The Anorthite–Diopside System: Structural and Devitrification Study. Part I: Structural Characterization by Molecular Dynamic Simulations

Molecular dynamic simulations of glasses belonging to the anorthite-diopside system have been performed in order to ob- tain an atomistic description of the material's structure. The structural parameters obtained by the simulations allow to con- firm that the glass materials are characterized by a very similar short-range environment. The main differences have been ob- served in the intermediate-range order of the structure that de- scribe the distribution and the packing of the tetrahedra constituting the three-dimensional networks. It is shown that the glass materials with composition close to the two extremes, corresponding to the pure glass anorthite or pure glass diopside, display the typical structural features of the tectosilicates and inosilicates subclass of minerals, respectively, to which anorthite and diopside crystals belong. largely used in the last few years for industrial applications. 1-3 The knowledge of the atomistic arrangement in glass materials is an essential requirement for the interpretation of their physical and chemical properties. Unfortunately, while the structure of a crystalline solid can be considered as the repetition of a basic unit cell, no long-range order can be detected in the glass and the structure shows only a short- and an intermediate-range order (IRO). The assumption of dominant disorder in glasses has been in- vestigated by several methods; generally a good approach con- sists in the analysis of the similarities between microscopic and macroscopic properties of glasses and correspondent crystals. 4 The structure of a glass material is constituted by SiO4-based networks distorted by the presence of the network modifier ions that break the three-dimensional (3D) framework to satisfy their coordination. It is well known that the structure of a single- chain metal silicate lattice depends on the size and the charge of the metal atoms: the number of SiO4 tetrahedra forming the re- peating unit, and their relative disposition, is a sensitive function of these parameters. 4 In this sense, the metal cations exert an important, perhaps even dominant, influence on the structure. The diopside mineral belongs to the subclass of silicates called inosilicates. This subclass contains two distinct groups: the sin- gle- and double-chain silicates. In the single-chain group, the tetrahedra share two oxygens with two other tetrahedra and form a seemingly endless chain. The ratio of silicon to oxygen is thus 1:3. The tetrahedra alternate to the left and then to the right along the line formed by the linked oxygens although more complex chains seem to spiral. In cross-section, the chain forms a trapezium and this shape produces the angles between the crystal faces and cleavage directions. In the single-chained sili- cates, the two directions of cleavage are at nearly right angles (close to 901), forming nearly square cross-sections. The anorthite belongs to the tectosilicates subclass of miner- als. This subclass is often called the ''Framework Silicates'' be- cause the structure is composed of interconnected tetrahedra going outward in all directions forming an intricate framework analogous to the framework of a large building. In this subclass all the oxygens are shared with other tetrahedra, giving a silicon- to-oxygen ratio of 1:2. In the near-pure state of only silicon and oxygen, the mineral is quartz (SiO2). But the tectosilicates are not that simple. It turns out that the aluminum ion can easily substitute for the silicon ion in the tetrahedra up to 50%. In other subclasses this substitution occurs to a more limited extent, but in the tectosilicates it is a major basis of the varying struc- tures. While the tetrahedron is nearly the same with an aluminum at its center, the charge is now a negative five (5) instead of the normal negative four (4). Since the charge in a crystal must be balanced, additional cations are needed in the structure and this is the main reason for the great variations within this subclass. 5 In this work, the study of the glass materials has been carried out by means of an elaboration of a theoretical model, subse- quently validated with the experimental data available, in order to obtain a detailed characterization of the glass network. It is well known that computer simulations, exploring the structure and the behavior of materials at the atomic level of detail, allow the static features determination and the quantification of the relationships existing between the structure and properties of materials. 6