Relationship between topological order and glass forming ability in densely packed enstatite and forsterite composition glasses

Abstract

The atomic structures of magnesium silicate melts are key to understanding processes related to the evolution of the Earth's mantle and represent precursors to the formation of most igneous rocks. Magnesium silicate compositions also represent a major component of many glass ceramics, and depending on their composition can span the entire fragility range of glass formation. The silica rich enstatite (MgSiO 3 ) composition is a good glass former, whereas the forsterite (Mg 2 SiO 4 ) composition is at the limit of glass formation. Here, the structure of MgSiO 3 and Mg 2 SiO 4 composition glasses obtained from levitated liquids have been modeled using Reverse Monte Carlo fits to diffraction data and by density functional theory. A ring statistics analysis suggests that the lower glass forming ability of the Mg 2 SiO 4 glass is associated with a topologically ordered and very narrow ring distribution. The MgO x polyhedra have a variety of irregular shapes in MgSiO 3 and Mg 2 SiO 4 glasses and a cavity analysis demonstrates that both glasses have almost no free volume due to a large contribution from edge sharing of MgO x -MgO x polyhedra. It is found that while the atomic volume of Mg cations in the glasses increases compared to that of the crystalline phases, the number of Mg-O contacts is reduced, although the effective chemical interaction of Mg 2þ remains similar. This unusual structure-property relation of Mg 2 SiO 4 glass demonstrates that by using containerless processing it may be possible to synthesize new families of dense glasses and glass ceramics with zero porosity.Earth science | glass structure G lasses can be synthesized by making use of a network former, such as B 2 O 3 , As 2 O 3 , SiO 2 , GeO 2 , or P 2 O 5 which form continuous three-dimensional networks, according to Zachariasen's theory (1). On the basis of this theory many ideas on glass formation have been proposed. One such idea by Cooper and Gupta proposed that a topologically disordered network is the key to understanding the origin of glass formation (2, 3). Indeed, it is well known that the silica glass network exhibits a broad ring distribution which is topologically disordered, and is made up of a connection of SiO 4 tetrahedra sharing oxygen atoms at the corners (4, 5). In contrast the corresponding crystalline phase has only 6-fold rings.Recently the use of the aerodynamic levitation technique combined with a laser heating system has expanded the glass forming range of many oxides, due to the avoidance of contact between the container walls and high-temperature melt (6). Tangeman and coworkers succeeded in synthesizing MgO-SiO 2 glasses over a wide compositional range (7), covering the enstatite composition (MgSiO 3 ) to the forsterite composition (Mg 2 SiO 4 ), which can be considered as analogues of quenched melts from the Earth and Lunar mantle (8-10). The Mg 2 SiO 4 composition glass has been formerly synthesized using a rapid quench method and studied using vibrational spectroscopy (11, 12), however, the advantage of the levitat...

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