Recent work has demonstrated that the dynamic interplay between silica and carbonate during co-pptn. can result in the self-assembly of unusual, highly complex crystal architectures with morphologies and textures resembling those typically displayed by biogenic minerals. These so-called biomorphs were shown to be composed of uniform elongated carbonate nanoparticles that are arranged according to a specific order over mesoscopic scales. In the present study, we have investigated the circumstances leading to the continuous formation and stabilization of such well-defined nanometric building units in these inorg. systems. For this purpose, in situ potentiometric titrn. measurements were carried out in order to monitor and quantify the influence of silica on both the nucleation and early growth stages of barium carbonate crystn. in alk. media at const. pH. Complementarily, the nature and compn. of particles occurring at different times in samples under various conditions were characterised ex situ by means of high-resoln. electron microscopy and elemental anal. The collected data clearly evidence that added silica affects carbonate crystn. from the very beginning (i.e. already prior to, during, and shortly after nucleation), eventually arresting growth on the nanoscale by cementation of BaCO3 particles within a siliceous matrix. Our findings thus shed light on the fundamental processes driving bottom-up self-organization in silica-carbonate materials and, for the first time, provide direct exptl. proof that silicate species are responsible for the miniaturization of carbonate crystals during growth of biomorphs, hence confirming previously discussed theor. models for their formation mechanism.