Mg-Fe partitioning between biotite and a supercritical chloride solution

An experimental study of the partitioning of Mg and Fe between synthetic biotite and an aqueous chloride solution in the supercritical region as a function of temperature, pressure and concentration of Mg and Fe is reported. In the temperature range 500°–700° C and the pressure range 25–200 MPa, the Mg-Fe distribution between biotite and the chloride solution can be described by distribution curves based on the ideal solution model within a data scattering of 8%. Mg is preferentially partitioned into biotite, and Fe prefers the solution. This tendency is enhanced with increasing temperature. The distribution constants for the Mg-Fe exchange reactions in the system K(Mg,Fe)₃AlSi₃O₁₀(OH)₂-(Mg,Fe)Cl₂-KCl-H₂O have been determined. The present data favor a model in which the activity of Fe and Mg in biotite is close to the mole fraction at temperatures above 500° C. Comparison of the Mg-Fe partitioning between biotite-chloride solution and olivine-chloride solution reveals a slight enrichment of Fe in olivine relative to biotite.