ESR-determined manganese partitioning ratios in dolomite synthesized at 180° and 250°C

Abstract

In a continuing study of the relation between dolomite origin and its manganese partitioning ratios [ Mn PR = ( Mn) Mg site ( Mn) Ca site ] dolomite was synthesized hydrothermally at 180° and 250°C. Synthesis products include 100% to 0% calcite, 0% to 100% dolomite and 0% to 15% magnesite. Dolomites formed at 250°C averaged 52% Ca; those formed at 180°C averaged 54.1% Ca. Mn PR at 180°C ranged from 5 to 10; at 250°C Mn PR's were 10–20. The high Ca contents and low Mn PR's of dolomites formed in short-duration experiments and at the lowest temperatures are due to the temperature dependence of the rates of dissolution and precipitation. Composition trends toward stable values (low Ca, high Mn PR) as thermodynamic equilibrium is approached. Mn PR's in synthetic dolomites vary within the range of the Lumsden-Lloyd group- A dolomites. The collective results are consistent with a hypothesis that Mn PR's reflect two distinct modes of formation for dolomite.