Halite-sylvite thermoelasticity

Abstract

Unit-cell volumes of four single-phase intermediate halite-sylvite solid solutions have been measured to pressures and temperatures of ~28 kbar and ~700 °C. Equation-of-state fitting of the data yields thermal expansion and compressibility as a function of composition across the chloride series. The variation of the product α₀·K₀ is linear (ideal) in composition between the accepted values for halite and sylvite. Taken separately, the individual values of α₀ and K₀ are not linear in composition. α₀ shows a maximum near the consolute composition (X_NaCl = 0.64) that exceeds the value for either end-member. There is a corresponding minimum in K₀. The fact that the α₀·K₀ product is variable (and incidentally so well behaved as to be linear across the composition series) reinforces the significance of the complementary maxima and minima in α₀ and K₀ (significantly, near the consolute composition). These extrema in α₀ and K₀ provide an example of intermediate properties that do not follow simply from values for the end-members.

Cell volumes across this series show small, well-behaved positive excesses, consistent with K-Na substitution causing defects through lattice mismatches. Barrett and Wallace (1954) showed maximum defect concentrations in the consolute region. Defect-riddled, weakened structures in the consolute region are more easily compressed or more easily thermally expanded, providing an explanation for our observed α₀ and K₀ variations. These compliant, loosened lattices should resist diffusive transfer less than non-defective crystals and, hence, might be expected to show higher diffusivities. Tracer diffusion rates are predicted to peak across the consolute region as exchange diffusion rates drop to zero.