A SIMPLE-MODEL FOR REFRACTIVE-INDEXES IN MIXED-CRYSTALS AND GLASSES

The "simple model" for the refractive indices of mixtures of silicate crystalline phases or glasses is based on the use of the "excess volume" formulation for a "simple mixture" (in the sense defined by Guggenheim), together with the Lorentz-Lorenz formulation of molar refractivity. The resulting application to refractive index and/or volume data yields a parameter, psi, the pressure derivative of the so-called mismatch energy of the mixture. This model explains precisely the indices for the thallium halides for which a preliminary attempt at such a model was made by Barth in 1930, and works rather well for a number of silicate systems and for at least one glass mixture. A free parameter, the Lorentz-Lorenz" overlap field" term, is fixed at the Lorentz-Lorenz value of 4-pi/3 in the molal refractivity expression but could actually be a variable for different types of silicate mixtures. I also solve the old, old problem posed by BOWEN and SCHAIRER (1935): The linearity of refractive index vs. composition plots on a wt% basis and its later use to eliminate "kinks" in plots with complicated endmembers.