BORATE MINERALS. II. A HIERARCHY OF STRUCTURES BASED UPON THE BORATE FUNDAMENTAL BUILDING BLOCK

A hierarchical structural classification is developed for borate minerals, based on the linkage of (BQ) triangles and (BO+) tetrahedra to form FBBs (fundamental building blocks) that polymerize to form the structural unit, a tightly bonded anionic polyhedral array whose excess charge is baianced by the presence of large low-valence interstitial cations. Thirty-one minerals, with nineteen distinct structure-types, contain isolated borate polyhedra. Twenty-seven minerals, with twenty-five distinct structure-types, contain finite clusters of borate polyhedra. Ten minerals, with ten distinct structue-types, contain chains of borate polyhedra. Fifteen minerals, with thirteen distinct structue-types, contain sheets of borate polyhedra. Fifteen minerals, with thirteen distinct sEucture-types, contain frameworks of borate polyhedra. It is only the close-packed structures of the isolatedpolyhedra class that show significant isotypism