Spectroscopic standards for four- and five-coordinated Fe2+

in oxygen-based minerals

Division of Geological and Planetary Sciences
California Institute of Technology
Pasadena, CA  91125, USA

American Mineralogist 86, 896-903


    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 -> 5T2g transition, appear in the spectral range 3,000 to 7,000 cm-1. In the spectra of gillespite and eudialyte, minerals with square-planar coordination, the bands shift to higher energies, appearing in the range 7,000-20,000 cm-1.  The amount of band splitting depends mainly on the distortion of the ligands surrounding four-coordinated Fe2+. It is 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 is well-correlated 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×liter×mol-1 to ~90 cm-1×liter×mole-1. For eudialyte and gillespite, due to the centrosymmetric character of the ligand environment, the e values ranges from about 0.5 to 10 cm-1×liter×mole-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 those of Fe2+ in octahedral coordination.

Data files from the paper

Mineral Idealized Formulas Spectrum Data Files
 eudialyte Na15Ca6Fe3Zr3Si25O73(OH)5 Eudialyte.gif 0.039 mm eudialyte160.a
0.039 mm eudialyte160.c
 gehlenite Ca2(Al,Fe)2SiO7  Gehlenite8013.gif 0.164 mm gehlenite8013.a
0.164 mm genlenite8013.c
 genthelvite* (Zn,Fe)4Be3(SiO4)3S Genthelvite9305.gif 0.041 mm genthelvite9305
 gillespite BaFeSi4O10 Gillespite1787.gif 0.100 mm gillespite1787.a
0.100 mm gillespite1787.c
 pellyite Ba2Ca(Fe,Mg)2Si6O17  0.030 mm Pellyite334.gif 0.032 mm pellyite334.a
0.028 mm pellyite334.b
0.028 mm pellyite334.c
 spinel (Mg,Fe)Al2O4 Spinel153.gif 0.434 mm  spinel56163-151
0.848 mm  spinel56163-152
0.410 mm  spinel56163-153
0.330 mm  spinel62047-6
0.966 mm  spinel62047-7
 staurolite (Fe,Mg)2Al9Si4O22(OH)2 Staurolite47.gif 0.062 mm staurolite47.alpha
0.064 mm staurolite47.beta
0.062 mm staurolite47.gamma
 grandidierite (Mg,Fe)Al3(BO4)(SiO4)O 0.60 mm Grandidierite441.gif

Errata: the labels for the beta and gamma spectra were reversed in Fig. 9 in the American Mineralogist paper. The spectra in the link above are correct.

0.579 mm grandidierite441.alpha
0.579 mm grandidierite441.beta
0.760 mm grandidierite441.gamma
 joaquinite Ba2NaCe2FeTi2Si8O26(OH)•H2O 0.050 mm Joaquinite158.gif 0.050 mm joaquinite158.alpha
0.050 mm joaquinite158.beta
0.050 mm joaquinite158.gamma

* Genthelvite, Zn4Be3(SiO4)3S, forms a solid-solution series with danalite, Fe4Be3(SiO4)3S, a red mineral.

Individual color figures from the paper in TIF format (~170 K each).

Figure 1    Figure 2    Figure 3  Figure 4    Figure 5  Figure 6  Figure 7  Figure 8   corrected-Figure 9   Figure 10   Figure 11   Figure 12