EFFECT OF BIOTITE (001) SURFACE ON THE PRECIPITATION OF CALCITE

Carbonate mineralization is one of the important issues for long-term CO₂ sequestration in geological environments. Hence it is critical to understand how non-carbonate minerals such as silicates affect the nucleation and growth of Ca-Mg-carbonates. We investigated calcite crystallization on biotite using SEM and XRD, and electron backscatter diffraction (EBSD) methods to explore the template effect by determining the orientation relationship between calcite and biotite substrate.

Our results show that biotite has a strong controlling effect on the orientation of calcite nucleation. The orientation relationship between the growing calcite crystals and biotite substrate are:

calcite (001) ~// biotite (001) and calcite (010) ~// biotite (010).

Our analysis indicates that calcite crystals initially nucleate from the Ca²⁺ layers on the biotite surface and occupy the original K⁺ sites. CO₃^2- groups on the surface then attach to the Ca²⁺ ions and form the first calcite layer. The geometry of this Ca²⁺ layer, which matches the calcite rhombus structure, could be the control factor on calcite a-axis direction. Pseudo-hexagonal symmetry on biotite (001) surface leads to the observed (001) twinning in calcite. The lattice mismatch along the a-axis of biotite (0.534nm) with the a-axis of calcite (0.499nm) is -4.3%, with the overall area mismatch of +1%. The slight difference can be accommodated through distortion of the first calcite layer.

A second orientation was also observed: calcite {104} ~// biotite (001). A similar relationship was also observed in ankerite-chlorite interface, which have similar structures to calcite and biotite, respectively.

The effect of biotite on the Mg incorporation into calcite was also studied. The results show that oriented calcite nucleation may lead to orientational incorporation of Mg²⁺, which may result in a larger tolerance of calcite for Mg²⁺; as much as 19% MgCO₃ can incorporate into the calcite lattice using solutions with high Mg/Ca (=5) ratios. However, high Mg²⁺ incorporation results in calcite morphology changes. As more Mg²⁺ ions enter the calcite lattice, crystal size decreases and elongated features are developed along the c-axis. Substrate effect becomes weaker as Mg²⁺ incorporation increases.