Saldi, Köhler, Marty and Oelkers2007; Galí et al., Reference Galí, Soler, Proenza, Lewis, Cama and Tauler2012), resulting in the Si in chlorite dissolving more easily than that in lizardite and talc. Additionally, the dissolution concentration of Si in lizardite is greater than that of talc because the silica tetrahedral sheet in lizardite has small amounts of Al and Fe instead of Si. By contrast, the silica tetrahedral sheet in talc has almost no Al and Fe and there are few defect sites. Therefore, the dissolution concentration of Si in talc is the lowest during the mineral–water interfacial reaction in acidic medium.

The Mg dissolution concentration decreases in the following order: lizardite > chlorite > talc (Fig. 2b). The difference in the dissolution concentration of Mg is mainly due to the varying structures of the minerals. Lizardite is the 1:1-type mineral, while talc and chlorite are 2:1-type minerals. When H⁺ in the sulfuric acid solution enters the interlayer domain of lizardite, Mg–OH on each octahedral sheet reacts fully with H⁺, and a great amount of Mg is released from the octahedral sheet. Furthermore, when H⁺ attacks (OH)^– on the octahedral sheet by the mineral edge (end face) of chlorite and talc, the outer octahedral sheet is destroyed and a small amount of Mg is dissolved. During the talc–water interfacial reaction, the dissolution of Mg gradually occurs from the outside to the inside at the mineral edge (end face). Due to the dissolution of Mg, the silica tetrahedral sheet on both sides gradually collapses into the middle, resulting in the dissolution of Mg from Mg–OH into the solution (Okada et al., Reference Okada, Temuujin, Kameshima and Kenneth2003). Therefore, the dissolution concentration of Mg in the 2:1-type mineral is lower than that in the 1:1-type mineral.

The Fe dissolution concentration decreases in the following order: lizardite > chlorite (Fig. 2c). The dissolved concentration of Fe in talc is lower than the detection limit of the ICP test and can be ignored. Fe in chlorite exists only in octahedral sheets. Therefore, the dissolution pattern of Fe in chlorite is similar to that of Mg. However, compared to Fe, Mg is more soluble in lizardite and chlorite, and Mg is preferentially dissolved. This phenomenon is associated with ionic polarization and bond strength (Heller-Kallai & Rozenson, Reference Heller-Kallai and Rozenson1981; He et al., Reference He, Guo, Xie, Lin and Li2002). When Mg and Fe adopt an octahedral coordination with oxygen, the cation’s ability to polarize affects the ionic bond strength. The ionic radii of Fe and Mg are 0.055 and 0.072 nm, respectively, indicating that the order of polarization ability is Fe > Mg; thus, the resulting ionic bond strength with oxygen follows the order Fe–O > Mg–O.

Table 3. Al elemental analysis of the chlorite samples.

In minerals, the macroscopic dissolution rate of ions is typically related to the normalization of the specific surface area of reaction. Various reaction-specific surface area models have been proposed for different minerals: (1) the reaction-specific surface area is usually assumed to be the BET specific surface area of the mineral or proportional to the BET surface area (Ganor et al., Reference Ganor, Mogollon and Lasaga1999; Villanova-de-Benavent et al., Reference Villanova-de-Benavent, Cama, Soler, Domènech, Galí and Proenza2022); or (2) the reaction-specific surface area is limited to certain crystal faces or is mainly controlled by surface defects (Nagy, Reference Nagy1995). This work uses the BET surface area as the mineral–water interfacial reaction surface area. The measured BET specific surface areas were found to be 17.8, 4.2 and 3.5 m² g^–1 for lizardite, chlorite and talc, respectively. The specific surface area of lizardite is large (Lacinska et al., Reference Lacinska, Styles, Bateman, Wagner, Hall, Gowing and Paul2016), so the dissolution rates of Si and Fe are lower than those of chlorite after normalization (Fig. 3a,c). After 24 h of the mineral–water interfacial reaction, the lizardite dissolution rate of Mg was higher than those of chlorite and talc (Fig. 3b). With the progress of the talc–water interfacial reaction, Mg on the edge (end face) of the talc gradually dissolves. Furthermore, the difficulty of H⁺ attacking the (OH)^– on the octahedral sheet gradually increases. Therefore, the dissolution rate of Mg in the talc is lower than those of lizardite and chlorite after 12 h. During the mineral–water interfacial reaction, the dissolution rates of Si and Mg of chlorite were found to be always greater than those of talc. Furthermore, in the tetrahedral sheet, Al replaces Si; thus, Al occupies ∼35% of the tetrahedral sheet. When chlorite reacts with the sulfuric acid solution, Al on the tetrahedral sheet dissolves before Si, resulting in defect sites on the tetrahedral sheet. Then, H⁺ enters the mineral from these defect sites and reacts with the octahedral sheet, speeding up the dissolution rate of Mg (Rozalen et al., Reference Rozalen, Ramos, Gervilla, Kerestedjian, Fiore and Huertas2014). This also causes the Mg dissolution concentration of chlorite to be greater than that of talc. During the lizardite–water interfacial reaction, the dissolution rate of Mg in lizardite consistently exceeds that of Fe. Within the initial 72 h period of the chlorite–water interfacial reaction, the dissolution rate of Mg in chlorite surpasses that of Fe. Subsequently, after 72 h, the dissolution rate of Mg in chlorite falls below that of Fe.

