Precipitation kinetics and carbon isotope partitioning of inorganic siderite at 25°C and 1 atm

Siderite was precipitated from NaHCO3 and Fe(ClO4)2 solutions under anaerobic conditions at 25degreesC and 1 atm total pressure using a modified version of the chemo-stat technique and the free-drift technique. Samples of solution and solid were withdrawn at different time intervals during time course experiments to determine the bulk and isotope composition of the solution and solid, and the morphology and mineralogy of the solid. A series of metastable precursors precipitated and dissolved sequentially, culminating in well-crystallized siderite rhombohedra having an average edge of similar to 2 mum and a limited size distribution. Siderite precipitation rate ranged from 10(0.23) to 10(2.44) mumol.m(-2).h(-1) for saturation states (with respect to siderite) ranging from near equilibrium to 10(3.53). Calculated carbon isotope fractionation factors (10(3)lnalpha) averaged 8.5 +/- 0.2 (1sigma; n = 4) for the siderite-CO2(g) system and 0.5 +/- 0.2 (1sigma; n = 4) for the siderite-HCO3(aq)- system. Copyright (C) 2004 Elsevier Ltd.