Witherite (BaCO3)/-quartz epitaxial nucleation and growth. Experimental findings and theoretical implications on biomineralization

Witherite (BaCO3) crystals have been synthesized by mixing BaCl2 ·2H2O and NaHCO3 aqueous solutions; their growth morphology is characterized by single and twinned individuals both [001] elongated and limited by {110} prism and {hhl} bipyramids. When quartz single crystals are used as substrates, heterogeneous nucleation of witherite occurs onto the {101j0}, {101j2}, and {011j2} forms of quartz and {001} witherite lamellae grow along the [100], [001], and [121j] directions of quartz. This witherite/ quartz three-dimensional epitaxy is the first evidence that two-dimensional epitaxial layers of quartz can form as well on witherite nucleating from aqueous solutions containing Na-metasilicate. As it is known, nano-polycrystalline structures, the “silica biomorphs”, are precipitated from such solutions: this implies a very high nucleation frequency of witherite. We propose that this is caused by the lowering of the specific surface energy of witherite for the two-dimensional localized adsorption of silica chains.