IRON-MAGNESIUM INTERSITE PARTITIONING IN AMPHIBOLES SYNTHESIZED NEAR THE TREMOLITE-FERRO-ACTINOLITE JOIN

Knowledge of iron - magnesium distribution in amphiboles is necessary for quantitative calculations involving amphibole-bearing rocks. We have investigated the iron-magnesium partitioning in four actinolite samples (Ca(y)((Mg(5.2 -x),Fe(x))Si8O22(OH)2 where y ranges from 1.8 to 1.6 and x is 0 (Trem), 0.52, 1.04, and 1.56 (10, 20, and 30 mol% ferro-actinolite). These were synthesized by sealing reagent-grade oxides and metallic iron in Au capsules and treated in an internally heated gas vessel at 825-600°, 0.6 GPa, and using a H2 source to bring the fO2 below the Co-CoO buffer. High yields of amphibole were only produced by severe grinding and multiple (6 times) treatments of about 100 hours each. Phases present were dominantly actinolite with small amounts of quartz, cummingtonite, and cpx. Grain size of the samples was extremely small (1-3 µm width). Samples were analyzed using an electron microprobe to establish the bulk composition, X-ray Rietveld structure refinement to determine the cell dimensions and to provide a measure of the Fe-Mg site partitioning, infrared spectroscopy to provide a qualitative measure of iron in the M1&M3 (=M13) sites, and room-temperature Mössbauer spectroscopy to provide quantitative information on the partitioning of Fe between the M13, M2, and M4 sites. Additionally, Mössbauer spectroscopy indicated all iron as ferrous (Fe2+). Fe-Mg distribution according to the Mössbauer results indicates a slight preference of Fe on the M4 over M13 and M2 sites and a slight preference for M13 over M2. For the 20 mol% sample Mössbauer gives area% of 45.1=M4, 47.9=M13, 6.9=M2 and via Rietveld the distribution coefficient Kd=(MgM13)(FeM2)/(FeM13)(MgM2)=1.09 ± .08, and 1.68 Ca and .32 Fe apfu with no Mg on the M4 site. Rietveld results showed very little site-partitioning preference for the M1, M2, and M3, with strong preference for Fe on the M4 site. Thermodynamic treatment of the data using the 3-site cation partitioning model for quadrilateral amphiboles of Ghiorso and Evans (2001, Amer. Min., in press) will be presented.