Witherite (BaCO3)/α-Quartz Epitaxial Nucleation and Growth: Experimental Findings and Theoretical Implications on Biomineralization

Witherite (BaCO3) crystals have been synthesized by mixing BaCl2 center dot 2H(2)O 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 11010), 110121, and {01 (1) over bar2} forms of quartz and {001} witherite, lamellae grow along the [100], [001], and [12 (1) over bar] 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.