Near-Infrared Spectroscopy Study of OH Stretching Modes in Pyrophyllite and Talc

Abstract

Pyrophyllite and talc are both tetrahedra–octahedra–tetrahedra (TOT)-type phyllosilicates, but differences can be found in the stacking mode of the layers and the ion occupation. Fourier transform infrared spectroscopy was used to differentiate between pyrophyllite and talc. In the 400–600 cm⁻¹ region, pyrophyllite exhibits six peaks, while talc only exhibits five peaks. In the 1000–1200 cm⁻¹ region, pyrophyllite exhibits three clear peaks at approximately 1051, 1070, and 1121 cm⁻¹; while talc only exhibits one strong peak near 1020 cm⁻¹. The differences between pyrophyllite and talc in the near–infrared (NIR) region are clear in the 4000–4700 cm⁻¹ region, and pyrophyllite exhibits an intense peak around 4615 cm⁻¹, which is attributed to the combination of the OH and Si-O-Si stretching bands. Talc has a maximum peak located near 4324 cm⁻¹, which is attributed to the OH stretching vibration. In addition, talc has a secondary peak near 4366 cm⁻¹. Talc has two other weaker peaks around 4054 and 4180 cm⁻¹. The 7000–7250 cm⁻¹ region exhibits the first fundamental overtone of the OH group stretching vibrations. The common characteristic band of these two minerals is 7175–7183 cm⁻¹. The first overtone of OH stretching vibrations can also be generated by adjacent peaks in the fundamental overtones. The peaks of these two minerals around 7094 cm⁻¹ appear to be a combination of 3630 (±5) cm⁻¹ and 3642 (±3) cm⁻¹.The factor of the first fundamental overtone of the OH group stretching vibration is 1.95 (±0.003). Therefore, the characteristic peaks in the mid-infrared (MIR) and NIR regions can be used to distinguish between pyrophyllite and talc, providing a research basis for further exploration in related geological areas.

1. Introduction

Pyrophyllite (Al₂[Si₄O₁₀](OH)₂) and talc (Mg₃[Si₄O₁₀](OH)₂) are both 2:1 tetrahedra–octahedra–tetrahedra (TOT)-type phyllosilicates. Pyrophyllite has a dioctahedral layered structure in which a sheet of octahedrally coordinated Al ions is sandwiched between two sheets of linked SiO₄ tetrahedra [1], while talc consists of an octahedral layer with Mg²⁺ between two silicate tetrahedral layers, in which each SiO₄ shares three corners with the adjacent tetrahedra [2,3]. However, due to the lattice mismatch and ion replacement between the octahedral(O) sheets and tetrahedral(T) sheets [4], the two minerals exhibit different structures [5] (Figure 1). Pyrophyllite exists in aluminum-rich acid effusive rocks and tuff, it is also commonly associated with epithermal systems that produce gold mineralization. Talc is generally related to magnesium-rich ultramafic rocks, dolomite, and products of the metasomatism of dolomitic limestone. Pyrophyllite and talc are commonly used in industrial applications, such as ceramics and rubber [6]. Due to its high absorbability and floatability, talc can be applied in environmental remediation, especially in the treatment of heavy metal-contaminated sewage. Many carving crafts and seals are made of high quality pyrophyllite. Raman spectroscopy and electron microprobe analysis (EMPA) have been widely used to distinguish between pyrophyllite and talc in previous studies, but from the perspective of OH vibrations, infrared spectroscopy is a better choice. The vibrational properties of OH groups can reflect changes in the cationic environment, so detailed information about OH bond vibrations, together with the distribution of the OH groups and metal ions can easily be obtained by analyzing infrared spectroscopy data.

