Low-temperature infrared spectrum and atomic-scale structure of hydrous defects in diopside

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.

First-principles modeling of the infrared spectrum of Fe- and Al-bearing lizardite

“…The theoretical correlation between bond length and OH frequencies (Tables 1 and 2) for the series of investigated models (Fig. 6) is also consistent with those previously determined at the same theoretical level on a series of hydroxylated defects in diopside and corundum (Balan et al, 2020;Balan, 2020). This relation between the vibrational frequency and a geometrical parameter rules out a contribution of the mass of the surrounding cations to the stretching dynamic of the OH groups, which is decoupled from the motion of other atoms.…”

“…The comparison reveals an overestimation of the theoretical stretching frequencies of interlayer-OH groups amounting to 50 cm −1 , which is consistent with the observations made on antigorite (∼ 57 cm −1 , Balan et al, 2021b). It is also in the range expected from previous investigations on hydrous defects in oxides and silicates (Balan et al, 2020;Balan, 2020). The difference between theoretical and experimental frequencies results from the partial cancellation of two types of errors, one being due to the use of an approximate exchange-correlation functional, which tends to underestimate the vibrational frequencies, and the other to the use of the harmonic approximation, which artificially increases (Balan et al, 2002); full squares: Raman frequencies of Monte Fico lizardite (Compagnoni et al, 2021).…”

Machine Learning for Identification of Primary Water Concentrations in Mantle Pyroxene

“…Similarly, it is also impossible to distinguish the occurrence of H diffusion based on correlations between major element components in cpx (Figure 2). Mineralogical studies indicate that the substitution mechanism of H in cpx is highly complex (Balan et al., 2020). During crystallization, the H 2 O content of cpx is coupled with its major element composition, but this relationship breaks down when subsequent H diffusion occurs.…”

“…The complexity of many magmatic systems means that the lack of a H composition profile in cpx does not exclude H diffusion or vice versa (e.g., Wade et al., 2008). Moreover, multiple substitution mechanisms for H in cpx (e.g., associated with Al for Si substitution, associated with Na for Ca substitution, four H atoms substituted for T site in lattice, two H atoms substituted for M1 or M2 site in the lattice and so on) have been proposed on the basis of high temperature experiments and theoretical properties (e.g., Andrut et al., 2007; Balan et al., 2020; Bromiley & Keppler, 2004; Skogby & Rossman, 1989; Smyth et al., 1991). Accordingly, H 2 O contents may not be related to a single specific element or component in cpx (Figure ).…”

“…Changes in these intensive variables are also reflected in the compositional characteristics of cpx phenocrysts crystallized from the magma. Complex substitution mechanisms for H in cpx indicate that H is related to all the lattice‐forming major elements in cpx (Balan et al., 2020). Clinopyroxene crystallization from silicate melt establishes the primary relationship between H and the major element composition of cpx, and the measured chemical composition of cpx grains is therefore expected to record information regarding the initial H 2 O content.…”