Figure 3. (a) Si, (b) Mg and (c) Fe (c) dissolution rates of lizardite (Lz), chlorite (Chl) and talc (Tlc) in sulfuric acid solution.

The relative release ratio (RRR) of the dissolved ions to describe the stoichiometry of alkali feldspar dissolution at various pH values is calculated according to Equation 2 (Holdren & Speyer, Reference Holdren and Speyer1985):

(2)\begin{equation}{\text{RR}}{{\text{R}}_{{\text{Al}}/{\text{Si}}}} = {\left( {{\text{Al}}/{\text{Si}}} \right)_{{\text{solution}}}}/{\left( {{\text{Al}}/{\text{Si}}} \right)_{{\text{solid}}}}\end{equation}

where RRR_Al/Si is the RRR of Al to Si, such that its value is 1 for congruent dissolution, <1 when Si is released preferentially and >1 when Al is released preferentially. In lizardite, chlorite and talc, Mg could preferentially dissolve. In chlorite, Al could preferentially dissolve from the silica tetrahedral sheet compared to Si. This result is verified by relevant studies in the literature (Jurinski & Rimstidt, Reference Jurinski and Rimstidt2001; Kalinowski & Schweda, Reference Kalinowski and Schweda2007; Rozalen et al., Reference Rozalen, Huertas, Brady, Cama, Garcia-Palma and Linares2008; Bibi et al., Reference Bibi, Singh and Silvester2011). Then, the (Mg/Si) ionic dissolution concentration ratio of chlorite decreases slightly with increasing reaction time and gradually tends to the ratio of the stoichiometric coefficient, which is consistent with the literature (Hamer et al., Reference Hamer, Graham, Amrhein and Bozhilov2003). The (Mg/Si) ionic dissolution ratio of lizardite increases with increasing reaction time, indicating that the dissolution rate of Mg is consistently greater than that of Si during the mineral–water interfacial reaction (Fig. 4). At the initial stage of the reaction at the water interface, H⁺ preferentially reacts with Mg–OH at the edge of the octahedral sheet structure, and the dissolution rate of Mg is higher than that of Si. With the progress of the reaction, H⁺ further reacts with Mg–OH inside the octahedral sheet, slowing down the dissolution rate of Mg. Therefore, the (Mg/Si) ionic dissolution ratio of talc first increases and then decreases with increasing reaction time.

Figure 4. Changes in Mg/Si concentration ratios of lizardite (Lz), chlorite (Chl) and talc (Tlc) samples dissolved in sulfuric acid medium with reaction time.

