*2.3. Vibrational Spectra at Room and High Temperatures*

Single grains of a diameter less than 150 μm were chosen for in-situ high-temperature Raman measurement, using a Horiba LabRAM HR Evolution system (HORIBA JobinYvon S.A.S., Paris, France) with a Ar<sup>+</sup> laser excitation source (λ = 532 nm) and a micro-confocal spectrometer. Each crystal piece was loaded on a sapphire plate in a Linkam TS 1500 heating stage (Linkam Scientific Instruments Ltd., Tadworth, Surrey, UK). High temperatures were generated by a resistance heater from 300 K up to 1500 K, with an increment of 50 K and a heating rate of 20 K/min. To further test the temperature dependence of these lattice vibrational modes, we also chose another grain from R503 for low-temperature Raman measurement. The sample piece was loaded on a sapphire window in a Linkam THMS 600 heating/cooling stage, and low temperatures were cooled down to 80 K by liquid nitrogen with a cooling rate of 15 K/min. The temperatures were automatically controlled with uncertainties less than 5 K. Each target temperature was maintained at least 5 minutes before measurement to guarantee thermal equilibrium.

To analyze the water contents in these synthetic coesite samples, 7–9 cleaned crystal pieces (in a diameter of 100–160 μm) were selected from each sample source for Mid-FTIR measurement at ambient condition. All these crystals were double-side polished to a thickness of 60–80 μm before measurements, and the water contents for each of these coesite samples are estimated as an average of these measured pieces in the following discussion. The IR spectra were collected using a Nicolet iS50 FTIR instrument (Thermofisher, Madison, WI, USA) coupled with a Continum microscope, a KBr beam-splitter, and a MCT-A detector cooled by liquid N2. For in-situ high-*T* FTIR measurement, four polished sample pieces (R503, R663, R694 and R749) was selected and loaded at the sapphire window of a custom HS1300G-MK2000 external heating stage (INSTC, Boulder, CO, USA). The FTIR spectra were obtained in the wavelength range above 3200 cm<sup>−</sup>1, with a resolution of 4 cm−<sup>1</sup> and an accumulation of 256 scans. Temperatures were measured from room temperature to about 1200 K with an interval of 50 K and a heating rate of 15 K/min. Background was also obtained after the measurement on the sample for each step.


**Table 3.** Refined atomic position coordinates.

The O1 site is at the inversion center with *x* = *y* = *z* = 0.
