Abstract

Povondraite, previously the tourmaline ferridravite, is redefined. It is rhombohedral, R3m, with a = 16.186(2) and c = 7.444(1) Å. The new chemical formula, derived by crystal structure analysis, is (Na0.80K0.26)Σ1.06(Fe2.283+Fe0.272+Mg0.53)Σ3.08(Fe4.293+Mg1.36Al0.32)Σ5.97B3Si5.96O27.88(OH)3.12 and the ideal end-member formula is NaFe33+Fe63+(BO3)3(Si6O18)(O,OH)4.

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