**4. Conclusions**


**Supplementary Materials:** The following are available online at http://www.mdpi.com/2073-4352/9/12/642/s1, Figure S1. Selected high-temperature Raman spectra as well as the pattern taken when quenched to room temperature for the samples of (A) R663, (B) R712 and (C) R749. Figure S2. The frequencies for the Raman-active modes as a function of temperature for the sample of *R712*: (A) 0–400 cm<sup>−</sup>1, (B) 400–800 cm−<sup>1</sup> and (C) 800–1200 cm<sup>−</sup>1. Linear regression is fitted for each dataset. Figure S3. The frequencies for the Raman-active modes as a function of temperature for the sample of *R663*: (A) 0–400 cm<sup>−</sup>1, (B) 400–800 cm−<sup>1</sup> and (C) 800–1200 cm<sup>−</sup>1. Linear regression is fitted for each dataset. Figure S4. The frequencies for the Raman-active modes as a function of temperature for the sample of *R749*: (A) 0–400 cm<sup>−</sup>1, (B) 400–800 cm−<sup>1</sup> and (C) 800–1200 cm<sup>−</sup>1. Linear regression is fitted for each dataset. Figure S5. Representative FTIR spectra obtained at high temperatures as well as when quenched to room temperature for the samples of (A) R663, (B) R694 and (C) R749. Table S1. Anisotropic displacement parameters (Å2) for the synthetic coesite samples in this study. Table S2. The frequencies of the Raman-active modes at ambient condition, as well as the temperature dependence and γ*iP* parameters. Table S3. The frequencies of the OH bands by FTIR measurement at ambient temperature if not noted, as well as their temperature dependence. The cif files are for the single-crystal structure refinements for the five coesite samples (R503, R663, R694, R712 and R749).

**Author Contributions:** Conceptualization, Y.Y.; methodology and investigation, Y.M., Y.P., D.L., X.W., and X.Z.; writing—original draft preparation, Y.M.; writing—review and edition, Y.Y., J.R.S., and J.Z.; visualization, Y.M. and Y.Y.; supervision, Y.Y.; project administration, Y.Y.; funding acquisition, Y.Y. All authors discussed the results and commented on the manuscript.

**Funding:** This research was funded by the National Key Research and Development Program of China (Grant No. 2016YF0600204), the National Natural Science Foundation of China (Grant Nos. 41590621 and 41672041).

**Acknowledgments:** The multi-anvil press synthesis, fs-LA–ICP-MS, EPMA, and high-temperature Raman measurements were conducted at China University of Geosciences (CUG) at Wuhan; the single-crystal X-ray diffraction experiments were carried out at Huazhong University of Science and Technology (HUST); while the high-temperature FTIR spectra were collected at Zhejiang University. Many thanks to Tao Luo (CUG), Yan Qin (HUST), and Xiaoyan Gu (Zhejiang University) for their experimental assistance.

**Conflicts of Interest:** The authors declare no conflict of interest.
