Molecular dynamics simulation of the effect of the Al-O-Al linkage on the P(1)over-bar-I(1)over-bar phase transition of anorthite

Molecular dynamics (MD) simulations of the influence of the Al-O-Al linkage on the <P(1)over bar>-<I(1)over bar> phase transition of pure anorthite (A n) were carried out using two different types of structures with fully orde red (FO) and partially disordered (PD) arrangements of Al/Si in tetrahedral sites. Discontinuous changes in unit cell volumes and structure factors at the transition temperature were observed in FO-An but not in PD-hn. These results show that the orders of the transitions of FO-An and PD-An are firs t and non-first, respectively. In both structures, the motions of the Ca at oms and the framework are strongly correlated with each other during the tr ansition and Ca atoms dominate the system. Since high-temperature X-ray stu dies have shown that the transition of natural anorthite is non-first order , it is suggested that the natural anorthite has a partially disordered arr angement of Al and Si atoms.