Crystal chemistry of the natural vanadium bronzes

The crystal chemistry of the natural vandium bronze minerals is reviewed on the basis of published information and new studies (mainly by X-ray powder-diffraction methods), using type material wherever possible. The known V bronze minerals are divided into three categories: (1) the hewettite group with layer structures analogous to synthetic Li3V6016, including hewettite, metahewettite, barnesite, hendersonite, and grantsite, (2) the straczekite group with layer structures analogous to synthetic Ags.44 VS020, including straczekite, corvusite, fernandinite, bokite, and bariandite, (3) other structure types including navajoite, schubnelite, fervanite, shcherbinaite, bannermanite, and melanovanadite. All known structures associated with the fibrous V bronzes (fiber spacing 3.6 A) can be considered as various lateral linkages (into sheets or networks) of only two types of polyvanadate chains: (1) a divanadate chain (VP6)n consisting of alternating square pyramids, and (2) a tetravanadate chain (V40l2)n consisting of four highly condensed single octahedral chains.