**3. Behavior of Minerals at Non-Ambient Conditions**

Comodi et al. [5] studied the transformation of the crystal structure of galenobismutite, PbBi2S4, under pressure up to 20.9 GPa. The structure undergoes reversible and completely elastic transitions. The size and the shape of Bi- and Pb-centered polyhedra suggest that the high-pressure structure of galenobismutite can host Na and Al in the lower mantle, which are incompatible with the periclase or perovskite crystal structures.

Hydrous coesite crystals, a high-pressure SiO2 polymorph, were synthesized with various B3<sup>+</sup> and Al3<sup>+</sup> contents and in situ high-temperature Raman and FTIR spectra were collected at ambient pressure by Miao et al. [6]. Crystals were observed to be stable up to 1500 K. Al substitution significantly reduces the H<sup>+</sup> concentration in coesite, so the mechanism is controlled by oxygen vacancies, while the B incorporation may prefer the electrostatically coupled substitution (Si4<sup>+</sup> = B3<sup>+</sup> + H+).
