A predictive equation for the refractive indices of silicate melts containing alkali, alkaline earth and aluminium oxides

Refractive indices of multicomponent silicate melts containing alumina have been determined using a high temperature ellipsometer and the data used to propose a predictive equation for the refractive indices of silicate melts. Refractive indices were measured at a wavelength of 632.8 nm over the temperature range 1100-1800 K and the standard deviation of the refractive indices measured was within ±0.0012. The samples studied contained 2-6 components from the oxides SiO 2 , Li 2 O, Na 2 O, K 2 O, MgO, CaO and Al 2 O 3 , where the mole fraction of SiO 2 was at least 0.4 and the mole fraction of A l 2 O 3 was smaller than or equal to the sum of those of alkali (R 2 O) and/or alkaline earth oxides (RO). Assuming that the structure of the aluminosilicate melts consists of only three kinds of chemical bond, i.e. Si-BO (BO: bridging oxygen), Al-BO with a charge compensating cation and Si-NBO (NBO: nonbridging oxygen) with a charge compensating cation, a predictive equation for the refractive index of the aluminosilicate melts is proposed to be n=a Si-BO n Si-BO +Σb Si-NBO(Ri) n Si - NBO(Ri) +Σc Al-BO(Ri) n Al-BO(Ri) where a Si-BO , b Si-BO(Ri) and c Al-BO(Ri) are, respectively, the numbers of Si-BO, Si-NBO and Al-BO bonds, where (R i ) represents the charge compensating ion for Si-NBO and Al-BO bonds (either an R i2 O or an R i O oxide). Parameters n Si-BO , n Si-NBO(Ri) and n Al-BO(Ri) are, respectively, values to express refractive indices per mole of Si-BO, Si-NBO and Al-BO bonds and have been determined as functions of temperature from the measured values of refractive index. It has been found that this equation can predict refractive index values with small error of ±0.65 % even in six component silicate melts.