Silicon Tetrahedral Sheet in Stilpnomelane

Abstract

A CONTINUOUS hexagonal sheet of silicon tetrahedra (as in kaolinite) has lateral dimensions too small to permit articulation with a trioctahedral sheet in which the octahedral cations are larger than those of magnesium. In micas and chlorites where iron may occur in the octahedral sheet, the tetrahedral sheet is expanded by the substitution of aluminium for silicon; in astrophyllite¹ insertion of TiO₆ octahedra in the tetrahedra sheet permits coordination to the iron-bearing octahedral sheet. Expansion by inversion of tetrahedra has been found in the iron silicate zussmanite². Stilpnomelane is a trioctahedral layer silicate with a layer thickness of about 12 Å. The octahedral cations are largely iron, which can range in oxidation state from all ferrous to almost all ferric³, whereas the tetrahedral cations are nearly all silicon. In this paper the tetrahedral sheet of stilpnomelane is described.