47-10 Measuring dolomite precipitation rates in dissolved silica-bearing solutions using flow-through fluid cell and in-situ XRD method

Session: Minerals in Motion: Tracking Mineral Reactions Using In Situ and Synchrotron Techniques, A Celebration of the Career of Peter Heaney

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Abstract:

Recent study shows that dolomite can precipitate from seawater directly in the presence of dissolved silica (Fang and Xu, 2022: https://doi.org/10.2110/jsr.2018.35).  Dissolved silica (aqueous Si(OH)₄) serves as a catalyst to lower the dehydration energy barrier for hydrated Mg²⁺-surface complexation on dolomite crystals and promote dolomite growth. The scarcity of sedimentary dolomite in Cenozoic rock record may result from depletion of dissolved silica in seawater due to diatom growth.  Measuring both dolomite growth and nucleation rates are critical for understanding sedimentation of primary dolomite in dolomite rock records.

We used a flow-through fluid cell with a hot gas heating component and in situ XRD (APS 13BMC) to study the dolomite precipitation / growth on dolomite nano-crystals (seeds) in a solution with concentrations of 80 mM of Mg and 10 mM of Ca at different temperatures. Average compositions of the (Ca,Mg)CO₃ precipitates (in the solid solution series of calcite and disordered dolomite) are calculated based on their d₁₀₄ values (Fang and Xu, 2019: https://doi.org/10.2110/jsr.2019.29). Weight percentages of the precipitates and dolomite seeds are calculated using Rietveld method. The amount of Mg incorporated into the (Ca,Mg)CO₃ precipitates increased from ~ 30 mol % of MgCO₃ at 35 °C to ~ 40 mol % of MgCO₃ at 43 °C and to ~ 50 mol % of MgCO₃ at 50 °C. However, their growth rates decreased from 35  °C to 50  °C. There is an “induction” (no detectable crystallization) and slow growth time period during the first 60 minutes. The growth rate of (Ca,Mg)CO₃ precipitate is related to solution temperature and its Mg content in the presence of dissolved silica. Average precipitation rates on the dolomite seeds are ~5 nm/hr for the disordered dolomite at 50 °C, and ~ 7 nm/hr for the Ca-rich disordered dolomite at 43 °C. Considering steady state nucleation of dolomite in sedimentation basins / lakes, the measured dolomite growth rates can explain the observed dolomite sedimentation rates (ranging from 100 μm/year to ~ 500 μm/year) recorded in Green River Formation stromatolites and an Ordovician carbonate with oscillatory dolomite-calcite micro-laminations (Fang and Xu, 2018: https://doi.org/10.2110/jsr.2018.35).

Geological Society of America Abstracts with Programs. Vol. 57, No. 6, 2025

doi: 10.1130/abs/2025AM-4364

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