Infrared spectroscopy is highly efficient, fast, and non-destructive tool for determining local cationic environments and fine structural features. The presence, intensity, wavelength position, and shape of the observed absorption peaks are diagnostic of the types and amounts of the minerals that are present in the surface material [6]. The mid-infrared region (MIR, 400–4000 cm⁻¹ (25–2.5 μm)) reflects the basic vibration modes of the functional groups [7], while the near-infrared region (NIR, ordinarily 4000–12,500 cm⁻¹ (2.5–1 μm)) is often used to detect the vibration characteristics of the combination bands of metal ions and OH groups, as well as the overtone bands of water and some functional groups in minerals [8]. Any change in the crystalline structure can significantly affect the overtone vibrations. Madejova studied the infrared [9] and Raman [10] spectra of pyrophyllite and talc, however, there were no additional experiments. At present, the study on differentiating pyrophyllite and talc using infrared spectroscopy is very few and has not formed a theory of large-scale systems internationally. In this article, more differentiation methods (micro X-ray fluorescence, X-ray diffraction, and so on) were used to supplement and improve the previous infrared study comprehensively.

The purpose of this study is to identify the differences in the mid-infrared and near-infrared spectra of pyrophyllite and talc systematically, and to speed up and simplify the process of distinguishing between the two minerals. In addition, this differentiation method can quickly and correctly detect soil on alien planets in the future. Related results can be used in the fields of ceramics, archaeology, and geology. Furthermore, the assignment and combination of the OH group vibration peaks will be discussed later, and we will attempt to explain the origins of these bands in detail. The thorough description and explanation of phyllosilicates may be significant for further exploration in related geological areas.

2. Materials and Methods

2.1. Materials

Among plenty of massive pyrophyllite and talc samples, four samples were finally chosen to analyze in this study for their high purity. The four samples were provided by the National Infrastructure of Mineral, Rock, and Fossil Resources for Science and Technology (NIMRF). Pyrophyllite samples DH-D-4 and BY-PD-4 were collected from Qingtian, Lishui City, Zhejiang Province, China, which is a famous pyrophyllite origin due to the high degree of quality; and the origin of talc samples is unknown. All the samples that were analyzed in this study were natural and specially selected. The specific gravity of the pyrophyllite samples was around 2.81, and that of the talc samples was approximately 2.76. During the related tests, the samples were shattered in moderation, and a uniform portion of each sample, i.e., without other phases, was chosen for the experiments.

The phenomenon of isomorphic substitution is quite common in phyllosilicates, so element impurities are also common, which may have different degrees of influence on the results of spectral experiments. Micro X-ray fluorescence (μ-XRF) analysis was performed before further study, and the results showed that the contents of the major elements, such as Si, Mg (in talc), and Al (in pyrophyllite), were very close to the theoretical values and were distributed uniformly (Figure 2). The contents of the impurity elements were very low; most samples contained iron, and a small number of samples contained other elements, such as magnesium and chlorine (

Table 1. Mineral characteristics of the pyrophyllite (DH-D-4 and BY-PD-4) and talc (xs-2-2-9 and ys81-98) samples.

Sample	Minerals	Color	Specific Gravity (SG)	Isomorphism (ω%)
DH-D-4	Pyrophyllite	Yellow green	2.82	Mg/0. 19%, Fe/0.10%, Ti/0.05%, S/0.02%
BY-PD-4	Pyrophyllite	Yellow green	2.81	Mg/0.51%, Cl/0.25%, Fe/0.14%, S/0.02%
xs-2-2-9	Talc	White	2.76	Fe/0.10%, Cr/0.01%, Ti/0.01%
ys81-98	Talc	Light yellow	2.77	Fe/0.28%, Al/0.09%, Cl/0.02%, Ti/0.01%

). It is considered unlikely that the impurity elements affected the results of this study since their contents in the samples were very low (Table 1).

The ionic radius of Al (coordination number = 6) is 0.054, and that of Mg (coordination number = 6) is 0.072. There is a certain difference between the ionic radii of these elements. From the perspective of geometry, replacements between ions are more likely to occur when the ionic radii of the mutual substitution ions are similar; considering the difference in the ionic radii of Al and Mg, it is difficult for ions to achieve isomorphism in minerals. In nature, only complete isomorphism occurs in pyrophyllite and talc series minerals, so it is difficult to find minerals with compositions between the Al and Mg endmembers.

