A DETAILED RE-EXAMINATION OF CA-MG-CARBONATE SEDIMENTS IN LAGOA VERMELHA USING RIETVELD REFINEMENT

Lagoa Vermelha, a coastal lagoon in Brazil, is well-known among carbonate geologists as the birthplace of the microbial dolomite model [1,2]. This model attributes dolomite precipitation to direct microbial mediation by sulfate-reducing bacteria that removes sulfate ions from the fluids, thus removing a known kinetic inhibitor to dolomite formation [2]. The model is based in large part on the results from low-temperature laboratory experiments using microbial cultures from Lagoa Vermelha, which claim to have produced ordered dolomite [1,2]. The validity of the microbial dolomitization model has been challenged recently on the basis that the published mineralogical and crystallographic data from these experiments and the lagoon itself are of insufficient quality to adequately determine if the precipitates are dolomite or very high-magnesium calcite [3,4].

Dolomite and very high-magnesium calcite (VHMC) are both Ca-Mg-carbonate minerals, but differ significantly in their structural properties. Dolomite belongs to the R crystallographic space group and is characterized by alternating cation layers of calcium and magnesium arranged perpendicular to the crystallographic c axis. VHMC, in contrast, is characterized by a random distribution of calcium and magnesium in the cation layers. This lack of cation ordering puts VHMC into the Rc crystallographic group [3].

The current study aims to investigate the degree of cation order in sediment samples collected from Lagoa Vermelha and the neighboring Brejo do Espinho lagoons by acquiring detailed X-ray diffraction data. Crystallographic parameters obtained by Rietveld refinement of powder samples are studied to reveal trends in the natural evolution of cation ordering in these calcium-magnesium carbonates. We compare the results to high-temperature experimental studies that showed that VHMC forms in the early stages of dolomitization as a precursor to recrystallized ordered dolomite [5].

1 Vasconcelos, C. et al., Nature 377, 220–222, 1995

2 Vasconcelos, C. & McKenzie, J.A., Journal of Sedimentary Research 67/3, 378–390, 1997

3 Gregg, J.M., et al., Sedimentology 62, 1749–1769, 2015

4 Kaczmarek, S.E. et al., SEPM Special Publication 109, 7–20, 2017

5 Kaczmarek, S.E. & Sibley, D.F., Sedimentology 61, 1862–1882, 2014