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 6A1g → 4T1g (4G) and 6A1g → 4T2g (4G) bands resemble the behaviour of spin-allowed dd-bands of the other 3dN 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 6A1g → 4A1g4Eg (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 αloc 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 6A1g → 4T1g (4G) and 6A1g → 4T2g (4G) bands, αloc of FeO6 is much higher in epidote than in andradite, being comparable to values in AlO6- or CrO6-octahedra. This indicates that αloc 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 6A1g → 4T1g4Eg (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 6A1g → 4T2g (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+.