2.2. Methods

The X-ray diffraction (XRD) analysis was performed at room temperature (approximately 22 °C) using the Smart lab X-ray powder diffractometer at the Institute of Earth Sciences and Resources, China University of Geosciences, Beijing (CUGB). This system was equipped with a conventional copper target X-ray tube (set to 45 kV and 200 mA) and a graphite monochromator, with a stepping scanning mode, a scanning speed of 4°/min, and a step length of 0.02° in the range of 3–90°. The humidity was around 22%. An agate mortar was used to pulverize the samples to 200-mesh powder, and then the powder was immediately stored in plastic tubes to diminish contamination and oxidation. The results were normalized using the OriginPro 9.0 software and were analyzed using the MDI Jade 6.5 software and the International Center of Diffraction Database (ICDD).

The Fourier transform infrared spectroscopy (FTIR) data were acquired using the Bruker Tensor II spectrometer at the NIMRF. The temperature was approximately 20 °C, and the humidity was less than 22%. The spectra were collected in the frequency regions of 400–4000 cm⁻¹ (MIR) and 4000–8000 cm⁻¹ (NIR) using the KBr compression transmission method, the sample to KBr weight ratio was 1:150, and the resolution was 4 cm⁻¹. Each spectrum was averaged from 64 scans to enhance the signal-to-noise ratio. The OriginPro 9.0 software was used to remove the baseline and to normalize the NIR data.

The original FTIR spectra of the infrared hydroxyl vibration are usually analyzed by taking the second derivative to obtain accurate peaks, due to the small peak spacing and the coincidence of multiple spectral peaks. Therefore, highly efficient data processing software is necessary. The PeakFit v4.12 software can help determine the exact locations and forms of the component peaks. All of the spectra that were processed were fitted using a combined Gauss–Lorentz area function, with r² > 0.96.

3. Results

3.1. X-ray Diffraction

Figure 3 shows the XRD patterns of the four samples, as well as the powder diffraction file (PDF) cards for pyrophyllite and talc downloaded from the ICDD^® database. Samples BY-PD-4 and DH-D-4 are compatible with the features of pyrophyllite, exhibiting similar strong peaks at d(002) = 9.600–9.616° and d(006) = 29.099–29.100°. Samples xs-2-2-9 and ys81-98 are consistent with the characteristics of talc, exhibiting the same peaks within this range: d(002) = 9.421–9.479° and d(006) = 28.581–28.683°. The ranges of 18–22°, 37–41°, 54–58°, 60–64°, and 66–70° (gray area in Figure 3) are the obvious region for distinguishing between pyrophyllite and talc by observing the number and intensity of the peaks. Samples BY-PD-4 and DH-D-4 have four peaks within this range, while samples xs-2-2-9 and ys81-98 only have two. The four samples are verified to be pure minerals after comparison with the PDF cards.

3.2. MIR Characteristics

There are four characteristic spectral bands in the mid-infrared range that can be examined to differentiate between the two minerals: 400–600 cm⁻¹, 650–800 cm⁻¹, 850–1200 cm⁻¹, and 3500–3750 cm⁻¹. The spectral peaks are extremely compatible with the standard spectra of the minerals (Figure 4), i.e., with no other peaks [11].

The presence, intensity, wavelength position, and shape of the peaks in the low-frequency region of 400–1200 cm⁻¹ can be analyzed to differentiate between talc and pyrophyllite. In the 400–600 cm⁻¹ region, pyrophyllite exhibits six peaks around 419, 457, 482, 518, 540, and 575 cm⁻¹, while talc exhibits five peaks near 423, 439,452, 466, and 535 cm⁻¹. In the 650–950 cm⁻¹ region, pyrophyllite exhibits four weak peaks around 812, 834, 852, and 950 cm⁻¹, while talc only exhibits one strong peak near 781 cm⁻¹. In the 1000–1200 cm⁻¹ region, pyrophyllite exhibits three clear peaks at approximately 1051, 1070, and 1121 cm⁻¹, and talc exhibits one strong peak near 1020 cm⁻¹. More vibration modes can be attained in the 2800–3700 cm⁻¹ region (Figure 4 and Figure 5;

Table 2. The MIR band assignments of the pyrophyllite (DH-D-4 and BY-PD-4) and talc (xs-2-2-9 and ys81-98) samples (cm⁻¹).

