*2.2. Single-Crystal X-ray Di*ff*raction (XRD)*

A single grain (with a diameter of 100–120 μm) from each synthetic sample was selected for XRD at ambient conditions. The unit-cell parameters (Table 2) were refined on a Rigako XtalAB mini diffractometer (Rigaku, Akishima, Japan) with a 600-W rotating Mo-anode X-ray source, which is operated at 50 kV and 20 mA. A Saturn 724 HG CCD detector (with a resolution of 1024 × 1024) was mounted on this diffractometer. The average wavelength of Mo *K*α1–*K*α<sup>2</sup> was calibrated to 0.71073 Å, and intensity data were collected in the 2θ scanning range of up to 52◦. The refinements of atomic positions (Table 3) and anisotropic displacement parameters (Table S1) were conducted using the software package of CrysAlisPro/Olex2 [33]. The data collection parameters are also listed in Table 2, including the numbers of the measured equivalent and unique reflections, as well as the model fit values for Goof, *R*1, and *R*int. For all these synthetic single crystals, the Goof parameters remain below 1.1, while the *R*<sup>1</sup> and *R*int values are lower than 2.9% and 1.5%, respectively. The Si4<sup>+</sup> [34] and O2<sup>−</sup> [35] ionic scattering factors were adopted, and the Si1 and Si2 occupancies were fixed at 1 (full) during the structural refinement procedures.


**Table 2.** Intensity data collection and unit-cell parameters for the synthetic coesite samples.

The unit-cell angles: α = γ = 90◦.
