Insights into the water status in hydrous minerals using terahertz time-domain spectroscopy

The determinations of water status incorporated in hydrous minerals are of considerable significances in geoscience fields. Coincidentally, the aqueous sensitivity of terahertz radiation has motivated numerous explorations in several cross-domain applications. Terahertz time-domain spectroscopy is employed as a major probing technique coupling of traditional detecting methods to uncover the mask of water status in copper sulfate pentahydrate as well as mineral quartz in this article. Based on the quantitative identification of water status in copper sulfate pentahydrate, the water incorporated in mineral quartz is verified qualitatively. Notable differences of optical constants originating from the water content are obtained for copper sulfate pentahydrate and mineral quartz. These present works indicate that terahertz technology can be considered as a promising method to satisfy the ever-increasing requirements in hydrous mineral analyses.

www.nature.com/scientificreports www.nature.com/scientificreports/ has prevailed in biology [10][11][12][13][14] ; oils 15,16 ; and medical [17][18][19] because it can provide sufficient optical information of molecular level interactions by obtaining amplitude and phase data simultaneously 20,21 . THz-TDS adopts the terahertz-wave detection with higher signal-to-noise ratio (SNR) locating in frequencies (0.1-10 THz) between the far infrared and microwave 22,23 . Terahertz wave is of lower energy 24,25 , and can be absorbed by the LFM of water molecules [26][27][28][29] . Moreover, water molecules' dynamics driven by diverse types of thermally excited intermolecular motions are so strong that the network of hydrogen bonds (blue dashed lines in the enlarged part of Fig. 1) rearranges on sub-picosecond (sub-ps) time scales, which is in agreement with the bandwidth of terahertz wave. Consequently, THz-TDS can be deemed as an ideal tool for sensitive water determinations of hydrous minerals.
Here, we perform both the quantitative assignments of water molecules in copper sulfate pentahydrate (CuSO 4 ·5H 2 O) and the characterizations of trace amounts of hydrous components that occur in mineral quartz using THz-TDS as well as other conventional analytical methods. On the one hand, it reports the terahertz frequency-dependent absorption coefficients and refractive indices of CuSO 4 ·5H 2 O at various temperatures, while the water content of CuSO 4 ·5H 2 O is quantitatively identified. These important optical constants manifest high correlations with the number of crystal water molecules. On the other hand, it qualitatively characterizes the water content of mineral quartz by enabling the combinations of THz-TDS with PXRD and Fourier transform infrared (FTIR) spectroscopy. The results encouragingly show that the temperature-dependent water content of mineral quartz can be characterized at terahertz frequencies. It is discovered that the dehydration process in mineral quartz presents the dependence of frequencies. These works reveal that THz-TDS will act as a potent candidate for quantitatively and qualitatively analyzing the water content and status.  Fig. 2a. In contrast to the reference signal, the signals of four selected temperatures are time delayed and amplitudes are decayed due to the sample refraction and absorption. Almost the appearance of no phase shift under four temperatures ensures that THz-TDS is not to be distorted by the thickness of the sample. The amplitude variances of THz-TDS at different temperatures are clearly observed in the inset. Upon increasing the temperature from 23 to 260 °C, the amplitudes are seen to decrease. As a matter of fact, the amplitude of the transmission terahertz time-domain signal is proportional to the water content. In CuSO 4 ·5H 2 O, crystal water molecules either coordinate with copper ions or form hydrogen bonds 30 . The variances of terahertz absorptions are mainly caused by two contributions. One is that the increase in temperatures brings about the decomposition www.nature.com/scientificreports www.nature.com/scientificreports/ in crystal water. Another is that the status of hydrates of copper sulfate pentahydrate has various crystal symmetry and coordination. The variances of dielectric properties will affect the optical responses to the terahertz wave 31 . Moreover, it is noted that the amplitude of the temporal signal of 260 °C attenuates significantly compared to the reference owing to the intrinsic absorption