Location and quantification of hydrogen in Ca- and Sr-anorthite

Abstract

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 ( P 1) 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 H 2 O 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: ν 3 at 3629 cm −1 , ν 2 at 3597 cm −1 and ν 1 at 3500 cm −1 . We interpret the broad band to be due to stretching vibrations of structural H 2 O and/or molecular water in inclusions. From the pleochroic behaviour we assigned ν 1 and ν 2 to O C (z)–H1 (O C (z)–H2) dipoles bonded both to O D (z) oxygens but occurring in different chemical environments Al (Si). Vibration ν 3 is assigned to an O B (0)–H3 dipole. The hydroxyl plus water content of the synthetic Sr-anorthite is 1100 ppm and hydroxyl concentration about 400–500 wt. ppm.