A hierachial classification of perovskite super-group minerals

English Abstract:

Some naturally-occurring oxides, fluorides, hydroxides, arsenides and silicates adopt crystal structures based upon, or which are derivatives of, the aristotypic cubic ternary perovskite structure (ABX₃), as exemplified by the synthetic compounds SrTiO₃ and KMgF₃ and their mineral counterparts, tausonite and parascandolaite. On the basis of the extensive studies of synthetic perovskite-structured compounds it is possible to derive a hierarchy of hettotype structures which are produced by: (1) tilting and distortion of the BX₆ octahedra; (2) ordering of A- and/or B-site cations; (3) formation of A-, B- or X-site vacancies. This hierarchical scheme as applied to naturally-occurring minerals results in the proposed recognition of a perovskite super group. Sub-groups include: (1) single ternary (ABX₃) ideal (tausonite, isolueshite) and distorted (bridgmanite, perovskite, loparite, lueshite,lakargiite, megawite, neighborite, parascandolaite) perovskites, and others which exhibit second order Jahn-Teller distortions (macedonite, barioperovskite); (2) B-site cation ordered double (A₂BB’X₆) perovskites (elpasolite, cryolite, simmonsite, vapnikite); (3) Anion-deficient (A₂B₂O₅) perovskites (brownmillerite, srebrodolskite); (4) A-site vacant single- and double hydroxyperovskites (dzhalindite, bernalite, schönfliesite, stottite and mopungite subgroups); (5) A-site vacant quadruple perovskites (skutterudite subgroup); (6) B-site vacant double perovskites (diaboleite). Common ternary oxide perovskites typically exhibit space group variations resulting from solid solution between potential end-member compositions, and complete characterization requires single-crystal structure determination. In some ambiguous cases vibrational spectroscopy (Raman, infrared) can provide critical additional information that allows a correct choice of space group.