ELECTRONIC ABSORPTION SPECTRA OF PHOSPHATE MINERALS WITH OLIVINE-TYPE STRUCTURES: I. MEMBERS OF THE TRIPHYLITE-LITHIOPHILITE SERIES, M1[6]LIM2[6](FEX 2+ MN1-X 2+)[PO4]

Abstract

Electronic absorption spectra of the orthorhombic olivine-type phosphate minerals of the triphylite-lithiophilite series, M1[6]LiM2[6](Fex 2+ Mn1-x 2+)[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 triphy lite-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. Energies and polarization behavior of band I. correspond closely to those of the strong Fe2+ -band in M2-octahedra of silicate olivines when the permutation of axes in Pmnb (ortho-phosphates) compared to Pbnm (orthosilicates) are taken into account. Band no. II. originates also from Fe2+, in M2, as no other transition metal ions are present eventually giving rise to absorptions in the NIR. Both bands, I. and II. are assigned to the 5A1(5T2g) → 5A1(5Eg), → 5B1(5Eg) transitions of Fe2+, in M2 under pseudosymmetry mm2, which is usually adopted for M2 in silicate olivines. The energy difference, δe, between I. and II. represents the low-symmetry splitting of the excited state 5Eg of octahedral Fe2+ under m3m. Polarization-averaged energies of the above transition I. and of δe increase with iron contents XFe as v̄I.=8906.5 + 669.7 · XFe (r=0.949) ̄δe=1749.2 + 525.7 · XFe (r=0.923) In contrast to silicate olivine s, where energies of the spin-allowed bands of both Fe2+(M1) and Fe2+(M2) decrease with Fe-content, the energy of the barycenter (v̄I + v̄II)/2 increases with XFe in the phosphates studied. This is evidently caused by the fact that Fe2+ substitutes the larger ion, Mn2+, in triphylite-lithiophilite, but the smaller one, Mg2+, in forsterite-fayalite. Adopting the energies of the 5T2g ground-state splitting, δg, in Fe2+ (M2) of silicate olivines also for the phosphates, then the crystalfield parameter, 10Dq, of Fe2+(M2), is obtained as 10Dq =v̄I.̄δc/ 2-2δg/3 with values increasing on XFe, as 10DqFe2+ = 7031.9+406.7 · XFe (r=0.948). Using this 10Dq =f(XFe) relation, local mean octahedral (Fe2+-O) distances in the phosphate olivines are calculated as R̄(Fe-O) local = 2.175-0.024 · XFe(r=0.972) From this, we obtain the local relaxation parameter ε = -0.40. The splitting ̄δe ≈ 2000 cm-1 found here for Fe2+(M2) in phosphate olivines is much higher than ̄δe ≈ 400 cm-1 published for silicate olivines, although the geometry of the M2-octabedra in these phosphate olivines and in (Mg, Fe)-silicate olivines are very similar, especially the (Fe-O) distances. This discrepancy needs further study. © 2006 E. Schweizerbart'sche Verlagsbuchhandlung.