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

Abstract

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 {1010}, {1012}, and {0112} forms of quartz and {001} witherite lamellae grow along the [100], [001], and [121] 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.