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 (XNaCl = 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.