THE THERMAL STABILITY OF THE SODIUM CARBONATE MINERAL SHORTITE FROM THE GREEN RIVER FORMATION, GREEN RIVER BASIN, WYOMING, USA

Preliminary experiments in the system Na₂CO₃-CaCO₃-H₂O show that the sodium carbonate mineral shortite (Na₂Ca₂(CO₃)₃) forms from the reaction of pirssonite (Na₂Ca(CO₃)₂•2H₂O ) and calcite at temperatures above 60 °C via the reaction Na₂Ca(CO₃)₃•2H₂O + CaCO₃ = Na₂Ca₂(CO₃)₃ + 2H₂O. This reaction is significant because shortite is volumetrically abundant (> 1.0 billion tons) in the saline units (i.e. Wilkins Peak Member) of the Eocene Green River Formation, Green River Basin, Wyoming. Shortite in the Green River Formation formed diagenetically by replacement, as displacive crystals in mud layers, and as fracture filling veins that crosscut sedimentary mud layers. Shortite is commonly associated with diagenetic pyrite and trona. Shortite has been identified in kimberlite dikes, carbonatites, and is interbedded with volcaniclastics in the Miocene sodium carbonate deposit of Beypazari, Turkey.

Shortite has been previously formed experimentally at high temperatures (75 to > 300 °C) and pressures (> 2 kbars) (Bradley & Eugster, 1969; Mckie & Frankis, 1977). In this study, experiments were conducted over the range of 40 to 140 °C at pressures of 1 atm or at water-vapor saturation using pure phases made synthetically (pirssonite made at 70 °C and 1 atm; shortite at 200-250 °C and 2 kbar). Synthetic shortite, pirssonite, calcite, and water were then sealed in PerFluoroAlkoxy (PFA) capsules for periods of up to several weeks. This study produced shortite at 60 °C and higher and pressures of 1 atm. X-ray diffraction was used to determine the reaction direction by measuring the pirssonite and shortite ratios following experimental runs.

Understanding the thermal stability of shortite will constrain the burial temperatures and depths at which it formed in the Green River Basin, and establish whether it is a syndepositional or a burial diagenetic mineral. Regardless, elevated temperatures, above those normally found in surface sedimentary environments, are required to form shortite.