Effect of Al on the Structure of Silicate Liquids at High-Pressure

Brent Poe (brent.poe@uni-bayreuth.de)¹, Claudia Romano (romano@uniroma3.it)² & Nikolay Zotov¹

¹ Bayerisches Geoinstitut, Universität Bayreuth, D-95444 Bayreuth, Germany

² Dipartimento di Scienze Geologiche, Università degli Studi Roma Tre, I-00146 Roma, Italy

Many of the properties of silicate liquids at ambient pressure can be linked to compositionally-based parameters such as degree of polymerization (or NBO/T) and Al/(Al+Si) under the generally safe assumption that Si and Al behave as network-forming cations and occupy tetrahedrally coordinated sites in the framework structure of the liquid. However, at high pressures, this assumption breaks down, as both Al and Si may undergo transformation to both five- and six-coordination, thereby limiting our understanding of structure-property relationships in such terms. In this study, our efforts have focussed on determining the structure of silicate liquids at high pressure by examining glasses quenched isobarically from the superliquidus state at pressures up to 10 GPa. We have synthesized several glasses ranging in composition from binary silicates to multicomponent aluminosilicates in the system CMAS, and have analyzed them by Raman, XANES and NMR spectroscopy and density determination. For the silicates at low pressures, before any evidence of cation coordination increases is observed, we find that Si-O-Si bond angle narrowing is the likely mechanism of compression, correlated with a continuous increase in density. However, for the glasses containing Al, T-O-T bond angle narrowing does not appear to be favored, until possibly at higher pressures, according to our interpretation of the spectroscopic results. Instead, we suspect that compression of aluminosilicate liquids initially involves the formation of either triclusters, in which a bridging oxygen atom becomes coordinated to a third tetrahedral cation, or stronger interactions between bridging oxygen atoms and divalent cations. At higher pressures, increasing coordination of Al is clearly evident in the NMR spectra of these glasses and the onset of this compression mechanism is identified by a discontinuity in the density of the glass as a function of the pressure of its formation.

The Structure of Glasses/melts in the System CaO-Al₂O₃-SiO₂

Lorenzo Pratico' (paris@camserv.unicam.it)¹, Gabriele Giuli, Claudia Romano², Eleonora Paris & Wu Ziyu

¹ Dip. di Scienze della Terra, Università di Camerino, Camerino 62032, Italy

² Dip. di Scienze Geologiche, Terza Università di Roma, Roma, Italy

The structure of aluminosilicate melts/glasses plays a key role in the Earth sciences for the understanding of rock-forming igneous processes, as well as in the material sciences for their technical applications. The alkaline-earth aluminosilicate glasses are an extremely important group of materials, with a wide range of commercial applications, as well as serving as analogs of basaltic melts. Definition of their structure and properties is still controversial and the role and effect of Al has long been object of debate. Here we report experimental XANES (X-ray absorption near-edge structure spectroscopy) spectra recorded at the Ca, Si, and Al K-edge on synthetic glasses of peralkaline composition in the CaO-Al₂O₃-SiO₂ system. They were prepared by quenching high-temperature melts obtained at 1 atm rapidly to the glassy state. The XANES spectra were recorded at LURE (F) and SSRL (USA) synchrotron radiation centers. Both Si and Al spectra show a strong and continuous variation of spectral features as a function of silica content. Multiple scattering calculation at the Al edge (Wu et al., Phys. Rev. B 60 9216, 1999) made possible to reproduce the spectra and interpret the variation of spectral feature as a function of T-O bond length and T-O-T angle. The T-O bond length was observed to increase and the T-O-T angle to decrease as a function of increasing aluminum content of the glass. Ca-edge XANES spectra for the same series of glasses suggest almost constant Ca­O bond lengths along the series, demonstrating that calcium atoms are almost unaffected by the strong chemical variation occurring in the tetrahedral network.

Vapour Compositions in Equilibrium with Silica-Undersaturated Magmas in the System Na₂O-Al₂O₃-SiO₂-H₂O: Clues to the Composition of Fenitizing Fluids

Robin Ferguson Preston (065robbo@cosmos.wits.ac.za)¹, Gary Stevens (065gary@cosmos.wits.ac.za)¹, Terence McCarthy (065mct@cosmos.wits.ac.za)² & Deon de Bruin (ddbruin@geoscience.org.za)³

¹ Economic Geology Research Institute, University of the Witwatersrand, Private Bag 3, PO Wits, Johannesburg, 2050, South Africa

² Department of Geology, University of the Witwatersrand, Private Bag 3, PO Wits, Johannesburg, 2050, South Africa

³ Council for Geoscience, 280 Pretoria Street, Silverton, Pretoria, South Africa

Fenites result from alkali metasomatism associated with silica-undersaturated alkaline intrusions, and are characterised by alkali and aluminium addition, albitization and nephelinization. In an attempt to constrain the fluid compositions involved, we have investigated the vapour phases in equilibrium with various silica-undersaturated alkaline magmas at conditions of 850°C and 1 kb. Experiments were run for five days in cold-seal bombs, housing a platinum capsule containing an Al₂O₃-Na₂O-SiO₂ mixture and 40 wt% deionised water. Starting compositions straddled the nepheline-albite join, and included peralkaline and alkali-granitoid compositions.

The quenched run products all contained a homogeneous glass and an aqueous fluid, and, in most cases, a radial crystalline phase which often occurred as spherical beads. These never occurred as inclusions in the glass and are interpreted to be a vapour quench phase. Several glasses also contained albite or nepheline crystals.

Starting compositions plotting to the peralkaline side of the albite-nepheline join in Al₂O₃-Na₂O-SiO₂ space fractionated to a more Si-enriched glass and Na-enriched vapour. Vapour compositions, in crystal free runs, were calculated from microprobe data on glass compositions and carefully determined weights of the run products and starting materials. A typical calculated vapour composition, produced in conjunction with a melt consisting of SiO₂ = 33.91, Al₂O₃ = 30.23 and Na₂O = 19.98 (all in wt%), is: 16 wt% SiO₂, 12 wt% Al₂O₃, 27 wt% Na₂O and 45 wt% H₂O.

This vapour may provide an analogue for natural fenitizing fluids as complete reaction of this vapour with an average granite (Clarke, 1992) at a fluid:rock ratio of approximately 0.7:1 would produce a nepheline-bearing rock. Further, most alkali intrusive and extrusive rocks appear not to accurately represent the composition of the magmatic system from which they were derived. Highly charged vapours of the type reported here may represent a considerable fraction of the entire chemical system.

Clarke DB, Granitoid Rocks, Chapman and Hall, 283 pp, (1992).