Systematic laboratory approach to produce Mg-rich carbonates at low temperature.

Zulfa Ali Al Disi,Mazen Al-Asali,Fadhil Sadooni,Essam Attia,Maria Dittrich,Tomaso R R Bontognali,Nabil Zouari,Hamad Al Saad Al-Kuwari

doi:10.1039/d1ra06206a

Zulfa Ali Al Disi, Mazen Al-Asali + Show 6 more

Open Access

https://doi.org/10.1039/d1ra06206a

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Abstract

Dolomite is a common Mg-rich carbonate in the geological record, but the mechanism of its formation remains unclear. At low temperature, the incorporation of magnesium ions into the carbonate minerals necessary to form dolomite is kinetically inhibited. Over the decades, several factors that possibly allow for overcoming this kinetic barrier have been proposed, and their effectiveness debated. Here, we present the results of a large number of laboratory precipitation experiments that have been designed to identify and compare the factors that promote the formation of Mg-rich carbonates. Under the tested conditions, the most interesting observations include: (1) from solutions that mimic evaporitic seawater, the maximum mol% of Mg incorporated in high Mg calcite is 35, (2) carbonates with a mol% of Mg above 40 were obtained exclusively in the presence of organic molecules, (3) no correlation was observed between the charge of the organic molecules and the incorporation of Mg, (4) the mode (i.e., slow vs. fast mixing) used to add carbonate to the solution obtaining supersaturation has a significant impact on the forming mineral phase (aragonite vs. nesquehonite vs. high Mg calcite) and its Mg content. These findings allow for a more informed evaluation of the existing models for dolomite formation, which are based on the study of natural environments and ancient sedimentary sequences.

Highlights

Dolomite – MgCa(CO3)2 – is a common carbonate mineral in ancient sedimentary sequences
The most interesting observations include: (1) from solutions that mimic evaporitic seawater, the maximum mol% of Mg incorporated in high Mg calcite is 35, (2) carbonates with a mol% of Mg above 40 were obtained exclusively in the presence of organic molecules, (3) no correlation was observed between the charge of the organic molecules and the incorporation of Mg, (4) the mode used to add carbonate to the solution obtaining supersaturation has a significant impact on the forming mineral phase and its Mg content
Besides con rming the results of the abovementioned previous studies, our experiments provided novel insight on the factors in uencing the incorporation of Mg into carbonate minerals at low temperature, which we discuss in more detail below

Summary

Introduction

During the last two centuries, many models for dolomite formation have been proposed, without providing an unanimously accepted solution to the problem. To make the results of the experiment as relevant as possible for studies of natural environments, we performed experiments with arti cial solutions that mimic the pore waters composition of the Dohat Faishakh sabkha in Qatar[28,30] (see Map in ESI†) This sabkha has been the focus of previous studies, which led to the isolation of microbes capable of precipitating Mg-rich carbonates in the laboratory[14] and the characterization of their EPS.[15] Because the method used in laboratory experiments to reach supersaturation with respect to carbonate minerals may affect the kinetic of the reaction resulting in the formation of different phases[31] we have used and compared three methods: ammonium free-dri (AFD), slow mixing with sodium bicarbonate method (BSM) and fast mixing with sodium bicarbonate method (BFM). This approach allowed us to test the hypothesis[5,33,34,35] that negatively charged carboxyl groups are essential to promote Mg dehydration and consequent incorporation in Mgrich carbonates

Precipitation solutions

Organic compounds

Mineral precipitation experiments

Results and discussion

Conclusions