The Structure of Silicate Melts: A Glass Perspective

Abstract

Glasses are widely used as analogues for the study of silicate melts. Although they are solid materials, their structure is inherently complex and diffi cult to study. However, progress has been made in elucidating this structure, its relationship to composition, and how it behaves at high temperatures and pressures. Recent research has brought to light some new fi ndings with important implications for natural melts. These include the observation that the structure is dependent upon the type and size of alkali or alkaline-earth cations incorporated into the glass network, with Li behaving very differently than other alkali cations. In addition, the presence of more than one type of alkali cation can cause non-linear physical behavior. Elements such as Al, Ti and Fe, which play important roles in petrological phenomena, can exhibit coordination environments (e.g., 5-fold) or polyhedral arrangements (e.g., triclusters) that are not common in crystalline phases. Furthermore, polyamorphic phase-transitions may occur at high temperature and pressures; the application of new theoretical models like mean fi eld-constraint hypothesis suggests that several glass phenomena, and probably melt phenomena as well, may be related to stress-rigid to fl oppy transitions. The state of understanding of glass structure, although still rudimentary relative to crystalline materials, has grown exponentially over the last few decades, often with geological researchers at the forefront.Keywords: melts, structure, glasses, petrology, spectroscopy, behavior, model. SOMMAIREOn se sert couramment des verres pour étudier les bains fondus silicatés. Quoiqu'ils sont des matériaux solides, leur structure est naturellement complexe et diffi cile à étudier. Toutefois, des progrès ont été faits dans l'étude de cette structure, sa relation à la composition, et son comportement à températures et pressions élevées. Les projets de recherche récents font ressortir les implications importantes aux bains fondus naturels. A titre d'exemple, la structure dépendrait de la sorte et la taille d'ion alcalin ou alcalino-terreux qui se trouve incorporé dans le réseau du verre. Le Li se comporte de façon très différente des autres alcalins. De plus, la présence de plus d'une sorte d'alcalin peut causer des comportements physiques non-linéaires. Les élements tels Al, Ti et Fe, dont l'importance pétrologique est évidente, peuvent adopter une coordinence inhabituelle, cinq par exemple, et un arrangement polyédrique, des groupements par trois, par exemple, qui ne sont pas communs dans les phases cristallines. De plus, des transitions de phases polyamorphiques pourraient avoir lieu à températures et à pressions élevées. L'application de nouveaux modèles théoriques, par exemple le modèle de contrainte moyenne du champ, fait penser que plusieurs phénomènes impliquant les verres, et tout probablement les bains fondus, pourraient résulter de transitions de matériau rigide face aux contraintes à maté-riau déformable. Quoique les connaissances de la structure du ve...

Structure and Properties of Slags Used in the Continuous Casting of Steel: Part 1 Conventional Mould Powders

“…Silicate Structure Much of our knowledge of the structure of silicate melts is based on the spectroscopic work carried out by the geological and glass fraternities. [26][27][28] The standard building block of silicate slags is the Si-O tetrahedron. In pure SiO 2 , each Si 4 + ion is surrounded by 4 O − ions and each O − is connected covalently to another Si-tetrahedron ( Fig.…”

“…27,28) The size of the cation is important since it affects (i) the strength of the ionic bond formed with the NBO, where the bond strength is usually represented by the cation field strength (z/r 2 ) (ii) the coordination number (N coord ) since N coord tends to increase with increasing cation size 27,28) and (iii) the movement of cations since large cations tend to move more sluggishly through melts than small cations (leading to lower diffusion coefficients and electrical conductivities). In general, cation effects are smaller than the effects of polymerisation; this can be seen from Fig.…”

“…Increasing cation field strength (K + < Na + < Li + < Ca 2 + < Mg 2 + ) has been reported to cause the following structural changes in slags: 27,28) (i) A wider distribution of polymeric species (Q n ) (ii) A lower coordination number (N coord ) which, in turn, affects the numbers of BOs and NBOs present (iii) A more compact distribution of inter-tetrahedral bond angles (Si-O-Si or Si-O-Al in alumino-silicates) which affects the amount of disorder present (i.e. configurational entropy).…”

“…19,42) The Ti 4 + ions could possibly act as (i) network formers (ii) act as network breakers (i.e. Ti ions adopt 5-( (5) Ti, 4 + ) or 6-fold coordination 27) and (iii) form TiO 2 -like clusters (since there is a tendency for Ti 4 + ions to prefer the company of their own kind (i.e. Ti-O-Ti and Si-O-Si are preferred to Ti-O-Si) as witnessed by the tendency for phase separation in the CaO-SiO 2 -TiO 2 system).…”

“…the "configurational entropy" is higher than that in the equivalent crystalline phase). Disorder between BOs and NBOs (or disorder between various Q n species) makes significant contributions to the configurational entropy (50% of the total comes from ideal mixing 27) ). Certain slag components promote the formation of the glass phase (e.g.…”