Mid- and Far-infrared spectra of a natural ilvaite sample were collected in situ in a diamond-anvil cell (DAC) as a function of pressure. The composition of the natural material is Ca2+0.94(Fe2+0.61Mn2+0.40)(Fe3+1.01Fe2+0.96Al3+0.02Mg2+0.01)[Si2.03O7/O/ (OH)] as determined by electron microprobe. One series of DAC experiments was performed at GFZ in the Mid-IR using argon as pressure medium and a Globar as light source. Four pressure series were performed at Bessy II in the Far-IR region using petroleum jelly as pressure medium and synchrotron light. For the Far-IR region we used a custom-made vacuum microscope adapted to an FTIR spectrometer. Pressure-induced changes in the Mid- and Far-IR spectra were analysed via the autocorrelation method for all five pressure series. All five series confirm the monoclinic to orthorhombic phase transition at about 2.3 GPa already known from X-ray diffraction studies. Our results show that incorporation of Mn2+ does not lower the transition pressure as reported previously. However, the magnitude of the initial monoclinic β angle and the concentration of impurities seem to control the transition pressure. The first evidence of a second pressure-induced phase transition at much higher pressure (between 10 and 11 GPa) is reported here. Both transitions are clearly visible throughout the whole spectral ranges: in the OH-stretching region, the Mid-IR region from 1400 to 400 cm−1 and in the Far-IR region down to 50 cm−1. The structural change at 10.5 GPa observed via infrared spectroscopy may reflect a pressure-induced suppression of the Jahn–Teller effect of the strongly distorted M2 octahedron. Above 11 GPa and up to 20 GPa no further discontinuities could be detected. One series going in pressure up to 31 GPa may indicate an additional structural change above 20 GPa.

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