Abstract
The Raman spectra of barite and celestine were recorded from 25 to 600 °C at ambient pressure and both minerals were stable over the entire temperature range. Most of the Raman bands of barite decreased in wavenumber with increasing temperature with the exception of the ν2 modes and the ν4 band at 616 cm−1, which did not exhibit a significant temperature dependence. These vibrations may be constrained by the lower thermal expansion along the a-axis and b-axis of barite. Similar to barite, most of the Raman bands of celestine also decreased in wavenumber with increasing temperature, with the exception of the ν2 modes and the ν4 band at 622 cm−1, which showed very little variation with increasing temperature. Variations of Raman shift as a function of temperature and FWHM (full width at half maximum) as a function of Raman shift for the main, ν1 modes of barite and celestine show that both minerals have almost identical linear trends with a slope of −0.02 cm−1/°C and −0.5, respectively, which allows for the prediction of Raman shifts and FWHM up to much higher temperatures. The calculated isobaric and isothermal mode Grüneisen parameters and the anharmonicity parameters show that the M–O modes (M = Ba2+ and Sr2+) in barite and celestine exhibit much higher values of both mode Grüneisen parameters and anharmonicity than the SO4 tetrahedra. This indicates that the S–O distances and S–O–S angles are less sensitive to pressure and temperature increase than the M–O distances in the structure. Furthermore, the generally higher anharmonicity in celestine is due to the smaller size of the Sr2+ cation, which causes the celestine structure to be more distorted than the barite structure.
Highlights
Barite (BaSO4 ) and celestine (SrSO4 ) are essential for many modern industries
Plots of temperature variation of the Raman bands of barite show that most of the Raman bands decreased in wavenumber with increasing temperature with the exception of the ν2 modes observed at 452 cm−1 and 461 cm−1 and the ν4 band at 616 cm−1, which did not exhibit a significant temperature dependence
Most of the Raman bands of celestine decreased in wavenumber with increasing temperature, with the exception of the ν2 modes at 454 cm−1 and 461 cm−1 and the ν4 band at 622 cm−1, which is the same as for barite
Summary
Barite (BaSO4 ) and celestine (SrSO4 ) are essential for many modern industries. The high density and low oil absorption properties of barite make it useful in many industrial applications [1]. Barite is used in the plastics industry to improve the stiffness, intensity, and abrasive strength of plastics and in the paper-making industry to improve the whiteness of paper products It is used in the paint industry for increasing the stability of the paint and adding more brightness to the paint and in the rubber industry to make the rubber acid- and alkali-proof. The larger Ba2+ cation forms a longer average Ba–O distance, and the charge on the O atoms is larger and the average S–O distance is shorter. This results in the celestine structure being more distorted than the barite structure [19]. In this paper, we report the Raman spectra of barite and celestine at temperatures up to 600 ◦ C at atmospheric pressure and consider how temperature effects the crystal structure of barite and celestine
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