Stable isotope fractionation during experimental formation of norsethite (BaMg[CO3]2):: A mineral analogue of dolomite

Stable oxygen and carbon isotope fractionation during the experimental formation of ordered norsethite (BaMg[CO3](2)) from the reaction of anhydrous BaCO3 (witherite) with relatively low concentrated sodium-magnesium bicarbonate solutions has been studied between 20 degrees and 135 degrees C. In the investigated temperature range, O-18 and C-13 are enriched in norsethite with respect to water and gaseous carbon dioxide, respectively. Whereas O-18/O-16 partitioning is intermediate between those of the BaCO3-H2O and MgCO3-H2O systems, C-13/C-12 partitioning is more similar to that for BaCO3-CO2. Between 20 degrees and 90 degrees C, the temperature dependences of the O-18/O-16 and C-13/C-12 fractionation factors are represented by the equations (T in degrees K): 10(3) ln alpha(BaMg[CO3)](2-H2O) = 2.83 10(6) T-2-2.85, and 10(3) ln alpha(BaMg[CO3]2)- (CO2)(gas) = 1.78 10(6) T-2-10.16. The later equation considers carbon isotope fractionation between the dissolved carbonate ion and carbon dioxide measured by Halas et al. (1997). Under standard state conditions (25 degrees C) the fractionation factors in the system BaMg[CO3](2)-CO2-H2O are: Oxygen isotopes: alpha(BaMg)(CO3)(2-H2O) = 1.02941, alpha(BaMg(CO3)2-OH-(aq)) = 1.07059, alpha(BaMg(CO3)2-CO2(gas)) = 0.98868, and alpha(BaMg(CO3)2-H2CO3+) = 0.98843; carbon isotopes: alpha(BaMg(CO3)2-CO2(gas)) = 1.00992, alpha(BaMg(CO3)2-H2CO3*) = 1.01099, alpha(BaMg(CO3)2-HCO3-) = 1.00194, alpha(BaMg(CO3)2-CO32-) = 1.00491 or 1.00150. The spontaneous precipitation of a BaMg[CO3](2) gel at 20 degrees C, followed by the alteration of the products at 20 degrees or 60 degrees C for 31 days, demonstrated isotope exchange reactions between solids and mother solutions due to recrystallization. Isotope equilibrium, was not reached within run time.