The beryllophosphate mineral pahasapaite, (Car rl.ir.K, ,Nao rE,r r)LirBeroProOnu.38HrO is cubic, I23,with a: 13.781(4 A and Z: l.Its structure, which has been refined to residuals of 0.047 (unweighted) and 0.035 (weighted) for all 560 reflections, contains ordered BeOo and POo tetrahedra sharing vertices to form a three-dimensional array of distorted truncated cubo-octahedra (d cages) linked through octagonal prisms (double S-rings). Pahasapaite has a distorted zeolite rho-type framework and is therefore structurally related to the faujasite group. Eight Li* and 32 HrO are situated within the cages, and the remaining 6 HrO and the 10.5 (Ca,+,Li*,K*,Na*) are located within and just outside of the double 8-rings, where they obstruct passage between neighboring cages. The double 8-rings consist of two elliptically distorted single 8-rings rotated 90 with respect to one another. Together they define a 2.2 x 2.2 A aperture (ignoring occluding cations and HrO), whereas the cages have a free diameter of about 8 A. There is a reduction in framework symmetry from the ideal Im3m in hydrated zeolite rho to I43m in dehydrated zeolite rho Io 123 in pahasapaite. In pahasapaite, this is due to ordering of the framework cations Be and P. Pahasapaite is the first example of a zeolite-like beryllophosphate and may also be considered a relative of the synthetic aluminophosphate molecular sieves.
Abstract The relationships between polytypism and OD structures are discussed, with presentation of some examples in different classes of compounds: polymorphism and polytypism in 9,10 phenantrenequinone; OD structures in latiumite and tuscanite family; polytypism in kaolinite family. The basic elements of OD theory are given, both for structures consisting of OD layers all of the same kind and for structures consisting of two or more distinct kinds of OD layers, and the advantages of an OD approach to polytypism, together with its limitations, are presented and discussed.
Abstract 'Clinobarylite', BaBe 2 Si 2 O 7 , was defined as a monoclinic dimorph of orthorhombic barylite. Subsequently, its crystal structure was also proved to be orthorhombic, differing from barylite in terms of the space group symmetry, Pmn 2 1 instead of Pmnb , and in unit-cell dimensions. Through the order-disorder (OD) theory, the polytypic relationships between 'clinobarylite' and barylite are described. 'Clinobarylite' corresponds to the MDO 1 polytype, with unit-cell parameters a = 11.650, b = 4.922, c = 4.674 Å, space group Pmn 2 1 ; barylite corresponds to the MDO 2 polytype, with a = 11.67, b = 9.82, c = 4.69 Å, space group Pmnb . The re-examination of the holotype specimen of 'clinobarylite' confirmed its orthorhombic symmetry. Its crystal structure has been refined starting from the atomic coordinates calculated for the MDO 1 polytype and the refinement converged to R 1 = 0.0144 for 929 observed reflections [F o > 4σF o ]. Owing to their polytypic relationships, 'clinobarylite' and barylite should be conveniently indicated as barylite-1 O and barylite-2 O , respectively; the name 'clinobarylite' should be discontinued. This new nomenclature of the barylite polytypes has been approved by the Commission on New Minerals, Nomenclature and Classification of the International Mineralogical Association (IMA 13-E).
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