Characterization of the heat-treated sericite from Son Binh area, Vietnam

1. Introduction

Sericite is a fine-grained, soft mica mineral with a structure similar to muscovite and illite. The chemical formula of sericite is defined as (K, Na, Ca) (Al, Fe, Mg)₂(Si, Al)₄O₁₀(OH)₂ with SiO₂ = 43-49%, Al₂O₃ = 27-37%, K₂O+Na₂O = 9-11%, H₂O = 4-6% (Ciullo, 1996). With many special properties such as lightness, flexibility, electrical insulation, impermeability, non-toxicity, inertness to the chemical environment, and blocking ultraviolet rays, sericite is considered an important and widely used industrial material such as polymers (Liang et al., 2019) paints (Kim et al., 2017), and high-quality ceramics (Brasileiro et al., 2022). In the field of ceramic production, as well as other related application fields, it is essential to study the physicochemical characteristics and technical properties of materials under natural conditions as well as under firing conditions (Wang et al., 2016; Liang et al., 2017; Abubakar et al., 2020). Elgamour et al. (2019) characterized two types of clay minerals from the Safi region, Morocco, at various firing temperatures (250℃-1100℃). These clays have different oxide and mineral compositions. Results indicated that, at different calcination temperatures, values such as the specific surface areas or the porosity of the calcined samples are different. Especially at 700℃, the clay supports made from both clays' porosity was maximal, with values of 23.45 ± 0.66% and 21.61± 0.60% (Elgamour et al., 2019). Natural kaolin samples from Shijiazhuang Minerals Co., Ltd. (China) were calcinated from room temperature to 400℃, 700℃, and 1050℃. Wang et al. (2011) concluded that during calcination at 600℃-900℃, metakaolin was formed. The surface area of the samples decreased with increased calcination temperature. Pore volume and size also increased with temperature (Wang et al., 2011). From the literature, it can be seen that depending on the mineral, and chemical composition of the raw materials, new materials with different properties will be formed by calcination. Therefore, it is necessary to study the physicochemical properties of the raw materials under the firing conditions for each different material before use.

Son Binh area, Ha Tinh province, Vietnam, has excellent potential for sericite. Recent studies have shown that the sericite ore in this area was formed by the hydrothermal metamorphism between rhyolite and their tuff belonging to the Lower Dong Trau sub-formation, in the period from 130.1 to 117.9 million years (Nguyen and Ngo, 2014). In the area, nine industrial sericite ore bodies were identified with the inferred mineral resource (333) and the reconnaissance mineral resource (334a) of about 1,565 million tons (Xuân et al., 2013). The quality of sericite here was quite good, meeting the production requirements in some primary fields (Nguyen et al., 2010; Xuân et al., 2013; Nguyen and Ngo, 2014; Nguyen, 2019). However, up to the present time, there have not been any studies on the characteristics of the Son Binh sericite at different firing temperatures.

In this study, the Son Binh sericite calcined at different temperatures of 300℃-1100℃ was characterized by X-ray diffraction (XRD), Fourier transformation infrared (FT-IR), thermogravimetric thermal analysis (TG)/differential thermal analysis (DTA), Scanning electron microscopy (SEM) - Energy-dispersive X-ray spectroscopy (EDS) and BET specific surface area.

