Dissolution Kinetics of Iron Carbonate, Illite and Labradorite – CO2-Saline Fluid-mineral Experiments within the GaMin’11 Inter-laboratory Comparison Exercise

Abstract

Abstract 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 CO 2 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 CO 2 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.