Dissolved ammonia catalyzes proto-dolomite precipitation at Earth surface temperature

Abstract

• Inorganically-produced proto-dolomite precipitated successfully at normal temperatures. • Proto-dolomite precipitated at 40 °C exhibited a certain degree of ordering. • Dissolved ammonia accelerated the dehydration of Mg[(H 2 O) 6 ] 2+ . Marine carbonate rocks are the major reservoir of carbon through CaCO 3 and CaMg(CO 3 ) 2 precipitation extracting CO 2 /HCO 3 - from the atmosphere/oceans; hence dolomite is one of the major sinks in the global carbon cycle. Most ancient dolomite has been considered as mainly precipitated under Earth surface conditions. Previous studies have demonstrated that microbes can mediate dolomite formation. However, the “microbial dolomite” model is not sufficient to explain the dolomite that shows no or little evidence of a microbial origin. Although attempted for decades, the synthesis of inorganically-produced dolomite at normal temperatures (<50 °C) has been relatively unsuccessful. Hence, this "dolomite enigma" has been one principal research focus this century. Here we demonstrate that NH 3 catalyzes proto-dolomite precipitation inorganically with higher Mg/Ca molar ratios and CO 3 2- activity at normal temperatures of 30 °C and 40 °C. NH 3 dissolution increases the alkalinity of the solution and transformes into NH 4 + ions, which prefer to bond with H 2 O on Mg[(H 2 O) 6 ] 2+ rather than free H 2 O, thus releasing Mg 2+ to facilitate proto-dolomite nucleation. Furthermore, the low dielectric constant and low dipole moment allow NH 4 + absorbed on crystal surfaces to lower the energy barrier of Mg[(H 2 O) 6 ] 2+ dehydration, promoting proto-dolomite nucleis growth. The system for proto-dolomite precipitation in our experiments closely simulates the natural aqueous environment. This study brings new insights to understanding the mechanisms of dolomite precipitation in natural waters.

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