Electronic absorption spectra of Fe3+ in andradite and epidote at different temperatures and pressures

Temperature, T, and pressure, P, behaviour of absorption bands caused by spin-forbidden dd-transitions of Fe³⁺ in octahedral sites of the andradite and epidote structures were studied at T from 300 to 750 K and P up to 10 GPa. The temperature dependence of the integral intensity of the ⁶A_1g → ⁴T_1g (⁴G) and ⁶A_1g → ⁴T_2g (⁴G) bands resemble the behaviour of spin-allowed dd-bands of the other 3d^N ions: an increase on heating in case of the centrosymmetric position of Fe³⁺ in the Y-site of andradite and nearly constant values in the strongly distorted M3-site of the epidote structure. Increasing T lowers the intensity of the crystal field-independent band ⁶A_1g → ⁴A_1g⁴E_g (⁴G) in both minerals. The influence of P on the intensity of the dd-bands is insignificant.

In andradite garnet, the local thermal expansion coefficient α_loc of the Fe³⁺-centred octahedra Y, as derived from the high-T spectra, is much lower than that of Cr³⁺-centred octahedra Y in chromium-bearing garnets. On the other hand, judging from T-induced shifts of ⁶A_1g → ⁴T_1g (⁴G) and ⁶A_1g → ⁴T_2g (⁴G) bands, α_loc of FeO₆ is much higher in epidote than in andradite, being comparable to values in AlO₆- or CrO₆-octahedra. This indicates that α_loc values of Fe³⁺-centred octahedra may vary strongly between different structural matrices, possibly as a consequence of differences in covalent bonding of Fe³⁺ compared to Cr³⁺.

In andradites, T and P produce energy shifts of the crystal field-independent band ⁶A_1g → ⁴T_1g⁴E_g (⁴G) in opposite directions, thus showing a decrease and an increase, respectively, in the covalence of the Fe³⁺-O bonds. In epidote, the effect of both T and P on the energy of the crystal field independent transition is much smaller than in andradite. In epidote, on the other hand, P causes a strong high-energy shift of the ⁶A_1g → ⁴T_2g (⁴G) transition, indicating a strong increase of the crystal-field strength Dq, and thus significant compressibility of the Fe³⁺-centred M3-octahedra with k ≅ 75 GPa.

Up to ca. 10 GPa, there appear no additional bands in the NIR range of the high-P spectra of both minerals. This is indicative for the absence of P-induced reduction of Fe³⁺.