Shift of CO2-I absorption bands in diamond: A pressure or compositional effect? A FTIR mapping study

https://doi.org/10.1016/j.diamond.2021.108280Get rights and content

Highlights

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    FTIR signatures of solid CO2-I phase are found in single crystal diamonds.

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    Positions, FWHM and intensities of CO2 bands change regularly across the samples.

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    Observed behavior of CO2 bands can arise both from residual pressure and impurities.

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    Spectroscopic barometry of CO2 is reliable if Davydov splitting is evident.

Abstract

Infra-red maps and profiles with high spatial resolution were obtained for two single crystal diamonds with pronounced CO2 IR absorption peaks. Detailed examination allows unambiguous assignment of the spectral features to solid CO2-I phase. It is shown that the distribution of IR band positions, intensities and widths in the sample follows regular patterns and is not chaotic as was suggested in previous works where spectra of a few individual spots were analysed. Consequently, pressure effects alone fail to explain all observed features and shifts of the CO2 bands. Experimental data can be explained by presence of impurities (such as water, N2, etc.) in the trapped CO2. This implies that spectroscopic barometry of CO2 microinclusions in diamond may be subject to poorly controlled bias. However, barometry is still possible if Davydov splitting of the CO2-I ν2 band is unequivocally observed, as this indicates high purity of the CO2 ice.

Introduction

Carbon dioxide is an important metasomatic agent in Earth's mantle and plays a major role in the carbon cycle. The presence of CO2 in diamonds was inferred long time ago from mass-spectrometry studies [[1], [2], [3], [4]]. Presence of carbon dioxide in some gem quality single crystal diamonds follows from IR spectroscopy [5,6] and cryomicroscopy of fluid inclusions [[7], [8], [9]]. Based on the pressure-induced shift of CO2 peak positions Schrauder and Navon [6] inferred extremely high residual pressures reaching 5 GPa in one specimen, suggesting the presence of compressed solid CO2. Even higher residual pressures, up to 20 GPa, were mentioned in an extensive investigation of CO2-containing diamonds by Chinn [5]. In that work strong variations in the shape of the CO2-related bands between different samples and even between analysis points in the same specimen were reported.

Investigation of a large set of polished diamonds with CO2 IR bands (termed as “CO2 diamonds”) using a beam condenser, i.e. allowing analysis of relatively small spots, revealed high variability of positions, widths, and relative intensities of CO2 ν3 and ν2 bands [10,11]. These authors suggested that such a variability cannot be explained by the presence of CO2 phase inclusions and tentatively proposed integration of CO2-molecules in the diamond lattice. The presence of oxygen as a lattice impurity in diamond is plausible ([12] and references therein, [13]), but its correlation with the CO2-absorption is uncertain. In this work, we report the results of a detailed investigation of diamond single crystals possessing CO2 absorption bands using IR microscopy and discuss implications for diamond studies and for more general application of barometry of fluid inclusions in minerals based on spectroscopic data.

Section snippets

Samples and methods

Two CO2-rich diamonds — FN7112 and FN7114 — were studied. Diamond FN7114 was described in Hainschwang et al. [11]. The samples were obtained commercially and their source is unknown. The stones were laser cut from gems and subsequently polished to make a double-sided plate. Both samples were treated at 6 GPa and 2100 °C for 10 min; the treatment did not influence the color and CO2-related absorption [11]. The sample FN7112 is 3.16 mm in diameter and 0.89 mm thick (mass 0.08 ct); the FN7114

The samples

Both studied diamonds are single crystals with a light greenish-brown color. Diamond FN7112 shows faint yellow fluorescence under ultraviolet light excitation, while diamond FN7114 is inert. The diamonds contain tiny inclusions of roughly hexagonal shape with dimensions of 5–10 μm and thickness of less than a micron (Fig. 1). In sample FN7112 the inclusions are present in the whole body of the stone and are crystallographically oriented; in some regions larger plates appear to be surrounded by

Major CO2-I bands

At room temperature, CO2 crystallizes into a cubic phase I between 0.6 GPa and 2 GPa [15,27]; the variations in the transition pressure are ascribed to the size of the CO2 droplets and, possibly, the nature of the surrounding medium. In the pressure range of ~8–13 GPa a transition into orthorhombic CO2-III occurs [15]. Fundamental modes and, consequently, IR spectra of these phases differ. The IR spectrum of CO2-I shows two bending bands (ν2a, ν2b) and a single asymmetric stretching band (ν3).

Conclusions

A detailed FTIR investigation of polished plates cut from two CO2-rich single crystal diamonds reveals that changes of the CO2-related IR features are not as random as it might seem from examination of bulk samples. Regular evolution of the position of the bands, FWHM and intensity is recorded for one of the samples; for the second stone, several domains with highly variable spectra are observed. Consideration of various possible mechanisms responsible for blueshift of CO2-absorption bands

CRediT authorship contribution statement

E.P.B. – investigation, software, data analysis, writing draft and revised version; T.H. - resources, writing draft; A.A.S. – conceptualization, methodology, investigation, writing draft and revised version. All authors have contributed to preparation and agreed to the submitted version of the manuscript.

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgments

The study was partly supported by RFBR grant 13-05-91320-SIG-a to AAS. We thank Dr. A. Shapagin for access to FTIR microscope and Uladzislava Dabranskaya from Utrecht University for creation of the graphical abstract. We highly appreciate thorough consideration of the manuscript and highly useful comments made by two anonymous reviewers.

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