Original paper

Influence of Fe, Cr and Al on hydrogen incorporation in orthopyroxene

Stalder, Roland


Three series of enstatite crystals doped with different amounts of Fe, Cr or Al were synthesized under water-saturated conditions at 25 kbar. The OH-incorporation was studied by FTIR-spectroscopy on oriented crystal sections. Strongest absorptions (approximately 65 % of total absortion) occurred in all cases when n γ was parallel to the electric field vector E of the incident light, weakest absorptions were observed when nβ was parallel to E. In addition to the O-H absortion bands of pure enstatite (at 3070 and 3361 cm-1, respectively), IR spectra on orthopyroxenes containing trivalent cations reveal additional peaks between 3250 and 3350 cm-1 as well as above 3400 cm-1, consistent with the generation of new hydrogen sites and a distortion of the polyhedral chains and cationic effects influencing the local electric field around the H bond. The amount of the incorporated OH correlates well with the concentration of trivalent cations in the crystal, consistent to the exchange mechanisms Fe3+ + H+ ⇔ 2 Mg2+ and Cr3+ + H+ ⇔ 2 Mg2+. Comparison to natural samples from the Earth's upper mantle strongly suggests that coupled substitutions involving Al3+ dominate the H+ incorporation.


orthopyroxenehydrogen incorporationtransition metalir-spectroscopyhigh pressure