Acceleration of dolomitization by zinc in saline waters

Veerle Vandeginste,Elisabeth Steer,Oliver Snell,Matthew R Hall,Arne Vandeginste

doi:10.1038/s41467-019-09870-y

Veerle Vandeginste, Elisabeth Steer + Show 3 more

Open Access

https://doi.org/10.1038/s41467-019-09870-y

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Abstract

Dolomite (CaMg(CO3)2) plays a key role in the global carbon cycle. Yet, the chemical mechanisms that catalyze its formation remain an enigma. Here, using batch reactor experiments, we demonstrate an unexpected acceleration of dolomite formation by zinc in saline fluids, reflecting a not uncommon spatial association of dolomite with Mississippi Valley-type ores. The acceleration correlates with dissolved zinc concentration, irrespective of the zinc source tested (ZnCl2 and ZnO). Moreover, the addition of dissolved zinc counteracts the inhibiting effect of dissolved sulfate on dolomite formation. Integration with previous studies enables us to develop an understanding of the dolomitization pathway. Our findings suggest that the fluids’ high ionic strength and zinc complexation facilitate magnesium ion dehydration, resulting in a dramatic decrease in induction time. This study establishes a previously unrecognized role of zinc in dolomite formation, and may help explain the changes in dolomite abundance through geological time.

Highlights

Dolomite (CaMg(CO3)2) plays a key role in the global carbon cycle
We conducted calciteto-dolomite replacement batch reactor experiments at 200 °C in which 200 mg of calcite was reacted with 15 ml of simulated natural brines containing 2.00 M NaCl, 0.30 M MgCl2 and 0.20 M CaCl2 in the control series, and added ZnCl2 (0.01, 0.03, 0.05, 0.10, and 0.20 M) in fluids of the same ionic strength of 3.5 in the ZnCl2 experiments
Detailed powder X-ray diffraction (PXRD) analysis of the reaction products shows that the dolomitization reaction occurs in three stages: first an induction period; followed by a conversion stage of the reactant, Mg-poor calcite, to protodolomite; and a recrystallization stage of protodolomite to well-ordered dolomite

Summary

Introduction

Dolomite (CaMg(CO3)2) plays a key role in the global carbon cycle. Yet, the chemical mechanisms that catalyze its formation remain an enigma. Previous research investigating factors affecting the kinetics of dolomite formation predominantly built on the common association of dolomite with salt lakes and lagoons[14]. In addition to those environments, dolomite is often the host rock of zinc ores of the Mississippi Valley-type (MVT)[22,23,24]. Our findings may help explain the common spatial association of dolomite with MVT ore formation, establish a previously unrecognized link between the geochemical cycles of carbon and zinc with the involvement of abiotic carbonate formation, and may increase our understanding of dolomite abundance through geological time

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