‘Clinobarylite’–barylite: order-disorder relationships and nomenclature

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Abstract
'Clinobarylite', BaBe2Si2O7, 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,Pmn21instead ofPmnb, and in unit-cell dimensions. Through the order-disorder (OD) theory, the polytypic relationships between 'clinobarylite' and barylite are described. 'Clinobarylite' corresponds to the MDO1polytype, with unit-cell parametersa= 11.650,b= 4.922,c= 4.674 Å, space groupPmn21; barylite corresponds to the MDO2polytype, witha= 11.67,b= 9.82,c= 4.69 Å, space groupPmnb. 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 MDO1polytype and the refinement converged to R1= 0.0144 for 929 observed reflections [Fo> 4σFo]. Owing to their polytypic relationships, 'clinobarylite' and barylite should be conveniently indicated as barylite-1Oand barylite-2O, 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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