CaCO₃ Nanoparticle Polymorph Selection

Precipitation Conditions

Supersaturation ratio S 12.0

Mg²⁺ / Ca²⁺ ratio 0.5

Temperature 25 °C

pH 9.5

Live Readouts

Nucleation rate J

— /s

Particles nucleated

Mean particle size

0 nm

Calcite — Vaterite — Aragonite —

How it works

Mixing Ca²⁺ and CO₃²⁻ solutions (e.g. CaCl₂ + Na₂CO₃) drives classical nucleation once the supersaturation ratio S = [Ca²⁺][CO₃²⁻]/K_sp is high enough to overcome the nucleation barrier:

ΔG* = 16πγ³Vm² / (3(RT lnS)²)
J = A·exp(−ΔG*/RT)

Higher S and higher T shrink ΔG*, so the nucleation rate J (particles per second, shown live) rises steeply — this simulator drives it directly from your S and T sliders.

Which polymorph forms is governed by Ostwald's step rule plus a real inhibitor effect: freshly nucleated CaCO₃ tends to appear first as the least-stable, lowest-surface-energy phase — spherical vaterite — which normally redissolves and reprecipitates as the thermodynamically stable calcite (rhombohedral) within seconds to minutes. Dissolved Mg²⁺ ions adsorb onto calcite's growth steps and strongly poison that step, which is why seawater and biomineralizing organisms — both Mg²⁺-rich — form needle-shaped aragonite or stabilized vaterite instead of calcite, even though calcite is the more stable form in pure water.

S (supersaturation) — sets the nucleation rate J and the growth speed of every particle.
Mg²⁺/Ca²⁺ — raises the odds a nucleus forms as aragonite instead of calcite, and slows (or blocks) vaterite → calcite conversion, matching the real inhibition mechanism above.
Temperature — speeds growth and the vaterite → calcite transformation rate (an Arrhenius-type effect).
pH — more CO₃²⁻ is available at high pH, which raises the effective nucleation rate.

This is the same competition exploited to engineer CaCO₃ drug-delivery nanoparticles: colloidal or biomineralization-style synthesis routes tune S, Mg²⁺ and pH specifically to lock in the porous, higher-surface-area vaterite phase (good for loading drugs into pores) instead of letting it age into denser, less useful calcite.

Calcite — rhombohedral Vaterite — spherulitic Aragonite — acicular (needle)

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CaCO₃ Nanoparticle Polymorph Selection: Calcite, Vaterite & Aragonite