Band Assignment	Pyrophyllite [12]	Talc [13]	DH-D-4	BY-PD-4	xs-2-2-9	ys81-98
Si-O bending vibration	422		419	419
O-H bending vibration		450			423 439 452	423 438 451
Si-O bending vibration	459		457	457
O-H bending vibration		465			466	465
Si-O bending vibration	480		482	482
Si-O-Al bending vibration	516		518	518
O-H bending vibration		538			535	535
Si-O-Al bending vibration	541		540	540
	578		575	575
OH bending vibration		669			668	669
OH bending		690			692	694
OH species		781			781	781
Al-OH bending vibration	812		812	812
	833		834	834
	853		852	852
	949		950	950
Si-O stretching vibration					1020	1020
Si-O-Si stretching vibration	1052		1051	1051
	1068		1070	1070
Si-O stretching vibration	1122		1121	1121
H2O band	1632		1634	1634
Overtone of SiO4 vibrational mode of 2ν3	2850	2850	2855	2853	2857	2855
	2925		2926	2926	2928	2928
O-H stretching vibration	3590		3590	3588	3584	3584
	3630	3623	3630	3630	3625	3624
	3645	3644	3642	3644	3644	3645
		3652			3655	3658
	3675	3676	3674	3674	3675	3675
	3700		3703	3703	3707	3707

).

3.3. NIR Characteristics

The characteristic NIR spectral bands of pyrophyllite and talc can be observed in the 4000–4700 cm⁻¹ region (Table 3 and Figure 6). The differences between the NIR spectra of pyrophyllite and talc are clear. The two pyrophyllite samples (DH-D-4 and BY-PD-4) exhibit strong peaks around 4615 cm⁻¹, which is assigned to the combination of the OH and Si-O-Si stretching bands [14]. In pyrophyllite, the peak around 4309 cm⁻¹ can also be observed, but the intensity of 4309 cm⁻¹ is weaker than that of 4615 cm⁻¹. The other two talc samples (xs-2-2-9 and ys81-98) have a maximum peak that is located near 4323 cm⁻¹, which is assigned to the OH stretching vibration [4]. The three weaker peaks of talc are located around 4056, 4180, and 4368 cm⁻¹.

In the 5100–5500 cm⁻¹ and 7000–7500 cm⁻¹ regions, masses of disordered but clear peaks appear in all four spectra. Bishop [15] pointed out that the small amount of H₂O molecules that are trapped at grain boundaries or connected with impurities in the samples may cause related bands to appear in these regions. Thus, it is likely that these signals are due to the water in the crystal structure rather than the impurity elements. Cheng [6] suggested that the various absorption peaks near 5260 cm⁻¹ and 7150 cm⁻¹ are caused by the unbound water in the interlayer structure of clay minerals, which is generally not assigned.

The 7000–7250 cm⁻¹ region belongs to the first fundamental overtone of the OH stretching vibration, and an obvious peak and multiple small peaks can be observed after fitting (Figure 7). In this region, the strong peak of pyrophyllite is located at around 7175 cm⁻¹, and that of talc is located near 7183 cm⁻¹. Among the weak peaks, the talc samples exhibit a peak at 7152 cm⁻¹, but the pyrophyllite samples do not. The peaks near 7294 and 7300 cm⁻¹ can be attributed to a combination of OH and H₂O stretching vibration [16].

Table 3. The NIR band assignments of the pyrophyllite (DH-D-4 and BY-PD-4) and talc (xs-2-2-9 and ys81-98) samples (cm⁻¹).

Assignment	Pyrophyllite [17]	Talc [18,19,20]	DH-D-4	BY-PD-4	xs-2-2-9	ys81-98
Combination of OH and Si-O-Si stretching band	4615		4615	4617
Combination of OH stretching and bending vibration		4055			4056	4056
	4186	4182	4185	4185	4180	4177
		4323			4323	4323
		4369			4368	4368
	6908	6922	6916	6916	6916	6925
The first fundamental overtone of OH stretching vibration		7073	7069	7070	7069	7076
			7094	7094	7094	7089
	7100		7106	7106	7105	7111
		7118	7118	7118	7118	7118
		7136	7139	7139	7139	7132
		7153			7152	7152
	7175	7185	7175	7175	7183	7183
	7201	7201	7204	7204	7204	7209
	7234		7231	7231	7231	7236
	7327	7327	7327	7327	7327	7327
	7340		7341	7341	7341	7342
Combination of OH and H2O stretching vibration	7294		7294	7294	7294	7289
	7300		7309	7309	7308	7302

Table 3. The NIR band assignments of the pyrophyllite (DH-D-4 and BY-PD-4) and talc (xs-2-2-9 and ys81-98) samples (cm⁻¹).

