• journal_title：European Journal of Mineralogy
• Contributor：Klaus LANGER ; Michail N. TARAN ; André-Mathieu FRANSOLET
• Publisher：E. Schweizerbart'sche Verlagsbuchhandlung Science Publishers
• Date：2006-
• Format：text/html
• Language：en
• Identifier：10.1127/0935-1221/2006/0018-0337
• journal_abbrev：Eur J Mineral
• issn：0935-1221
• volume：18
• issue：3
• firstpage：337
• section：Articles

Electronic absorption spectra of the orthorhombic olivine-type phosphate minerals of the triphylite-lithiophilite series, ^M1[6]Li^M2[6](Fe_x²⁺Mn_1-x²⁺)[PO₄], were obtained on members with X_Fe = 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 X_Fe, (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(⁵T_2g)→⁵A₁(⁵E_g), →⁵B₁(⁵E_g) transitions of Fe²⁺ in M2 under pseudosymmetry mm2, which is usually adopted for M2 in silicate olivines. The energy difference, δ̄_e, between I. and II. represents the low-symmetry splitting of the excited state ⁵E_g of octahedral Fe²⁺ under m3m. Polarization-averaged energies of the above transition I. and of δ̄_e increase with iron contents X_Fe as In contrast to silicate olivines, where energies of the spin-allowed bands of both Fe²⁺(M1) and Fe²⁺(M2) decrease with Fe-content, the energy of the barycenter increases with X_Fe 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 ⁵T_2g ground-state splitting, δ̄_g, in Fe²⁺ (M2) of silicate olivines also for the phosphates, then the crystal-field parameter, 10Dq, of Fe²⁺(M2), is obtained as 10Dq = with values increasing on X_Fe as

Using this 10Dq = f(X_Fe) relation, local mean octahedral (Fe²⁺-O) distances in the phosphate olivines are calculated as

From this, we obtain the local relaxation parameter ϵ = −0.40.

The splitting δ̄_e ≈ 2000 cm⁻¹ found here for Fe²⁺(M2) in phosphate olivines is much higher than δ̄_e ≈ 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.