Role of Bulk pH during Witherite Biomorph Growth in Silica Gels

The precipitation of alkali-earth carbonates, such as barium carbonate (witherite), in mildly alkaline silica gels occurs in the form of crystalline aggregates that display external morphologies resembling those encountered in crystalline biomaterials and which have been named silica-carbonate biomorphs. In this work, we have followed, spatially and temporally, the evolution of pH inside silica gels during the precipitation of witherite biomorphs, in correlation with a photographical record of the spatial and temporal evolution of the morphologies of the aggregates. By combining the two resulting observations, we have determined that the formation of witherite biomorphs is most efficient under a pH that is less alkaline than it was previously believed. We discuss the implications of our findings and how they fit in the frame of the recently proposed model for the formation of these materials.