Abstract
Atomic force microscopy has been used to determine more precisely the mechanism of the initial stages of dolomite dissolution. Analysis of outflow solutions initially shows fluctuations of both Ca and Mg release with concentrations of Ca >> Mg. The dolomite surface dissolves congruently in the presence of slightly acidified water as confirmed by the regular spreading of characteristic rhombohedral etch pits. Direct in situ observations show that a new phase precipitates on the dissolving surface simultaneously. As the Ca and Mg release decreases with time, the precipitated phase can be seen to spread across the dolomite surface. These observations indicate that the apparent incongruent dissolution of dolomite is a two-step process involving stoichiometric dissolution with the release of Ca, Mg and CO3 ions to solution at the mineral–fluid interface coupled with precipitation of a new Mg-carbonate phase. The coupled element release confirms the interface-coupled dissolutionprecipitation mechanism.
Highlights
DOLOMITE (Ca,Mg(CO3)2) is present in significant amounts in the Earth’s crust and its dissolution together with that of other dominant carbonates, mostly calcite, partly control the Ca2+ and CO32À budget of superficial waters as well as the global carbon cycle (Mackenzie and Andersson, 2013)
The Atomic Force Microscopy (AFM) experiments were carried out using a Digital Instruments Nanoscope III Multimode AFM (Bruker) fitted with a fluid cell so that solutions of known composition could be passed over the dolomite surfaces within the O-ring sealed cell allowing no contact with air
A nucleation of 1À10 etch pits per 5 mm65 mm area, each dolomite surface had a large nucleation of etch pits that enlarged slowly by etch-pitedge retreat or spreading
Summary
DOLOMITE (Ca,Mg(CO3)2) is present in significant amounts in the Earth’s crust and its dissolution together with that of other dominant carbonates, mostly calcite, partly control the Ca2+ and CO32À budget of superficial waters as well as the global carbon cycle (Mackenzie and Andersson, 2013). Ca2+ and CO32À ions are released in equal quantities to solution. The measured elements released to solution are not in stoichiometric proportions to the solid phase. Ca2+ is well known to be released preferentially initially, especially at low pH where the dissolution rate is enhanced. This paper is published as part of a special issue in Mineralogical Magazine, Vol 78(6), 2014 entitled ‘Mineral–fluid interactions: scaling, surface reactivity and natural systems’.
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