Here we report a detailed exchange Monte Carlo study using a classical ionic model of the model perovskite parascandolaiteneighborite (K,Na)MgF3 solid solution and its end-members for temperatures in the range 300—1000 K and pressures from 0—8 GPa. <...> Full account is taken of the local environment of the individual cations, clustering and thermal effects. <...> Properties considered include the crystal structure, phase transitions, the thermodynamics of mixing and the non-ideality of the solid solution. <...> Clustering of the potassium ions is examined via a short-range order parameter. <...> INTRODUCTION Solid solutions and phase stability continue to present considerable challenges for theory. <...> Energy differences between different phases can be small and subtle cation ordering effects can be often crucial in determining phase stability and thermodynamic and chemical properties. <...> This solution is not only an excellent test of any theoretical model, but also serves as a useful analogue for the silicate perovskite (Mg, Fe)(Al,Si)O3, which is a dominant phase under the conditions of the lower mantle of the Earth (pressure > 25 GPa, temperature > 2000 K) [ 1—4]. <...> Neighborite NaMgF3 [ 11 ] is isoelectronic and isostructural with MgSiO3; the ratios of the formal cation charges are the same in both compounds (1:2) and the ratio of their ionic radii are about the same. <...> KMgF3 was until very recently known only as a synthetic crystal but has now been identified as the new mineral parascandolaite, found as a volcanic sublimate at Vesuvius [ 12 ]. <...> KMgF3 is a cubic perovskite (Fig. 1, a), while in NaMgF3 there is an orthorhombic distortion such that the Mg—F—Mg bridges linking the MgF6 octahedra are not linear (Fig. 1, b). <...> Cubic perovskite structure AMF3 (a), Representative tilting of MF6 octahedra in an orthorhombic perovskite (b) rules (the so-called tolerance factor) is possible [ 13, 14 ]. <...> The Na+ cation is too small to touch the twelve neighbouring anions in a cubic structure and the Mg—F—Mg links bend, tilting the MgF6 octahedra to bring further anions into contact with the A cations. <...> An extensive NMR and X-ray diffraction study of the effect of the A-site cation radius on the ordering of BX6 octahedra in (K,Na)MgF3 <...>