2. Geological setting of the study area

The geological characteristics of the study area are pretty simple, including formations of sedimentary and metamorphic rocks of the Paleozoic - Mesozoic age (Xuân et al., 2013; Nguyen and Ngo, 2014; Nguyen, 2019). The rocks of the Song Ca Formation (O₃-S₁ sc) are widely distributed and mainly consist of weakly metamorphosed terrigenous sediments. The Huoi Nhi Formation (S₂-D₁ hn) is composed primarily of sandstone, shale, siltstone, sandstone, and sericite quartz schist. The Dong Trau Formation is divided into two sub-formations: The lower Dong Trau sub-formation (T₂a đt₁) is mainly composed of rhyolite rocks, rhyolitic tuff, tuffaceous sandstone, tuffaceous siltstone, quartz porphyry. The thickness of the lower sub-formation is 250-950 m. The upper sub-formation (T₂a đt₂) consists mainly of siltstone, brownish-purple shale, thin layers of sand and gray-brown gravel. The thickness of the upper sub-formation is 700-800 m. Intrusive rock in the area is mainly sub-volcanic porphyritic granite of the Song Ma Complex (γτT₂ sm₁). This complex's porphyritic granite is closely related in space and originates from the Dong Trau Formation volcanic rocks. The distribution of the intrusive and sedimentary rocks is shown in Fig. 1. In the study area, the northwest-southeast fault system is dominant. The Son Binh sericite mineralization is distributed in the lower Dong Trau sub-formation rocks. Sericite mineralization is discontinuously stretched, forming various bodies in a northwest-southeast direction. The total length of these sericite bodies is more than 4,000 m, and their width ranges from 50 m to 150 m.

Fig. 1.

Simplified geological map of the Son Binh area (Modified from Tran et al., 1996).

3. Methodology

4. Results and Discussion

4.1 Characteristics of the Son Binh sericite

4.1.1 Mineral composition

The X-ray diffraction (XRD) pattern of the Son Binh sericite sample (S0) in Fig. 5 indicates that the mineralogical compositions are ~35 wt.% sericite, ~10 wt.% pyrophyllite, ~5 wt.% kaolinite, ~5 wt.% feldspars with ~40 wt.% quartz, and other minor minerals, such as gibbsite. The XRD peaks of these minerals are consistent with other reports (Moon and Moon, 1996; Li et al., 2014; Liang et al., 2019). This result shows that, SBS has similar sericite content as others (González-Miranda et al., 2018). However, the quartz content in the SBS sample is relatively high. It may be due to the origin of sericite deposits related to magmatic activity in the area (Nguyen and Ngo, 2014).

4.1.2 Chemical composition

The chemical composition of the SBS was determined by XRF and presented in Table 1. The results show that silica and alumina are the principal oxides with the highest percentage in the SBS sample at 64.94% and 24.53%, respectively. Other minor amounts of oxides are titanium oxide (TiO₂) (0.25%) and total iron content (T.Fe) (0.12%). The amount of alkaline elements (Na₂O+K₂O) is 7.47%. The loss on ignition (LOI) in the SBS sample due to physical combined water, chemically combined water, and volatile matter accounts for 2.0%. It is similar to the chemical compositions of the Son Binh sericite reported in the literature (Nguyen et al., 2010; Xuân et al., 2013; Nguyen, 2019). In addition, it is also consistent with the results of SEM-EDS analysis with the peaks of Al, Si, K, Na, and Fe (Fig. 2).

Table 1.

Chemical composition of the SBS sample.

Fig. 2.

SEM image (left) and EDS point analysis result (right) of the SBS.

4.1.3 Morphology and particle size distribution

The scanning electron micrograph (SEM) image in Fig. 2 presents the typical sericite morphology with thin sheets, relatively uniform and minor in size. The result of the particle size analysis of the SBS sample shows that the sericite sample has a high proportion of fine particles with main sizes of about 13.75 μm (Fig. 3). Thus, it can be seen that the SBS has a relatively fine grain size that can meet the standards of many different applications.

Fig. 3.

Particle size distribution curve of the Son Binh sericite.

4.2 Characteristics of the thermal-treated SBS

4.2.1 Thermogravimetric thermal and differential thermal analysis

The thermal inspection of the SBS sample by differential thermal analysis (DTA) and thermogravimetric (TG) analysis was carried out in an inert atmosphere to 1100℃ at a heating rate of 5℃/min, and temperature (T) curves were simultaneously recorded. The DTA analysis of the SBS sample shows various reaction peaks (endothermic and exothermic) during heating to 1100℃ (Fig. 4). The first endothermic peak at about 100ºC is due to the removal of physical combined water present in the sample. The endothermic peak at about 575°C may associate with the characteristic SiO₂ (α) à SiO₂ (β) quartz phase transition or polymorphic transformation (González-Miranda, et al., 2018). The exothermic peak at about 1015°C corresponded to the crystallization of the mullite phase. This temperature is higher than the results of previous publications (Manoharan et al., 2012; González-Miranda et al., 2018). It may be due to the amount and conditions of transformation of sericite and quartz in the SBS sample, impacting on the thermal transformation. The TGA analysis shows various reductions in mass caused by the change of materials. Mass deductions for the SBS sample in the temperature range from 300℃ to 900℃ (about 5%).

