Anorthite syntheses were performed at 2 GPa and 700 °C in a piston cylinder apparatus (Sr-anorthite) and at 200 and 500 MPa and 800–850 °C in a cold-seal hydrothermal apparatus (Ca- and Sr-anorthite). Furthermore a natural plagioclase sample from Monte Somma, Italy was included in the study.
The low-pressure anorthite samples consist of crystals about 10 μm in size, which were investigated by powder FTIR spectroscopy and powder X-ray diffraction. Rietveld refinement indicated single-phase anorthite products of triclinic symmetry (P1̅) for the Ca-anorthite experiments and a mixture of Sr-slawsonite, strontianite and Sr-anorthite of monoclinic symmetry (space group I2/c, Chiari et al., 1975) for the Sr-anorthite experiments. The 500 MPa Ca-anorthite exhibits an IR pattern very similar to those described for natural plagioclase, e.g. one broad OH-band observed at low energies (around 3200 cm−1). OH incorporation in Ca-anorthite seems to increase with pressure: at 500 MPa/800 °C it contains about 1300 wt. ppm H2O while at 200 MPa/850 °C no hydroxyl groups could be detected. The high-pressure Sr-anorthite sample consists of 50–300 μm large clear single crystals of monoclinic symmetry, which were investigated by polarised single-crystal FTIR spectroscopy, TEM using the FIB technique and powder X-ray diffraction. Polarised single-crystal infrared spectra in the OH stretching region showed a relative broad but weak band centred around 3400 cm−1 plus three sharp and intense bands: ν3 at 3629 cm −1, ν2 at 3597 cm −1 and ν1 at 3500 cm −1. We interpret the broad band to be due to stretching vibrations of structural H2O and/or molecular water in inclusions. From the pleochroic behaviour we assigned ν1 and ν2 to OC(z)–H1 (OC(z)–H2) dipoles bonded both to OD(z) oxygens but occurring in different chemical environments Al (Si). Vibration ν3 is assigned to an OB(0)–H3 dipole. The hydroxyl plus water content of the synthetic Sr-anorthite is 1100 ppm and hydroxyl concentration about 400–500 wt. ppm.