Assignment	Pyrophyllite [17]	Talc [18,19,20]	DH-D-4	BY-PD-4	xs-2-2-9	ys81-98
Combination of OH and Si-O-Si stretching band	4615		4615	4617
Combination of OH stretching and bending vibration		4055			4056	4056
	4186	4182	4185	4185	4180	4177
		4323			4323	4323
		4369			4368	4368
	6908	6922	6916	6916	6916	6925
The first fundamental overtone of OH stretching vibration		7073	7069	7070	7069	7076
			7094	7094	7094	7089
	7100		7106	7106	7105	7111
		7118	7118	7118	7118	7118
		7136	7139	7139	7139	7132
		7153			7152	7152
	7175	7185	7175	7175	7183	7183
	7201	7201	7204	7204	7204	7209
	7234		7231	7231	7231	7236
	7327	7327	7327	7327	7327	7327
	7340		7341	7341	7341	7342
Combination of OH and H2O stretching vibration	7294		7294	7294	7294	7289
	7300		7309	7309	7308	7302

4. Discussion

The relationships between the band frequencies of the MIR and NIR spectra were determined through trial-and-error summations. The obvious peak around 4615 cm⁻¹ in the pyrophyllite samples is a combination of the peaks around 3674 cm⁻¹ and 950 cm⁻¹. The peak at 3674 cm⁻¹ is attributed to O-H stretching vibration [21], and the peak at 950 cm⁻¹ is attributed to Al-OH in-plane libration [22]. In addition, the characteristic peak at around 4323 cm⁻¹ in the talc samples appears to be a combination of the peaks around 3655 cm⁻¹ and 669 cm⁻¹. The peaks at 3655 cm⁻¹ and 669 cm⁻¹ are both attributed to OH libration.

Table 4. The major NIR bands in the 4000–4700 cm⁻¹ region for the four samples and their corresponding MIR peaks (cm⁻¹).

Sample	Measured Peak	Fundamental Peaks	Theoretical Peak [24,25]	Δ
DH-D-4	4615	3674 + 950	4624	9
BY-PD-4	4617	3674 + 950	4624	7
xs-2-2-9	4056	3625 + 439	4064	8
	4177	3644 + 535	4179	2
	4323	3655 + 669	4324	1
	4368	3675 + 692	4367	1
ys81-98	4056	3624 + 438	4062	6
	4177	3645 + 535	4180	3
	4323	3658 + 669	4327	4
	4368	3675 + 694	4369	1

lists the detailed corresponding values for the four samples. The errors are less than 10 cm⁻¹ [23], and the strength is in agreement.

In the 7000–8000 cm⁻¹ region, the absorption corresponds to the first fundamental overtone of OH stretching vibration (2νOH) [26,27]. The most obvious peaks of the pyrophyllite samples are located near 7175 cm⁻¹, and that of the talc samples is located at 7183 cm⁻¹, which can be attributed to the OH stretching vibration [28,29].

Table 5. The major NIR bands in the 7000–7250 cm⁻¹ region for the four samples and their corresponding MIR peaks (cm⁻¹).

Sample	Measured Peak	Fundamental Peaks	Factor	Sample	Measured Peak	Fundamental Peaks	Factor
DH-D-4	7094	(3630 + 3642)/2	1.9510	BY-PD-4	7094	(3630 + 3644)/2	1.9505
	7118	3642	1.9544		7118	3644	1.9533
	7175	3674	1.9529		7175	3674	1.9529
	7231	3703	1.9527		7231	3703	1.9527
			Average 1.9528				Average 1.9524
xs-2-2-9	7094	(3625 + 3644)/2	1.9519	ys81-98	7089	(3624 + 3645)/2	1.9505
	7118	3644	1.9533		7118	3645	1.9528
	7183	3675	1.9546		7183	3675	1.9546
	7231	3707	1.9506		7236	3707	1.9519
			Average 1.9526				Average 1.9525

