Stable Isotope Fractionation during Experimental Formation of Norsethite (BaMg[CO3]2): A Mineral Analogue of Dolomite

Stable oxygen and carbon isotopefractionation during the experimental formation ofordered norsethite (BaMg[CO3]2) from thereaction of anhydrous BaCO3 (witherite) withrelatively low concentrated sodium-magnesiumbicarbonate solutions has been studied between20° and 135 °C. In the investigatedtemperature range, 18O and 13C are enrichedin norsethite with respect to water and gaseous carbondioxide, respectively. Whereas 18O/16Opartitioning is intermediate between those of theBaCO3–H2O and MgCO3–H2O systems,13C/12C partitioning is more similar to thatfor BaCO3–CO2. Between 20° and90°C, the temperature dependences of the18O/16O and 13C/12C fractionationfactors are represented by the equations (T in °K):103 ln αBaMg[CO3]2-H2O = 2.83 106T--2.85, and 103lnαBaMg[CO3]2-CO2(gas) = 1.78 106T--10.16. The later equation considers carbon isotope fractionationbetween the dissolved carbonate ion and carbon dioxide measured by Halaset al. (1997). Under standard state conditions (25 °C) the fractionation factors in the system BaMg[CO3]2-CO2-H2O are: Oxygen isotopes: αBaMg(CO3)2-H2O = 1.02941, αBaMg(CO3)2-OH-(aq) = 1.07059,αBaMg(CO3)2-CO2(gas) = 0.98868, andαBaMg(CO3)2-H2CO3* = 0.98843; carbon isotopes:αBaMg(CO3)2-CO2(gas) = 1.00992,αBaMg(CO3)2-H2CO3* = 1.01099,αBaMg(CO3)2-HCO3- = 1.00194,αBaMg(CO3)2-CO32- = 1.00491 or 1.00150.The spontaneous precipitation of aBaMg[CO3]2 gel at 20 °C,followed by the alteration of the products at20° or 60°C for 31 days,demonstrated isotope exchange reactions betweensolids and mother solutions dueto recrystallization. Isotope equilibrium, wasnot reached within run time.