Hydroxy-aluminosilicate interlayers in montmorillonite: implications for acidic environments

Abstract

Hydroxy-aluminosilicate (HAS) ions are one of the major forms of soluble aluminium and the precursor of non-crystalline aluminosilicates in acidic (pH<5.5) environments1–6. Acidification in acid-vulnerable areas7,8 would thus promote HAS formation. Over four decades the study of interactions of soluble aluminium species with clay minerals has been focused on the deposition of hydroxy-aluminium in interlayer spaces of expandable layer silicates9–12. Although the synthesis of 'hydroxy-silicoaluminium pillared smectites' as cracking catalysts has been recently attempted13, the nature of the dominant species adsorbed by smectites from the mixture of HAS, hydroxy-aluminium ions and silicic acid remains obscure. Here we provide evidence that HAS ions can be adsorbed in the interlayer spaces of montmorillonite, resulting in an expanded structure, and so the natural occurrence of HAS-interlayered layer silicates merits attention. Furthermore, the transport of HAS ions, their transformation to non-crystalline aluminosilicates and the levels of toxic solution aluminium species in acidic terrestrial and aquatic environments can thus be affected by naturally occurring expandable layer silicates such as montmorillonite.