Fig. 4.

Thermogravimetric thermal analysis (TGA-red line) and differential thermal analysis (DTA-blue line) thermal analysis of the SBS.

4.2.2 XRD Analysis

Fig. 5 shows the XRD patterns of the raw and the thermal-treated SBS samples (Fig. 5). The XRD patterns show the minerals in the SBS sample (S0) are sericite, pyrophyllite, kaolinite, feldspar, and quartz. The peaks representing these minerals are almost unchanged at the calcination temperature of 300℃ and 500℃, except for the disappearance of kaolinite peaks (S3 and S5). At 700℃, the intensity of these peaks tends to decrease, especially for sericite and pyrophyllite minerals (S7). That is, at this temperature, the structure of sericite and pyrophyllite fighting breaks down and transforms into a new phase. The most apparent change in minerals can be seen in the XRD pattern of the calcined sample at 110 0℃ (S11). Here, the sericite and pyrophyllite peaks disappeared, and the XRD pattern shows peaks of quartz and the new mineral mullite. These XRD results are consistent with the DTA analysis of the SBS sample above (Fig. 4) and other reports (Wang et al., 2016; González-Miranda et al., 2018).

Fig. 5.

XRD patterns of the SBS at different temperatures (at room temperature (S0), 300℃ (S3), 500℃ (S5), 700℃ (S7), 900℃ (S9), and 1100℃ (S11)). S-sericite, P-pyrophyllite, K-kaolinite, F-feldspar, Q-quartz, M-mullite.

4.2.3 FT-IR Analysis

The results of FT-IR analysis in Fig. 6 show the change of O-H, Si-O-Si, and Al-O-Al groups in the SBS samples at different temperatures (Fig. 6). It can be seen that the IR spectrum of the SBS sample at room temperature conditions (S0) is quite similar to previous studies for samples containing typical minerals of sericite, pyrophyllite, kaolinite, and quartz (Janek et al., 2009; Jovanovski et al., 2016; Tang et al., 2016; Selim et al., 2018). The absorption peaks at 3627 cm^-1 and 911 cm^-1 are assigned to the stretching vibration of the O-H group. The Si-O-Si stretching region comprises two absorption bands at 1061 cm^-1 and 998 cm^-1. The band at 834 cm^-1 is assigned to the stretching vibration of Al-OH (Janek et al., 2009). The stretching mode of Si (Al)-O and Si-O-Si (Al) are presented at the peaks of 797 cm^-1 and 754 cm^-1 (Tang et al., 2016). The band associated with SiO stretching of quartz was also observed at the height of 693 cm^-1 (Ekosse, 2005). The peaks at 531 cm^-1 and 473 cm^-1 reflect the bending mode of Si-O-Si groups (Jovanovski et al., 2016; Selim et al., 2018). As can be seen, all peaks in the spectrum are unchanged under calcination conditions at 300℃ and 500℃. However, at the firing temperature of 700℃, the O-H group's vibration peaks gradually disappeared. The Si-O-Si stretching region at 1061 cm^-1 and 998 cm^-1 moved to 1051 cm^-1 and 950 cm^-1, respectively. The split peaks shifted from 754 cm^-1 to 718 cm^-1, 531 cm^-1 to 558 cm^-1, and 473 cm^-1 to 485 cm^-1, respectively. In addition, a 655 cm^-1 peak of Si-O appeared and disappeared at 1100℃. At 1100℃, the apparent spectrum at 558 cm^-1, 693 cm^-1, and 797 cm^-1 represented the existence of the mullite phase. Thus, it can be seen that the shifts and appearances of the peaks in the IR pattern are due to the change of mineral phases during calcination, and it is consistent with the results of the XRD analysis (Fig. 5).

