Calcite Kinetics for Spiral Growth and Two-Dimensional Nucleation

Area of Science:

• Geochemistry
• Crystallography
• Materials Science

Background:

• Calcite crystals ({10.4} faces) grow via molecular steps.
• Growth mechanisms include stochastic two-dimensional (2D) nucleation and steady spiral hillocks from dislocations.

Purpose of the Study:

• To determine the kinetics of calcite growth mechanisms (2D nucleation vs. spiral growth) as a function of supersaturation.
• To elucidate the influence of crystal size and diffusion on the dominant growth mechanism.

Main Methods:

• Kinetic analysis of calcite growth.
• Investigation of supersaturation effects on nucleation and spiral growth.
• Evaluation of boundary layer diffusion impacts on surface supersaturation.

Main Results:

• Calcite crystals > ~1 μm predominantly exhibit spiral growth, independent of supersaturation.
• Sub-micron crystals favor 2D nucleation at high supersaturations.
• Boundary layer diffusion favors spiral growth in larger crystals by reducing surface supersaturation.
• Diffusion limitations affect nanoscopic crystal growth.
• Additives can alter the dominant growth mechanism.

Conclusions:

• Crystal size is a critical factor determining calcite growth mechanism dominance.
• Surface supersaturation, modulated by diffusion, dictates the prevalence of spiral growth over 2D nucleation for larger crystals.
• Additives offer a means to control calcite crystal morphology by influencing growth pathways.