Structural changes in lizardite, chlorite and talc during the mineral–water interfacial reactions in sulfuric acid solution

The characteristic diffraction reflection intensity of lizardite decreases with increasing reaction time after the mineral–water interfacial reaction in sulfuric acid medium (Fig. 5a). The characteristic reflection strength of chlorite decreased in the first 12 h after the mineral–water interfacial reaction but did not change significantly during the reaction from 12 to 168 h (Fig. 5b). The reflection diffraction intensity, Ic(002)/Ic(001), of the characteristic reflection strength of chlorite can generally be used to calculate the degree of conversion to vermiculite (Weaver, Reference Weaver1956; Rich, Reference Rich1968), such that the lower the ratio, the higher the degree of vermiculite. The strength ratio of Ic(002)/Ic(001) decreases after 72 h of reaction at the mineral–water interface, indicating that chlorite changes to vermiculite. The characteristic reflection intensity of talc hardly changed during the reaction (Fig. 5c).

Figure 5. XRD traces of (a) lizardite (Lz), (b) chlorite (Chl) and (c) talc (Tlc) after various reaction times in sulfuric acid medium.

The absorption reflections at 3690 and 3646 cm^–1 are attributed to the stretching vibrations of the external and internal hydroxyl groups, respectively. Mg–O(H) has a characteristic reflection peak at 610 cm^–1 (Chen et al., Reference Chen, Wang, Shi, Hu, Zhang and Li2023). The Si–O characteristic reflections at 1079 and 956 cm^–1 are attributed to the stretching vibration of the Si–O bond perpendicular to the structural layer and the Si–O–Si bond parallel to the structural layer, respectively (Fig. 6a; Lacinska et al., Reference Lacinska, Styles, Bateman, Wagner, Hall, Gowing and Paul2016; Beglaryan et al., Reference Beglaryan, Isahakyan, Zulumyan, Melikyan and Terzyan2023). The intensity of each characteristic reflection gradually decreases with increasing reaction time. This indicates that Mg and Si in the mineral are gradually dissolved, the structure is destroyed and the amorphous SiO₂ structure is transformed. The gradual disappearance of the reflection at 610 cm^–1 also confirms the dissolution of Mg in the lizardite structure (Liu et al., Reference Liu, Peng, Liu, Zhang and Wang2022). The chlorite reflections show Si–O bending and stretching and Mg–O(H) bending absorptions in the 1300–500 cm⁻¹ region (Meng et al., Reference Meng, Liu, Li, Zhang, Hu, Chen and Shi2018; Wang et al., Reference Wang, Zhang, Sun and Han2023). The bands at 987 cm^–1 were attributed to the absorptions of Si–O–Si and Si–O–Al (Madejová, Reference Madejová2003). The band at 762 cm^–1 was attributed to trioctahedral chlorite (Fig. 6b; Zhao & He, Reference Zhao and He2016). In the first 12 h after the reaction between chlorite and the sulfuric acid solution, the characteristic reflections of chlorite decrease with increasing reaction time. The characteristic reflection strength does not change significantly after the reaction at the water interface for 12 h. In talc, the stretching vibration reflection of Mg–OH at 3677 cm^–1, the Si–O stretching vibration reflection at 1018 cm^–1 (Kubicki et al., Reference Kubicki, Blake and Apitz1996; Madejová et al., Reference Madejová, Bujdák, Janek and Komadel1998) and the Si–O–Si bending mode at 669 cm^–1 (Yang et al., Reference Yang, Du, Hu, Yang, Tang and Avvakumov2006; Nied et al., Reference Nied, Enemark-Rasmussen, L’Hopital, Skibsted and Lothenbach2016) did not change significantly during the mineral–water interfacial reaction with the sulfuric acid solution (Fig. 6c), which is consistent with the XRD data.

Figure 6. FTIR spectra of (a) lizardite (Lz), (b) chlorite (Chl) and (c) talc (Tlc) as raw samples and after various reaction times in sulfuric acid medium.

Figure 7. SEM images: (a) original lizardite sample, (b) lizardite in sulfuric acid for 24 h and (c) lizardite in sulfuric acid for 72 h; (d) original chlorite sample, (e) chlorite in sulfuric acid for 24 h and (f) chlorite in sulfuric acid for 72 h; and (g) original talc sample, (h) talc in sulfuric acid for 24 h and (i) talc in sulfuric acid for 72 h.