lists the detailed corresponding values for the four samples. In the 7000–7250 cm⁻¹ region, the peaks of the talc samples are similar to the peaks of the pyrophyllite samples. Pyrophyllite and talc both have a TOT structure. With only one type of OH bond, the structure is extraordinarily stable, so it is difficult for the OH vibration to be influenced. Based on the results of this study, it can be concluded that the peaks in the 7000–7250 cm⁻¹ region cannot be used to distinguish between pyrophyllite and talc.

It is generally accepted that the average factor between the OH stretching vibration and its overtone should be twofold. In this study, the factor was calculated to be 1.95 (±0.003) (Table 5), which is more accurate than simply rounding to twofold and is a bit less. This has been illustrated via quantum mechanics [30].

In addition to the fundamental overtone that is generated by the OH stretching vibration, pyrophyllite and talc both have bands that cannot match the single OH stretching vibration band one-to-one. To achieve better matching results, the combination of the double OH stretching vibration is more accurate than the single vibration. No obvious change was observed when comparing the NIR spectra of the four samples before and after drying, so the effect of the adsorbed water was excluded [8]. The peak at around 7094 cm⁻¹ appears to be a combination of 3630 (±5) cm⁻¹ and 3642 (±3) cm⁻¹. The peaks at 3630 (±5) cm⁻¹ and 3642 (±3) cm⁻¹ are attributed to O-H stretching vibration. Adjacent peaks can cause combined absorption, and this combination is related to the band position of the OH stretching vibration [8].

5. Conclusions

• XRD data show that the samples that were studied in this article are consistent with the characteristics of minerals in PDF cards, which can verify that these samples are pure minerals. The ranges of 18–22°, 37–41°, 54–58°, 60–64°, and 66–70° are the obvious regions for distinguishing between pyrophyllite and talc by observing the number and intensity of the peaks.
• Obvious differences were identified between the MIR spectra of pyrophyllite and talc samples. In the 400–600 cm⁻¹ region, pyrophyllite exhibits six peaks, while talc only exhibits five. In the 1000–1200 cm⁻¹ region, pyrophyllite exhibits three clear peaks at approximately 1051, 1070, and 1121 cm⁻¹; while talc exhibits one strong peak near 1020 cm⁻¹.
• The NIR spectral characteristic peaks of pyrophyllite and talc are located in the 4000–4700 cm⁻¹ region, and they are more obvious than the MIR spectral characteristic peaks. Pyrophyllite exhibits a strong peak at around 4615 cm⁻¹, which is attributed to the combination of the OH and Si-O-Si stretching bands. Talc exhibits a maximum peak that is located near 4323 cm⁻¹, which is assigned to the OH stretching vibration. In addition, the talc samples exhibit a secondary peak near 4366 cm⁻¹. Talc also contains two other weaker peaks at around 4056 and 4180 cm⁻¹.
• The OH stretching vibration can be combined with the OH bending vibration and Si-O vibration. The OH combination vibration of pyrophyllite is located at 4615–4617 cm⁻¹, which is a combination of the peaks around 3674 cm⁻¹ and 950 cm⁻¹. The peak at 3674 cm⁻¹ is attributed to O-H stretching vibration, and the peak at 950 cm⁻¹ is attributed to Al-OH in-plane libration. Additionally, 7000–7250 cm⁻¹ is the first fundamental overtone region for the OH stretching vibration, including an obvious absorption peak and various small peaks. Pyrophyllite and talc each have a few combined overtones of the OH stretching vibration, which are composed of adjacent OH stretching vibrations. The peaks of the two minerals at around 7094 cm⁻¹ appear to be a combination of the 3630 (±5) cm⁻¹ and 3642 (±3) cm⁻¹ peaks. The 7000–7250 cm⁻¹ region cannot be used to distinguish between pyrophyllite and talc.
• Due to the effect of the non-ideal conditions, the theoretical peak position of the overtone peak is higher than that of the actual position. The factor of the first fundamental overtone of the OH group stretching vibration is 1.95 (±0.003).