ausblenden:
Schlagwörter:
Inter-laboratory comparison;
GaMin’11 exercise;
Experiments at GFZ Potsdam;
Mineral dissolution kinetics
Zusammenfassung:
Experiments to investigate individual CO2-brine-mineral interactions were designed to provide dissolution kinetics for rock-forming minerals. Separates of an iron carbonate, a clay mineral and a feldspar mineral were stored in flexible Titanium Grade-2 cells together with 2 M NaCl brine and pure CO2 at 80 °C and 20 (30) MPa for one (iron carbonate), two (clay) and three (feldspar) weeks, respectively. The carbonate separate consists of 96.3±3.2 wt% iron carbonate and 3.7±0.8 wt% quartz with the iron carbonate phase being composed of 72.3±1.4 wt% siderite and 27.7±1.2 wt% ankerite. During the experiments, siderite abundance increased to 83.3±1.5 wt%, while that of ankerite decreased to 16.7±1.4 wt%. The average empirical formula of untreated and treated siderite is Fe0.8Mg0.1Mn0.1CO3, that of untreated ankerite changed slightly from (Ca1.0Mg0.2Mn0.1Fe0.7)(CO3)2 to (Ca0.9Mg0.3Mn0.1Fe0.7)(CO3)2 during CO2 exposure. Fluid data obtained during these experiments show similar behavior for Ca2+ and Mg2+, as well as Fe2+ and Mn2+, respectively. The clay separate initially consists of 84.2±6.9 wt% illite, 11.9±0.4 wt% orthoclase and 3.9±0.2 wt% quartz with untreated illite actually being an illite-smectite mixed-layer mineral composed of 87.2±1.5 wt% illite and 12.8±1.5 wt% Ca-smectite. During the experiments using the clay separate it was found that the composition changed to 88.3±7.8 wt% illite, 9.2±0.5 wt% orthoclase, and 2.5±0.2 wt% quartz, with CO2-treated illite now consisting of 89.0±1.7 wt% illite and 10.5±1.6 wt% Ca-smectite.
Fluid data show, besides others, increase Ca2+ concentrations over time. Analyses of the feldspar separate reveal pure labradorite with a stoichiometric composition of Na0.5-0.6Ca0.4-0.5Al1.3-1.6Si2.4-2.6O8. During labradorite exposure experiments cation brine concentrations (e.g. Ca2+, Ba2+ and Al3+) increased. Based on the acquired geochemical data sets, the experiments using individual mineral separates indicate (i) dissolution of ankerite and stable siderite, (ii) preferred dissolution of the Ca-smectite component out of the illite-smectite mixed-layer mineral, and (iii) dissolution of labradorite.
