Abstract
Electrostatic energy calculations are made to investigate distortions in the mica structure. The use of the electrostatic model is valid to explain 99 percent of the variation of K–O bondlengths in structure refinements. The interlayer cation has little influence on tetrahedral rotation, because its site has a more favorable potential as the tetrahedral rotation decreases and the tetrahedra are elongated. It is suggested that the tetrahedral elongation is larger when the positive tetrahedral charge is lower. This is confirmed by phlogopite refinements. It is suggested that a larger octahedral cation diminishes the counter-rotation of upper and lower oxygen triads in the dioctahedral layer. This “octahedral rotation” deforms the tetrahedral layer, and determines the relative stability of the 1M and 2M, polytypes. The 1M polytype would be more stable by less than 0.2 kcal if the octahedral rotation remains less than 4°. The larger octahedral cations in glauconite and celadonite explain the exclusive occurrence of these minerals as 1M polytypes.