of anhydrous copper sulfate. Figure 2b shows the Fourier transforms of Fig. 2a in the time range from 0 to 16 ps. The corresponding amplitudes of THz-FDS augment as temperatures increase. The spectral range of the reference is shown up to about 3.0 THz while it narrows down to about 2.4 THz for 100 and 260 °C as well as 2.0 THz for 23 and 60 °C owing to the different substantial absorption of the samples. The spectral range of each hydrate extends to a little higher frequency than before upon the dehydration. Figure 3 shows the frequency-dependent absorption coefficients and refractive indices of CuSO 4 ·5H 2 O in the temperature range from 23 to 260 °C. One can see the same trend as the temporal signals for both the absorption coefficients and refractive indices. They all decline upon the dehydration of crystal water molecules. Figure 3a shows that the absorption coefficients of four temperatures increase rapidly, especially the temperature of 23 and 60 °C at high frequencies. The value variances of absorption coefficients at 0.8 THz and 1.6 THz are recognized in the inset. The nonlinear slopes of the two curves denote that the rate of the dehydration process is various. It is observed that with the ambient temperature increasing from 23 to 100 °C, absorption coefficients at 0.8 THz vary from 8.1 to 5.5 cm −1 . At 1.6 THz, absorption coefficients descend from 46.9 to 22.0 cm −1 . This phenomenon illustrates that the water molecules are more sensitive to the higher terahertz frequency. They require higher energy to overcome the hydrogen bond binding, and start to appear as the mode of intermolecular vibrations under the driving of the stronger terahertz radiation. Theoretically, the number of crystal water molecules of copper sulfate pentahydrate can be regarded as the crystal water content under the macro case at different temperatures. During the heating process, the number of dehydrated water molecules follows the order of 2, 2 and 1. The absorption coefficients and the crystallization water content are in accordance with Lambert Beer's law. In experiments, CuSO 4 ·5H 2 O transforms into copper sulfate trihydrate (CuSO 4 ·3H 2 O) firstly upon heated from 23 to 60 °C. Another two water molecules are lost to form copper sulfate hydrate (CuSO 4 ·H 2 O) when heated to about 100 °C. The CuSO 4 ·H 2 O is converted to the copper sulfate (CuSO 4 ) till the temperature increases to about 260 °C. Those experimental absorption coefficients agree well with that of above discussed theory (Supplementary Materials). And monotonous temperature dependence of the refractive indices is observed in Fig. 3b. The lower refractive index occurs at the temperature of 260 °C. The curves' shapes continue to have a similar trend over all frequency bands.

Results and Discussions
The identical temperature-dependent trend between refractive index and absorption coefficients provides solid evidence that the two optical parameters arise from same mechanisms in CuSO 4 ·5H 2 O. In summary, THz-TDS has different feedback on four kinds of hydrates of CuSO 4 ·5H 2 O. The strong temperature-dependent absorption coefficients and refractive indices at terahertz frequencies can be used as enhanced quantitative clarifications of CuSO 4 ·5H 2 O and its other three hydrate forms.
Additionally, in order to exclude the effect of dielectric properties on terahertz absorptions, anhydrous copper sulfate is measured at different temperatures as shown in Fig. 4. It can be seen from Fig. 4(a) that the time-domain spectra of four temperatures are basically the same. Almost no distinction is observed in frequency-domain spectra (b); absorption coefficients (c) and refractive index (d). Those results prove that the dielectric properties of anhydrous copper sulfate do not vary in the temperature range from 23 to 260 °C.   be concluded that other similar minerals of which the chemical formulas contain a certain number of water molecules are likely to be characterized using THz-TDS. In this section, the mineral quartz will be determined by THz-TDS combining PXRD and FTIR to explore and analyze hydrous components. Mineral quartz is an earth-abundant mineral form of tetrahedrally bonded silica, and is one kind of NAMs first studied for the influence of water on its mechanical strength 32 . Compared to the crystal quartz, mineral quartz presents higher absorption coefficients at terahertz bands. (Supplementary Materials) The characterizations of water content in mineral quartz would pave the way for subsequent researches on other NAMs.
