SPECTROSCOPIC STANDARDS FOR FOUR- AND FIVEFOLD-COORDINATED FE2+ IN OXYGEN-BASED MINERALS

Abstract

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, grandidierite and joaquinite. Broad, intense spin-allowed dd bands of tetrahedrally coordinated Fe2+, originating from the 5E→ 5T2 transition, appear in the spectral range 3000–7000 cm–1. In the spectra of gillespite and eudialyte, minerals with square-planar coordination, the bands shift to higher energies, appearing in the range 7000–20 000 cm–1. The amount of band splitting depends mainly on the distortion of the ligands surrounding four-coordinated Fe2+. Splitting and distortion are minimal for spinel with a regular tetrahedral site, and maximal for eudialyte and gillespite. For the minerals in four-coordination, the barycenter of the split bands correlates with the sum of the bond-length and edge-length distortion parameters if the square planer sites are excluded from the correlation. Molar absorption coefficients (e) of the spin-allowed tetrahedral Fe2+ bands range from ~20 cm–1·L·mol–1 to ~90 cm–1·L·mol–1. For eudialyte and gillespite, 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 joaquinite, five-coordination causes spectra that resemble those of Fe2+ in highly distorted octahedral sites. The number of bands suggests, however, that the electronic level scheme of five-coordinated Fe2+ in grandidierite significantly differs from that of Fe2+ in octahedral coordination.