Electronic absorption spectra of the orthorhombic olivine-type phosphate minerals of the triphylite-lithiophilite series, M1LiM2(Fex2+Mn1-x2+)[PO4], were obtained on members with XFe = 0.195 (p, rs), 0.326 (sc, ps), 0.500 (p, rs), 0.564 (sc, ps), 0.708 (sc, ps), 0.750 (p, rs). Polarized spectra, ps, were measured on properly oriented single crystal slabs, sc, while powder remission spectra, rs, were scanned on unoriented fine grained powders, p. In all cases, microscope-spectrometric methods were used.
All triphylite-lithiophilite spectra show the following features: (i) a nearly unpolarized absorption edge at around 30000 cm−1, which shifts slightly towards higher energies on increasing XFe, (ii) very weak and sharp bands at 24200 and 23800 cm−1 (c > b > a) typical of spin-forbidden dd-bands in Mn2+, and a series of very weak, differently polarized bands in the range 19000-15000 cm−1, (iii) an intense band system in the NIR typical of spin-allowed dd-transitions in octahedral Fe2+, consisting of two strongly polarized bands, I. at around 9100-9400 cm−1 (c > > a > b) and II. at around 7400-7200 cm−1 (c > > a ≈ b) attached as a strong shoulder to I.
From this, we obtain the local relaxation parameter ϵ = −0.40.
The splitting δ̄e ≈ 2000 cm−1 found here for Fe2+(M2) in phosphate olivines is much higher than δ̄e ≈ 400 cm−1 published for silicate olivines, although the geometry of the M2-octahedra in these phosphate olivines and in (Mg, Fe)-silicate olivines are very similar, especially the (Fe-O) distances. This discrepancy needs further study.