The Role of Saturation in Controlling CaCO₃ Mineralogy, Crystal Morphology, and Carbonate Fabrics: New Insights Into an Unsolved Enigma

De Choudens, Vionette¹, Luis Gonzalez¹, Jennifer Roberts¹ (1) University of Kansas, Lawrence, KS

We have quantified the relationship between saturation state (Ω) and calcite and aragonite crystal morphology and fabrics. Experimental solutions ranged from Mg:Ca of 1 to 5, and Ω of 1 to 45. Increased fluid Mg:Ca ratios required progressively higher Ω for calcite precipitation. At lower Ω only aragonite precipitated. Data suggest that high Ω induce calcite growth rates that overcome the inhibitory effect of Mg⁺⁺ and allow calcite to compete with precipitating aragonite. Calcite crystal morphologies and calcite and aragonite fabrics were controlled by Ω and Mg:Ca ratios.

For calcite, shallow euhedral rhombs were observed at Mg:Ca = 1, and the lowest Ω (<2). Steep rhombs were observed in all experiments with solution Mg:Ca ≥ 2, but only at higher Ω (≥ 2) in solutions with Mg:Ca of 1. Scalenohedrons were formed from solutions with Mg:Ca of 5 and Ω > 20. Calcite interpenetration twins and resulting rosette fabrics were only observed in solutions with Mg:Ca of 1 to 3 at increasingly high Ω. Stacked rhombs and dendrites fabrics were observed at Ω > 6 in solutions with Mg:Ca = 1. Despite higher Ω calcite crystals were progressively smaller at higher Mg:Ca, suggesting that growth rates, although fast enough to compete with aragonite, were slower than those at lower Mg:Ca.

Aragonite rod, dumbbell, and rosette fabrics were observed in all Mg:Ca. Rods and dumbbells formed at the lowest Ω, while rosettes developed with increasing Ω. Spherulites formed from solutions with Mg:Ca of 4 and 5 at the highest Ω > 12 and 14. Rosettes, dumbbells, and spherulites increase in size with increasing Mg:Ca ratios and Ω.