The tetrahedral sheets consist of silica tetrahedra, which are silicon ions surrounded by four oxygen ions. In biotite, one in four silicon ions is replaced by an aluminium ion. The tetrahedra each share three of their four oxygen ions with neighboring tetrahedra to produce a hexagonal sheet. The remaining oxygen ion (the apical oxygen ion) is available to bond with the octahedral sheet.
The te:trahedral sheets consist of silica tetrahedra, which are silicon ions surrounded by four oxygen ions. In biotite, one in four s?ilicon ions is re?placed by an aluminium ion. The tetrahedra each sh.are three of their four oxygen ions with neighboring tetrahedra to produce a hexagonal sheet. The remaining oxygen ion (the apical oxygen ion) is, available to bond with the octahedral sheet.