Micromorphological changes of lizardite, chlorite and talc during reaction in sulfuric acid solutions

The lizardite sample (Fig. 7a) mainly presents a lamellar micromorphology, with some small mineral particles and rod-like lizardite attached to the surface. After the mineral–water interfacial reaction between lizardite and sulfuric acid for 24 h (Fig. 7b), the laminar main structure is retained, the outer surface is corroded, the large layer structure is broken down into small layers and a small amount of amorphous silica is attached to the mineral surface. After 72 h (Fig. 7c), the lizardite was significantly corroded, and most layered structures became amorphous silica aggregates. With increasing reaction time, the surface of lizardite was gradually destroyed. Chlorite originally had a smooth surface and whole structure (Fig. 7d). After 24 h of reaction (Fig. 7e), the chlorite layer structure collapsed and the edge was destroyed. After 72 h (Fig. 7f), the extent of corrosion increased, and obvious structural defects appeared. The dissolution of chlorite in acidic solution occurred at the grain edge and at cracks and structural defects (Ross, Reference Ross1969). For talc, after reaction for 24 h, the layered structure did not change significantly; the edge lamellae were attacked by H⁺, the structure was destroyed and corroded and part of the structure of talc collapsed. After 72 h, the lamellae were smaller and were found to be adhered to the surface. Talc showed no obvious structural changes compared to those observed with lizardite and chlorite. Thus, the acid resistance of talc was stronger than that of lizardite and chlorite.

The lizardite layer was obvious (Fig. 8a), indicating greater crystallinity. After 24 h of reaction (Fig. 8b), lizardite showed fewer lattice stripes, indicating reduced crystallinity. For chlorite, after 24 h of reaction (Fig. 8d), the decrease in the interlayer space indicates that the crystal layer structure was damaged and the structural layer collapsed and compressed. This is consistent with the SEM results for chlorite (Fig. 7e,f). Talc shows lattice stripes and high crystallinity (Fig. 8e), and the interlayer space decreased after 24 h of reaction (Fig. 8f).

Figure 8. TEM images: (a) original lizardite sample and (b) lizardite in sulfuric acid for 24 h; (c) original chlorite sample and (d) chlorite in sulfuric acid for 24 h; and (e) original talc sample and (f) talc in sulfuric acid for 24 h.

Mineral dissolution mechanism

The mineral–water interfacial reaction mechanisms of lizardite, chlorite and talc in acidic medium are summarized in Fig. 9. Lizardite has a TO layer structure with small particles. After H⁺ from the acidic medium enters the interlayer domain, it can react with the (OH)^– group on each octahedral sheet, and the Mg in the mineral structure can dissolve quickly. The Si–O bond in the tetrahedral sheet is stronger than the Mg–(O,OH) bonds in the octahedral sheet; thus, the dissolution concentration and dissolution rate of Si are smaller than those of Mg. The chlorite interlayer consists of (hydr)oxides, and the attack of H⁺ on the octahedral sheet can be carried out through three pathways: (1) at the Mg–(OH) octahedral sheet in the interlayer sandwich, where some Mg is replaced by Al; (2) at the Mg–(O,OH) octahedral sheet in the middle of the TOT layer, where the reaction proceeds from the outside to the inside; and (3) at the defect site generated due to the substitution of Si by Al in the silicon tetrahedral sheet, then Al is preferentially dissolved, resulting in more structural defects. Therefore, the dissolution rates of Si and Mg in chlorite are consistently higher than those of talc during the mineral–water interfacial reaction. Talc contains a TOT layer, wherein H⁺ enters the octahedral channel from the mineral edge and first reacts with Mg–OH at the edge, destroying the outer octahedral sheet and dissolving Mg. The talc–water interfacial reaction gradually occurred from the outside to the inside. As Mg is preferentially dissolved, the silica tetrahedral sheets on both sides gradually converge to the middle, blocking the reaction between H⁺ and Mg–OH. Therefore, talc is the most stable mineral regarding the mineral–water interfacial reaction in an acidic medium.

Figure 9. Schematic diagram of the mineral structures attacked by H⁺: (a) lizardite, (b) chlorite and (c) talc.