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)
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.
AAPG Search and Discover Article #90063©2007 AAPG Annual Convention, Long Beach, California