Original paper

Pressure-induced phase transitions in ilvaite studied by in situ micro-FTIR spectroscopy

Koch-Müller, Monika; Mrosko, Maria; Gottschalk, Matthias; Schade, Ulrich

Abstract

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

Keywords

ilvaitephase transitionmid/far-infrared spectroscopydiamond-anvil cellhigh pressure