Fig. 6.

FT-IR graphs of the SBS at different temperatures (at room temperature (S0), 300℃ (S3), 500℃ (S5), 700℃ (S7), 900℃ (S9), and 1100℃ (S11)).

4.2.4 SEM observation

The SEM images of the samples of S0, S3, S5, S7, S9, and S11 are shown in Fig. 7. From the SEM images, it can be seen that sericite and pyrophyllite have fairly straightforward layered structures at room temperature (S0). The layered structure of these plates is almost unchanged when the sample is calcined at 300℃-500℃ (S3-S5). However, it changed and gradually disappeared as the heating temperature increased from 700℃ to 1100℃ (S7-S11). In particular, SEM images for samples calcined at 1100℃ showed that the sericite minerals were almost melted and replaced by the mullite material. The morphological change of minerals according to calcination temperature is consistent with other analytical results such as XRD, FT-IR, and TG-DTA.

Fig. 7.

SEM images of the thermal-treated SBS at different temperatures (at room temperature (S0), 300℃ (S3), 500℃ (S5), 700℃ (S7), 900℃ (S9), and 1100℃ (S11)).

4.2.5 Physical properties of the thermal-treated SBS

Fig. 8 shows the adsorption-desorption isotherm of N₂ at liquid N₂ temperature of the SBS samples at 300℃, 500℃, 700℃, 900℃, and 1100℃. It can be seen that the isotherm shapes of the thermal-treated SBS at different temperatures are similar and show an almost reversible Type II (Fig. 8). This shape indicates that the thermal-treated SBS samples are dominantly macroporous (Kuila and Prasad, 2013).

Fig. 8.

Nitrogen adsorption/desorption isotherms of the thermal-treated SBS at different temperatures (at room temperature (S0), 300℃ (S3), 500℃ (S5), 700℃ (S7), 900℃ (S9), and 1100℃ (S11)).

The BET surface area, pore volume, and pore size of the thermal-treated SBS samples are listed in Table 2. It can be seen that all four values of the BET surface area, the BJH adsorption cumulative surface area of pores between 1.7 nm and 300 nm width, the BJH adsorption cumulative volume of pores between 1.7 nm and 300 nm width, and the BJH adsorption average pore width (4V/A) of the SBS samples decreased with an increase in the calcination temperature. This suggests the influence of calcination temperature on the binding of minerals in the compressed SBS sample with a pressure of about 40 MPa. This binding increases gradually with the rise of the calcination temperature.

Table 2.

Physical properties of the thermal-treated SBS.

5. Conclusion

In this study, X-ray fluorescence analysis (XRF), X-ray diffraction (XRD), Fourier transformation infrared (FT-IR), thermogravimetric thermal analysis (TG)/differential thermal analysis (DTA), Scanning electron microscopy (SEM) - Energy-dispersive X-ray spectroscopy (EDS) and BET specific surface area were used for characterization of the heat-treated Son Binh sericite samples. The sericite was pressed at 40 MPa and calcined from 300℃-1100℃. The analysis results show that the Son Binh samples are mainly layer-form sericite, pyrophyllite, and kaolinite, with an average particle size of about 13.75 μm. The sample's chemical compositions are primarily SiO₂ (64.94%) and Al₂O₃ (24.53%), followed by (Na₂O+K₂O) (7.47%), TiO₂ (0.25%), and total iron content (0.12%). The morphological properties of sericite were not changed during calcination at temperatures of 300℃ to 900℃. The XRD and FT-IR analysis results show a change in mineral phases at the calcination levels from 500℃ to 700℃ and 900℃ to 1100℃. At 1100℃, the appearance of mullite can be attributed to the transformation of sericite or other minerals in the sample. The BET surface area, pore volume, and pore size of the thermal-treated sericite samples decreased with an increase in the calcination temperature. The results presented in this study can be important information for manufacturers using the SBS sericite to produce traditional ceramics or other advanced materials.

Acknowledgments

This research was financially supported by the Hanoi University of Mining and Geology (HUMG) - Vietnam, under grant number T22-25.

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