The structure of high-Mg alkali-bearing aluminosilicate glasses investigated in situ at ambient and high pressure by multi-angle energy dispersive X-ray diffraction and infrared microspectroscopy

Abstract

The structural properties of synthetic high-Mg alkali-bearing aluminosilicate glasses analogues of natural picritic-to-komatiitic magmas were investigated in situ by multi-angle energy dispersive X-ray diffraction at ambient pressure and 2.1 GPa and by Fourier Transform infrared spectroscopy up to 5.4 GPa upon compression and decompression. Our results reveal that the intermediate range ordering of the glass structure at 2.1 GPa is 3.14 Å, increasing to 3.19 Å when decompressed to ambient pressure. The local structure at 2.1 GPa shows T-O lengths of 1.66 Å and T-T distances of 3.19 Å, resulting in a T-O-T angle of 147°. Upon decompression, T-O lengths shorten to 1.65 Å and T-T distances increase to 3.21 Å, widening the T-O-T angle to 154°. The deconvoluted infrared spectra result in the presence of Q¹, Q², Q³ populations in the aluminosilicate spectral region, whose proportions remain relatively unchanged up to 5.4 GPa. The structural response of the investigated glasses to cold compression does not involve changes in polymerization, but rather a shrinking and compaction of the structure as evidenced by the Qⁿ species shifting to higher wavenumbers as a function of pressure. The structural properties determined by X-ray diffraction for this glass composition are discussed along with those of glasses from previous studies to highlight a compositional dependence dictated by the amount of SiO₂ and Al₂O₃.