Low-temperature evolution of OH bands in synthetic forsterite, implication for the nature of H defects at high pressure

J. Ingrin*, J. Liu, C. Depecker, S. C. Kohn, E. Balan, K. J. Grant

*Corresponding author for this work

    Research output: Contribution to journalArticle

    30 Citations (Scopus)

    Abstract

    We performed in situ infrared spectroscopic measurements of OH bands in a forsterite single crystal between -194 and 200 °C. The crystal was synthesized at 2 GPa from a cooling experiment performed between 1,400 and 1,275 °C at a rate of 1 °C per hour under high silica-activity conditions. Twenty-four individual bands were identified at low temperature. Three different groups can be distinguished: (1) Most of the OH bands between 3,300 and 3,650 cm-1 display a small frequency lowering (<4 cm-1) and a moderate broadening (<10 cm-1) as temperature is increased from -194 to 200 °C. The behaviour of these bands is compatible with weakly H-bonded OH groups associated with hydrogen substitution into silicon tetrahedra; (2) In the same frequency range, two bands at 3,617 and 3,566 cm-1 display a significantly anharmonic behaviour with stronger frequency lowering (42 and 27 cm-1 respectively) and broadening (~30 cm-1) with increasing temperature. It is tentatively proposed that the defects responsible for these OH bands correspond to H atoms in interstitial position; (3) In the frequency region between 3,300 and 3,000 cm-1, three broad bands are identified at 3,151, 3,178 and 3,217 cm-1, at -194 °C. They exhibit significant frequency increase (~20 cm-1) and broadening (~70 cm-1) with increasing temperature, indicating moderate H bonding. These bands are compatible with (2H)Mg defects. A survey of published spectra of forsterite samples synthesized above 5 GPa shows that about 75 % of the incorporated hydrogen belongs to type (1) OH bands associated with Si substitution and 25 % to the broad band at 3,566 cm-1 (type (2); 3,550 cm-1 at room temperature). The contribution of OH bands of type (3), associated to (2H)Mg defects, is negligible. Therefore, solubility of hydrogen in forsterite (and natural olivine compositions) cannot be described by a single solubility law, but by the combination of at least two laws, with different activation volumes and water fugacity exponents.

    Original languageEnglish
    Pages (from-to)499-510
    Number of pages12
    JournalPhysics and Chemistry of Minerals
    Volume40
    Issue number6
    DOIs
    Publication statusPublished - Jun 2013

    Keywords

    • Band shift
    • Forsterite
    • H defects
    • Hydrogen
    • Infrared
    • OH bands
    • Olivine
    • Silicon vacancies
    • Water solubility laws

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