Effect of high pressure on the refractive index and density of natural aluminosilicate glasses of alkali basalt composition in the SiO2-Al2O3-TiO2-Fe2O3-P2O5-FeO-MnO-CaO-MgO-Na2O-K2O system

Abstract

The pressure dependence of the refractive index for a natural glass of composition 0.528SiO 2 · 0.019TiO 2 · 0.102Al 2 O 3 · 0.021Fe 2 O 3 · 0.062FeO · 0.002MnO · 0.081CaO · 0.072MgO · 0.068Na 2 O · 0.034K 2 O · 0.011P 2 O 5 at pressures up to 5.0 GPa is determined using a polarizing interference microscope and an apparatus with diamond anvils. The densities and compressibilities of the glass are calculated from the measured refractive indices within the theory of photoelasticity. The results are compared with the data obtained earlier for another glass that has a similar composition but contains a lower concentration of alkali metal oxides. A higher compressibility of the glass under investigation as compared to the compressibility of the glass studied previously is in agreement with the degrees of polymerization that are calculated taking into account the formation of triclusters.

Influence of pressure on the refractive index and relative density of glasses in the CaO-Al2O3-SiO2 system

“…This behavior is similar to the behavior observed in the pressure dependences of the relative changes in the density of other silicate glasses studied in our previous works [7,9,11,12]. The pressure coefficient of the relative change in the density initially increases, passes through a maximum in the pressure range 1.0-1.5 GPa, and then slowly decreases.…”

“…In the pressure range 1.0-6.0 GPa, the compressibility of the 0.167CaO · 0.167Al 2 O 3 · 0.666SiO 2 glass with the ratio of molar concentrations [13] and, more recently, Toplis et al [3] assumed that aluminum can be incorporated into silicate melts and glasses through the formation of triclusters composed of three tetrahedra joined via a common oxygen atom. In our earlier studies [11,14], we used this assumption as the starting point and proposed a model describing the formation of nonbridging oxygen atoms in the structure of aluminosilicate glasses and melts, in which excess charges in aluminum-oxygen tetrahedra are compensated at the expense of doubly charged cations, and a relationship for calculating the degree of polymerization of aluminosilicate glasses and melts with allowance for the formation of triclusters and the presence of 25% aluminum cations with a coordination number of more than four. Taking into account the recent experimental results obtained by Neuville et al [15], according to which the glasses with the ratio of molar concentrations [CaO]/([CaO] + [Al 2 O 3 ]) = 0.5 contain ~10% highly coordinated aluminum, we modified the aforementioned relationship in our previous paper [16].…”

“…The relative changes in the density of the aluminosilicate glasses were calculated from the experimental data on the refractive index in the framework of the Mueller theory of photoelasticity [10] according to the equation ∆d/d = 6n∆n/(n 2 -1)(n 2 + 2)(1 -Λ), where Λ = −(∆R/R 0 )/(∆d/d) is the Mueller strain-polarizability constant. The above equation was obtained by the differentiation of the Lorentz-Lorenz equation (n 2 -1)/(n 2 + 2) = dR. For the aluminosilicate glasses, the strainpolarizability constant was determined to be Λ ≅ 0.24 [11]. The experimental data on the refractive index for the glasses under investigation at pressures up to 6.0 GPa and the results of the calculations of the relative changes in the density are presented in Tables 1 and 2 and in the figure.…”

Degree of polymerization of the CaAl2Si2O8 aluminosilicate glass

“…The refractive index of aluminosilicate glasses is larger than that of silicate glasses due to the stronger polarizing effect of Al 3+ ions as compared to Si 4+ ions. For these glasses, the strain-polarizability constant was determined to be Λ = 0.24 [14]. The calculated pressure dependences of the density and the compressibility for the CaAl 2 Si 2 O 8 glass in the pressure range up to 5.0 GPa are presented in Fig.…”

“…Lacy [31] and Toplis et al [32] assumed that aluminum can be incorporated into silicate glasses and melts through the formation of triclusters composed of three tetrahedra joined via a common oxygen atom. By using this assumption as the starting point, a technique was proposed in our previous works [33,14] for calculating the degree of polymerization of aluminosilicate glasses and melts with allowance for the presence of triclusters. The structural-chemical parameter NBO/T was calculated with due regard for the type of (singly charged or doubly charged) cations contained in glasses in order to compensate for excess charges in aluminum-oxygen tetrahedra.…”