In order to characterize the water status of mineral quartz at different temperatures, PXRD results presented in Fig. 5a are used to analyze the crystalline degrees. Sharp diffraction characteristic peaks are exhibited in five plots, which indicate that the mineral quartz is highly crystalline. The positions and intensities of peaks have not altered significantly from 23 to 900 °C. Only a minor diverge is observed as shown in the inset. This suggests that mineral quartz has basically identical crystal form at those temperatures. With the temperature increasing from 600 up to 900 °C, the crystal gradually transforms from α-SiO 2 to cristobalite as shown in the enlarged part. However, it is worth mentioning that a great variance occurs under 1200 °C. It indicates that a new substance "cristobalite" has been formed at such a high temperature 33 .
To directly confirm the water status in mineral quartz, FTIR results of the samples at different temperatures are shown in Fig. 5b 34 . Broad asymmetrical stretching vibration bands of -OH at 3435 cm −1 are noticed at 23 and 300 °C, which are attributed to the HFM of water molecules. The spectrum of 300 °C has a little difference with that of 23 °C. But the intensities of -OH become weaker than that of 23 °C. This is because the water content in the sample has been gradually reduced by raising temperatures. With the temperature increasing to 600 °C, the asymmetric stretching vibration bands of -OH at 3435 cm −1 have disappeared. It means that water has been removed completely at such an ambient temperature. The spectrum of 900 °C presents almost the same position of the bands as that of 600 °C. At 1200 °C, the bands of Si-O at 650 cm −1 start to shift to a lower wavenumber of 643 cm −1 . And two split bands of Si-O have merged into one at around 470 cm −1 . Based on those PXRD results, it is known that the mineral quartz has been converted into cristobalite under this case. www.nature.com/scientificreports www.nature.com/scientificreports/ The LFM of water and thermal formation within mineral quartz are characterized using THz-TDS as shown in Fig. 6a. It can be seen that although the phase delay occurs at 23 and 300 °C, the distortions of effective terahertz absorptions can be negligible. The tendency remains consistent with that of Fig. 2a. The higher the temperature, the higher the detected intensity of the temporal signal (inset in Fig. 6a). From the amplitudes especial of 300, 600 and 900 °C, it shows that the amplitudes relate with the temperature due to the variance of water content. This is in agreement with the FTIR spectra in Fig. 5b. It is also found that the amplitudes of THz-TDS become intense with the temperature increasing from 600 to 900 °C, even to 1200 °C. This can be attributed to the transformation of the structure of mineral quartz at such a high temperature. The similar amplitudes of 900 and 1200 °C suggest that water content no longer decreases and the structure of mineral quartz tends to stabilize till the cristobalite is formed under such conditions.
The amplitudes of THz-FDS reduce upon the increase of temperatures as shown in Fig. 6b. Compared to that of CuSO 4 ·5H 2 O, the spectral limits of mineral quartz at five temperatures decay to just about 2.6 THz. The differences of the absorption between high and low temperatures are not obvious. This indicates that the relative steady structure of mineral quartz is not easily transformed with the increase of the temperature. Figure 7 demonstrates the corresponding frequency-dependent absorption coefficients and refractive indices of mineral quartz. The absorption coefficients presented in Fig. 7a can be well interpreted by water content. In addition, the absorption coefficients of five temperatures show a nonlinear increasing over all frequencies. At lower frequencies below 1.6 THz, the absorption coefficients of 900 and 1200 °C increase slightly with the increasing frequency, which corresponds to the little drop in the absorption of the terahertz temporal signals (Fig. 6a). Notably, because the bending vibrations of the hydroxyl group of mineral quartz are more pronounced at higher frequencies 35,36 . The absorption coefficients are rather difficult to distinguish from 1.6 THz. As the decomposition of water has been realized completely by heating to 900 and 1200 °C, absorption coefficients tend to maintain invariable at higher frequencies. To clearly present the differences between high and low frequencies, the absorption coefficients at 0.8 THz and 1.6 THz are selected as shown in the inset. The absorption coefficients at 1.6 THz attenuate more than that of 0.8 THz, which is similar to the situation of Fig. 3a. The refractive indices in Fig. 7b show that mineral quartz of 1200 °C is more transparent to the terahertz radiation than that of other temperatures. Meanwhile, the similar refractive indices in the case of 23 and 300 °C illustrate the same status of mineral quartz during this heating process. The uniform trend of refractive indices suggests that mineral quartz at different temperatures does not seem to have specific absorption bands that could be attributed to any features.
