ɐ vɚɹˈaɪəɾi ʌv ˈʌðɚ kɹˈɪstɐlˌaɪn fˈeɪzᵻz hɐvbɪn ɛkspˌɛɹɪmˈɛntəli əbzˈɜːvd, ɪŋklˈuːdɪŋ kjˈuːbɪk flˈʊɹɹaɪt ˌɛfˌɛmˈɛm, tˈɛtɹɐɡənəl pˈiː fˈoːɹɾi tˈuː slˈæʃ ˌɛnˌɛmsˈiː, mˌɑːnoʊklˈɪnɪk pˈiː twˈɛnti wˈʌn slˈæʃ sˈiː ænd ˌɔːɹθoːɹhˈɑːmbɪk pˌiːbˌiːsˌiːˈeɪ ænd pˌiːˌɛnˌɛmˈeɪ.
A variety of other crystalline phases have been experimentally observed, including cubic fluorite (Fmm), tetragonal (P42/nmc), monoclinic (P21/c) and orthorhombic (Pbca and Pnma).