Potassic cordierites with the chemical composition KxMgaA14+xSi s_xO18 (x = 0.00, 0.10, 0.20, and 0.25) were synthesized by annealing glasses at 1290 ~ C for different lengths of time. The procedure resulted in cordierites with different states of A1, Si-order for the tetrahedral sites in the structure. The dependence between the potassium-content and the state of order on one side and between annealing time and the state of order on the other side was then studied using 29Si MAS nuclear magnetic resonance (NMR) spectroscopy. The spectra show that the state of order is a continuous function of annealing time for all compositions considered, but the rate of ordering decreases with increasing K-content. Since the substitution K + A1 ~ Si leads to higher A1/Si-ratios; the lower rate of ordering is discussed as a consequence of changed statistics for A1,Si site exchanges. The A1 atoms replacing silicon in the structure to balance the charge of potassium cations are not located close to the potassium ion but at a maximum distance from it. This is shown to be a consequence of an improvement in coordination of all oxygen atoms in the cordierite flamework.
hexagonal symmetry . Annealing leads to an ordering of aluminium and silicon in the low-temperature polymorph with orthorhombic symmetry (Gibbs 1966). In this phase each six-membered ring contains two aluminium and four silicon atoms with a ratio A1/Si of 1:2 for the T2-tetrahedra. The respective ratio for the ring-connecting Tl-tetrahedra is 2:1. J