Microsommite: crystal chemistry, phase transitions, Ising model and Monte Carlo simulations

Microsommite, ideal formula [Na4K2(SO4)] [Ca2Cl2][Si6Al6O24], is a rare fel dspathoid that occurs in volcanic products of Vesuvius. It belongs to the c ancrinite-davyne group of minerals, presenting an ABAB... stacking sequence of layers that contain six-membered rings of tetrahedra, with Si and Al ca tions regularly alternating in the tetrahedral sites. The structure was ref ined in space group P6(3) to R = 0.053 by means of single-crystal X-ray dif fraction data. The cell parameters are a = 22,161 Angstrom = 3a(dav), c = 5 .358 Angstrom = C-dav; Z = 3. The superstructure arises due to the long-ran ge ordering of extra-framework ions within the channels of the structure. T his ordering progressively decreases with rising temperature until it is co mpletely lost and microsommite transforms into davyne. The order-disorder t ransformation has been monitored in several crystals by means of X-ray supe rstructure reflections and the critical parameters T-c approximate to 750 d egreesC and beta approximate to 0.12 were obtained. The kinetics of the ord ering process were followed at different temperatures and the activation en ergy was determined to be about 125 kJ mol(-1). The continuous order-disord er phase transition in microsommite has been discussed on the basis of a tw o-dimensional Ising model in a triangular lattice with nn (nearest neighbou rs) and nnn (next-nearest neighbours) interactions. Such a model was simula ted using a Monte Carlo technique. The theoretical model well matches the e xperimental data; two phase transitions were indicated by the simulated run s: at low temperature only one of the three sublattices begins to disorder, whereas the second transition involves all three sublattices.