Abstract
Abstract. Hydrous defects in diopside (CaMgSi2O6) play an important role in the water budget of the Earth's mantle. Related OH-stretching modes lead to a variety of infrared absorption bands observed in natural or experimental samples. In the present study, we report new low-temperature infrared spectra of reference natural diopside samples in the OH-stretching range. In parallel, the structure and vibrational properties of a series of OH-bearing defects in diopside are theoretically determined at the density functional theory level. The infrared spectra make it possible to resolve additional bands in the region above 3600 cm−1 and reveal that their anharmonic behavior differs from that of the bands at lower frequency. A comparison of theoretical results with experimental data makes it possible to propose atomic-scale geometries corresponding to observed OH-stretching bands. It confirms that the bands observed at 3620–3651 cm−1 are related to M3+ ions substituted for Si in tetrahedral sites, while the 3420 cm−1 band is associated with the Na+ for Ca2+ substitution. In both cases, H+ incorporation compensates the charge deficit due to the heterovalent substitution. The other major mechanism of water incorporation in diopside relates to the charge compensation of cationic vacancies, among which Ca vacancies play a central role. The 3357 cm−1 band corresponds to doubly protonated Ca vacancies in pure diopside. In experimental diopside-bearing trivalent cations, the bands at 3432–3460 cm−1 correspond to singly protonated Ca vacancies with a nearby octahedral M3+ ion, while the 3310 cm−1 band likely involves a more remote charge compensation by M3+ ions. More complex defects associating Ca vacancies with tetrahedral M3+ and octahedral Ti4+ ions are proposed for the bands observed between 3500 and 3600 cm−1 in natural diopside. The Fe2+ for Mg2+ and Fe2+ for Ca2+ substitutions are also found to affect nearby OH-bearing defects, causing a shift and broadening of OH stretching bands in chemically more complex diopside samples.
Highlights
Diopside is considered a major host of “water” in the upper mantle (e.g., Demouchy and Bolfan-Casanova, 2016) contributing to the Earth global budget of water
As in other nominally anhydrous minerals, water occurs as hydroxyl groups associated with cationic vacancies or chemical impurities, ensuring the electric neutrality of the defective crystal (e.g., Keppler and Smyth, 2006; Skogby, 2006)
We report new low-temperature infrared spectra of natural diopside samples previously investigated by Ingrin et al (1989), Andrut et al (2007), and Yang et al (2019)
Summary
Diopside is considered a major host of “water” in the upper mantle (e.g., Demouchy and Bolfan-Casanova, 2016) contributing to the Earth global budget of water. As in other nominally anhydrous minerals, water occurs as hydroxyl groups associated with cationic vacancies or chemical impurities, ensuring the electric neutrality of the defective crystal (e.g., Keppler and Smyth, 2006; Skogby, 2006). The occurrence of OH groups at low concentration in natural and experimental samples of diopside is attested by their characteristic OH stretching bands in Fourier-transform infrared (FTIR) spectra. The spectra display a prominent band observed at 3357 cm−1 and ascribed to divalent cation vacancies compensated by two protons (Stalder and Ludwig, 2007; Sundvall et al, 2009; Purwin et al, 2009). Natural samples, as well as experimental samples doped with chemical impurities, display a larger variety of signals. The spectra of diopside, containing trivalent cations and synthesized at 20 kbar under silica
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