Quartz family of silica polymorphs: Comparative simulation study of quartz, moganite, and orthorhombic silica, and their phase transformations

A quartz family of silica polymorphs is studied. The structure and phase transformations of quartz, moganite, and orthorhombic silica ($o$-silica) are examined using molecular dynamics simulations. Moganite and its high-pressure forms appear to be the most stable phases in the temperature range from 100 to 1100 K and the pressure range from 0 to 30 GPa. Upon applied pressure $o$-silica transforms first into a monoclinic phase (at about 15 GPa) and then into a high-density orthorhombic phase (at about 28 GPa). Moganite also exhibits two crystal-crystal phase transformations, at about 5 and 21 GPa. Quartz undergoes amorphization at approximately 21 GPa in accordance with previous studies. Moganite and $o$-silica are likely to be present among naturally occurring microcrystalline forms of silica; however, similarities in their x-ray-diffraction patterns would make it very difficult to distinguish between them. The implications of computer simulation results for crystal growth and metamorphism are discussed.