The influence of crystal field stabilization energy on Fe 2+ partitioning in paragenetic minerals

Abstract

In order to explore possible quantitative relations between crystal field stabilization energy, CFSE, and partitioning behaviour of the 3d 6-configured Fe2+ ion, a suite of 29 paragenetic rock-forming minerals from 12 high-grade metamorphic rock samples of the Ukrainian shield, including the parageneses garnet/orthopyroxene/clinopyroxene (2x), orthopyroxene/clinopyroxene, garnet/clinopyroxene, garnet/orthopyroxene/biotite, garnet/biotite, garnet/cordierite, garnet/cordierite/biotite, garnet/orthopyroxene/clinopyroxene/Ca-amphibole, Ca-amphibole/biotite (retrograde), was studied by electron microprobe analysis to obtain the respective K D Fe2+ (Ph1/Ph2) values and by polarized single crystal electronic absorption spectroscopy to evaluate the respective CFSEFe2+ values. Other than in the case of Cr3+, a clear quantitative relation between K D (Ph1/Ph2) and the ΔCFSE(Ph1/Ph2) was only observed when geometrical factors, mainly the volume of crystallographic sites and ionic radii of ions competing in the partitioning process, are similar in the respective two paragenetic phases to within 15–20%. In such cases, the ΔCFSEFe2+ contribution to K D (Ph1/Ph2) amounts to 0.1 to 0.2 log K D per 100 cm−1ΔCFSE. The conclusion is that ΔCFSEFe2+ plays only a secondary role after geometrical factors, in the partitioning behaviour of Fe2+. The reason for this is seen in the facts that, compared to the 3d 3-configured Cr3+ ion, CFSE of the 3d 6-configured Fe2+ amounts only to 20–25%, and that the former ion enters only octahedral sites with similar geometrical properties in the paragenetic mineral phases.