AbstractAs part of a wider study of the nature and origins of cation orderdisorder in micas, a variety of computational techniques have been used to investigate the nature of tetrahedral and octahedral ordering in phengite, K2[6](Al3Mg)[4](Si7Al)O20(OH)4. Values of the atomic exchange interaction parameters Jn used to model the energies of orderdisorder were calculated. Both tetrahedral AlSi and octahedral AlMg ordering were studied and hence three types of interaction parameter were necessary: for TT, OO and TO interactions (where T denotes tetrahedral sites and O denotes octahedral sites). Values for the TT and OO interactions were taken from results on other systems, whilst we calculated new values for the TO interactions. We have demonstrated that modelling the octahedral and tetrahedral sheets alone and independently produces different results from modelling a whole TOT layer, hence justifying the inclusion of the TO interactions. Simulations of a whole TOT layer of phengite indicated the presence of short-range order, but no long-range order was observed.