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

Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, California 91125, U.S.A.

Correspondence: ^* E-mail: grr{at}gps.caltech.edu

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 () 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 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.

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