Abstract

Synthetic carbonate fluorapatite (CFA) minerals produced with a range of manganese(II) and iron(II) concentrations exhibit varying intensities of luminescence when examined by cathodoluminescence (CL) petrography. Manganese(II) activates luminescence in CFA, with a rapid increase in relative luminescent intensity (RLI) between 40 and 100 ppmw Mn. A sample with less than 40 ppmw Mn is considered non-luminescent based on visual criteria. Above 100 ppmw Mn, the RLI value asymptotically approaches a maximum at about 400 ppmw Mn and remains constant to 1000 ppmw Mn. Iron(II) quenches luminescence and effectively reduces the RLI value of CFA samples with high Mn concentrations. This quenching behavior is not dependent on absolute Fe concentrations but rather on Mn/Fe ratios. Samples with Mn/Fe weight ratios below 1 have lower RLI values than samples with Mn/Fe ratios above 1, regardless of the absolute Mn or Fe concentrations. RLI values increase linearly as Mn/Fe ratios increase from 0 to 1, then remain approximately constant at a bright luminescence with Mn/Fe ratios increasing from 1 to 6. For the CFA samples examined here, a plot of Mn versus Fe concentrations with an overlay of observed luminescent intensities reveals fields of bright, dull, and non-luminescence. Although this is a common procedure in characterizing carbonate luminescence, this is the first such plot for CFA. Both Mn/Ca and Fe/Ca are higher in the solid phase (CFA) compared with the precipitating solution. The best estimate from these experiments of the effective partition coefficient (D = (metal/Ca in solid)/(metal/Ca in solution)) for Mn incorporation in CFA is 2, and that for Fe is 13. Due to changing metal/calcium solution concentrations during precipitations, these Ds were estimated using assumptions about precipitation rates and solution composition changes during the course of experiments and are not representative of equilibrium values.

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