Spectroscopic standards for four- and fivefold-coordinated Fe2+ in oxygen-based minerals

Optical spectra are presented for seven oxygen based, four-coordinated Fe2 bearing minerals, eudialyte, gehlenite, genthelvite, gillespite, pellyite, spinel, and staurolite, and two five-coordinated Fe2+ minerals, grandidier ite and joaquinite. Broad, intense spin-allowed dd bands of tetrahedrally c oordinated Fe2+, originating from the E-5 --> T-5(2) transition, appear in the spectral range 3000-7000 cm-l. In the spectra of gillespite and eudialy te, minerals with square-planar coordination, the bands shift to higher ene rgies, appearing in the range 7000-20000 cm(-1). The amount of band splitti ng depends mainly on the distortion of the ligands surrounding four-coordin ated Fe2+. Splitting and distortion are minimal for spinel with a regular t etrahedral site, and maximal for eudialyte and gillespite. For the minerals in four-coordination, the barycenter of the split bands correlates with th e sum of the bond-length and edge-length distortion parameters if the squar e planer sites are excluded from the correlation. Molar absorption coeffici ents (E) of the spin-allowed tetrahedral Fe2+ bands range from similar to 2 0 cm(-1).L.mol(-1) to similar to 90 cm(-1).mol(-1). For eudialyte and gille spite, due to the centrosymmetric character of the ligand environment, the E values ranges from about 0.5-10 cm(-1).L.mol(-1). For grandidierite and j oaquinite, five-coordination causes spectra that resemble those of Fe2+ in highly distorted octahedral sites. The number of bands suggests, however, t hat the electronic level scheme of five-coordinated Fe2+ in grandidierite s ignificantly differs from that of Fe2+ in octahedral coordination.