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 (ε) 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 ε 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.

Polarized optical absorption spectra of a rare Fe²⁺, Fe³⁺-bearing silicate mineral, tuhualite, (Na,K)₂Fe₂²⁺Fe₂³⁺Si₁₂O₃₀, were measured at room temperature in the range 350–4000 nm (ca. 28500–2500 cm⁻¹). The spectra display a number of strongly pleochroic absorption bands in the visible and NIR range, which are attributed to ferric and ferrous ions distributed over octahedral and tetrahedral sites in the structure. No absorption bands caused by H₂O or OH stretching vibrations have been observed in the 3000 to 4000 cm⁻¹ range. A narrow, weak absorption line at ~422 nm (γ ≈ β) is attributed to the ⁶A_1g → ⁴A_1g,⁴E_g spin-forbidden transition of six-coordinated Fe³⁺ ions that occupy the octahedral Fe2 position of the tuhualite structure. A broad, intense band at ~573 nm (γ > β >> α) is assigned to a Fe²⁺/Fe³⁺ intervalence charge transfer transition (IVCT) between tetrahedral Fe²⁺ and octahedral Fe³⁺. Together with the high-energy absorption edge, the band at 573 nm causes the intense violet color and spectacular pleochroism (γ > β >> α) of tuhualite. An intense band at ~1040 nm (γ > β >> α) and a much weaker band ~ 2150 nm (α) are attributed to spin-allowed dd transitions of Fe²⁺ in the strongly distorted tetrahedral Fe1 sites. On the basis of the data obtained for tuhualite, optical spectra of the structurally related minerals beryl, cordierite, and osumilite are discussed and re-assigned. The spectra are re-interpreted on the basis of the distribution of Fe²⁺ in these minerals over structural octahedral and tetrahedral positions.