Similarly, as shown in Fig. 8, terahertz spectra present approximately same values for anhydrous mineral quartz at different temperatures. Consequently, the effect of dielectric properties of dehydrated mineral quartz could be neglected.

Conclusions
In conclusion, based on THz-TDS, we have demonstrated the quantitative water determination of CuSO 4 ·5H 2 O within the temperature range from 23 to 260 °C. Based on such results, water status incorporated in mineral quartz is characterized. The presented results show that in terms of hydrous minerals, the dehydration can be analyzed by the optical parameters of absorption coefficients and refractive index, even accompanying the structural transformation. This proposed study not only highlights the effectiveness of THz-TDS in clarifying the www.nature.com/scientificreports www.nature.com/scientificreports/ mechanism of the dehydration process of hydrous minerals, but also provides a potential in estimating the water reservoir of mineral system.

Methods
Sample preparations. CuSO 4 ·5H 2 O crystals with high purity (≥99.99%) were used as raw materials in our experiments. Mineral quartz was the original mineral mined from Xinjiang area in China. Two kinds of minerals were crushed into powders by a grinder and the particle sizes approximate 75 μm were determined by a 200-mesh sieve to decrease the scattering absorption arising from the particles scattering to the terahertz wave. By comparing the literature data, when heating CuSO 4 ·5H 2 O crystals, it will lose two water molecules at 60 °C firstly. And another two is at 100 °C, then the last one is at 260 °C 25 . In order to get trihydrate, monohydrate, and anhydrate, CuSO 4 ·5H 2 O powders were heated to 60, 100 and 260 °C by the muffle furnace successively, with heating rate of 10 °C/min, and kept for 30 min to complete the phase transition. Similarly, heating treatments were applied to mineral quartz powders, but the temperatures of 300, 600, 900, and 1200 °C were selected. Subsequently, anhydrous copper sulfate and anhydrous mineral quartz were heated to corresponding temperatures as CuSO 4 ·5H 2 O and mineral quartz powders. Finally, all the sample powders were cooled down to room temperature.

THz-TDS.
After the dehydration process, sample powders were compressed into about 1 mm thickness pellets with the diameter of 13 mm to be measured using THz-TDS. Each sample had a mass concentration of relevant powders of 0.2 g/(0.2 g + 0.1 g) ≈ 66.7%. The relevant powders (such as 0.2 g copper sulfate pentahydrate powders or mineral quartz powders) and 0.1 g polytetrafluoroethylene (PTFE) were uniformly mixed.
The THz-TDS experiments were performed with a transmission configuration at room temperature, and the spectrometer was continuously purged with nitrogen to minimize absorption from atmospheric water and keep the humidity less than 5% as depicted in Fig. 1. Initially, a mode-locked femtosecond Ti-sapphire laser with the central wavelength of 800 nm, 100 fs pulse duration and 80 MHz pulse repetition rate was used for the terahertz radiation generation and detection. Then the femtosecond laser was divided into two beams by the beam-splitter (BS), one of which was used as the pump beam with the higher power while the other as the probe beam with the lower power. The pump beam passing through the time-delay stage excited the photoconductive GaAs crystal to generate terahertz radiation. After being focalized and reflected by a set of parabolic mirrors (PM1-PM4), the collimated terahertz radiation transmitted the sample carrying sample information. Finally, the probe beam with the pump beam reached the ZnTe detecting crystal at the same time after transmitting through the silicon wafer. The thickness of the ZnTe detecting crystal is 2 mm and the orientation is <110>. The timing between the probe and