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| Titel |
Hydrothermal replacement of calcite by Mg-carbonates |
| VerfasserIn |
Laura Jonas, Thomas Mueller, Ralf Dohmen |
| Konferenz |
EGU General Assembly 2014
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 16 (2014) |
| Datensatznummer |
250098409
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| Publikation (Nr.) |
 EGU/EGU2014-14082.pdf |
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| Zusammenfassung |
| The transport of heat and mass through the Earth’s crust is coupled to mineral reactions and
the exchange of isotopes and elements between different phases. Carbonate minerals are a
major constituent of the Earth’s crust and play an important role in different physical,
chemical and even biological processes.
In this experimental study, the element exchange reaction between calcite (CaCO3) and a
Mg-rich fluid phase is investigated under hydrothermal conditions. Single crystals of calcite
(2x2x2 mm) react with 1 ml of a 1 M MgCl2 solution at 200°C in a Teflon-lined steel
autoclave for different times between one day and four weeks. The reaction leads to
the formation of a porous reaction front and the pseudomorphic replacement of
calcite by dolomite [CaMg(CO3)2] and magnesite (MgCO3). Scanning electron
microscopy revealed that the reaction rim consists of small Mg-carbonate rhombs closely
attached to each other, suggesting that the replacement reaction takes place by a
dissolution-precipitation mechanism. Typically, the observed reaction front can be divided
into two different domains. The outer part of the reaction rim, i.e. from the mineral surface in
contact to the fluid inwards, consists of magnesite, whereas the inner part of the rim
surrounding the unreacted calcite core consists of Ca-rich dolomite. The formation of a
porous microstructure that varies in different parts of the reaction rim is a direct
result of the large molar volume change induced by the replacement of calcite by
magnesite and dolomite. The developing porosity therefore creates fluid pathways that
promote the progress of the reaction front towards the unreacted core of the single
crystal.
Compositional profiles measured perpendicular to the mineral surface across the
reactions rims using electron microprobe (EMPA) further revealed a compositional
gradient within the reaction rim with regard to the structure-forming elements Mg
and Ca. Here, the amount of Mg incorporated in both product phases increases
with increasing distance from the unreacted calcite core, countered by a decrease
of Ca incorporated. Both the coexistence of two different product phases and the
distinct compositional gradient within the forming reaction rim are unequivocal
signs of a chemical zonation of Ca and Mg in the fluid phase which mediates the
element exchange between the reaction interface and the bulk solution. Atomic
adsorption spectroscopy revealed that the Ca/Mg ratio in the reacted fluid increases as a
function of time, reflecting the progressive exchange of Mg and Ca between the
fluid and the solid phase. The time-dependence of the evolving Ca/Mg ratio can
be fitted with a square root of time relation that indicates a transport controlled
reaction.
We interpret the hydrothermal replacement of calcite to operate via a
dissolution/re-precipitation mechanism, whereas the reaction progress is controlled by the
transport of the structure forming elements through the developing reaction rim. |
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