Effects of Reduced Iron on Calcite Crystal Morphology and Implications for Tracing Paleoenvironmental Chemistry

Dawn Y. Sumner, Tracy Fenger, and Sarah A. Tourre

1999, AGU Annual Meeting, Abstracts

One approach to tracing interactions between early life and geochemical environments is to analyze fossil-mineral associations for clues to both environmental chemistry and biological processes. Carbonate precipitating environments are of great interest because calcite crystal morphology is influenced by the interaction of various ions and organic molecules with specific crystal faces and growth sites. These interactions may give rise to distinctive (but not necessarily unique) morphological characteristics. Here, we present experimental results that suggest calcite precipitating in the presence of Fe2+ may show development of specific crystal faces. Calcite grown in the presence of even low (Fe2+) show retention of {1 0 -1 4} rhombic faces and the growth of 6 irregular {1 0 -1 k} faces with prominent {1 0 -1 4} growth steps. In large crystals, platy overgrowths on the {1 0 -1 k} faces are slightly misaligned, and resulting composite rhomb faces are distinctly curved. This growth pattern may be found in natural calcite associated with anoxic, calcite supersaturated environments.
Experiments were performed in NaCl solutions with ionic strength of 0.6-0.9M. CaCl2 was added to produce (Ca2+)=0.04-0.1M and FeCl2 was added to give (Fe2+)=0-3mM. Pure CO2 was bubbled through the stirred solution. pH was raised to 6.4 at the beginning of experiments by addition of Na2CO3. pH was kept low to reduce the rate of Fe2+ oxidation; the oxidative rate constant is 3 orders of magnitude higher at pH=8.3 than at 6.4. Rhombic seed crystals were added, and pH was allowed to drift down as calcite precipitated. Samples for SEM imaging were taken at various intervals during the experiments ranging from 5 minutes to 5 hours after initiation of precipitation. Progressive development of the {1 0 -1 k} faces could be observed within the first half hour. Calcite overgrowths from experiments lacking Fe2+ ranged from well developed rhombs at low (Ca2+) to platy overgrowths on {1 0 -1 4} faces at high (Ca2+).

(Scale bars on all photos are 2 microns long.)

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Dawn Y. Sumner
Department of Geology
University of California
Davis, CA 95616