• journal_title：The Canadian Mineralogist
• Contributor：Igor V. Pekov ; Olga V. Yakubovich ; Werner Massa ; Nikita V. Chukanov ; Natalia N. Kononkova ; Atali A. Agakhanov ; Vladimir Yu. Karpenko
• Publisher：Mineralogical Association of Canada
• Date：2010-
• Format：text/html
• Language：en
• Identifier：10.3749/canmin.48.2.241
• journal_abbrev：Can Mineral
• issn：0008-4476
• volume：48
• issue：2
• firstpage：241
• section：Articles

Three old specimens, collected in the 19^th century and now deposited in Fersman Mineralogical Museum, Moscow, are labeled as rhodizite from the Sarapulka, Shaitanka (both cotype localities) and Alabashka granitic pegmatite fields, Central Urals, Russia. All are Cs-dominant (Cs > K) and must now be considered londonite. The crystal structure of londonite from Sarapulka was solved from single-crystal data collected at 193 K and refined to R = 0.0203. The mineral is cubic, space group P4̅3m, a 7.3149(7) Å. Its structure is based on a microporous quasi-framework formed by clusters of four edge-sharing AlO₆ octahedra linked by BO₄ and BeO₄ tetrahedra. Both Cs⁺ and K⁺ are ordered in the cages of the quasi-framework. The very short Cs–K distance, 0.51(3) Å, prevents simultaneous occupancy of these positions in the same cage. The Be and K atoms are also separated by an unallowable short distance of 2.76(3) Å, and thus their contents are coupled. The solid-solution system between rhodizite (K-dominant), londonite (Cs-dominant) and the hypothetical K- and Cs-free analogue, (□,H₂O){Al₄[Be₄B₁₂O₂₈]}, is complicated, with numerous coupled heterovalent substitutions. Taking into consideration chemical, structural and IR data, it can be presented as: (A,□,H₂O)₁(Al,Li)₄(Be,Li,Al,□ )₄(B,Be)₁₂[O_28−x(OH,F)_x], where A = K, Cs and x < 1; the species-defining elements are marked in bold. The rhodizite–londonite series is structurally related to pharmacosiderite- and sodalite-type compounds. The association of pharmacosiderite-type [Al₄O₄] clusters and a sodalite-type [B₁₂O₂₄] framework results in the formation of an original complex, [Al₄O₄B₁₂O₂₄], in rhodizite and londonite.