 The tetrahedral sheet is composed mainly of silicon (Si4+), but can have substantial (about 1 in 8) substitution of either Fe3+ or Al3+, or combinations of these two cations.Thus, nontronite typically is characterised by having most (usually greater than 60%) of the layer charge located in the tetrahedral sheet.The layer charge is typically balanced by divalent calcium (Ca2+) or magnesium (Mg2+).