Electrostatic lattice-site energies of alkali feldspars in relation with Si/Al ordering

Abstract

Abstract In electrostatic point charge models with formal charges, lattice-site energies and crystal energies have been calculated for alkali feldspars with different degrees of Si/Al ordering. In sequence of increasing ordering, i.e. high sanidine – ordered orthoclase – low microcline, the crystal energies become more negative. The differences in crystal energy between the various polymorphs is a measure of the energy needed for the Al ordering. This ordering energy amounts to −134 kcal/mol, being the difference between an ordered and a disordered potassium feldspar. In a point charge model with lowered charges (K1+T1.75+ O8 −) this energy decreases to −25 kcal/mol. Lattice-site energies are calculated for all ion sites of high sanidine, ordered orthoclase, low microcline and analbite. During the two-step ordering the Al concentrates first on both T1 sites. The lattice-site energy of the T1(0) site is almost linearly correlated with the degree of Si/Al ordering. For ordered orthoclase the T2 lattice-site energy has almost the same low value as the T1(m) and both T2 sites of low microcline. In addition, the lattice-site energies have been computed in three differently ordered hypothetical models, respectively with Al concentrated on T1(m), T2(0) and T2(m), and in a fourth model, also hypothetical, with disordered Al and the unit cell geometry of low microcline. The calculations show that the occupation of a T site by Al instead of Si results in a lattice-site energy which is about 400 kcal/mol less negative due to the larger T–O distances. When this geometrical correction factor is taken into account, Al must be ordered in the T1(0) site, as in this way the lowest negative lattice-site energy is obtained. Lattice-site energies and crystal energy of analbite do not differ much from those of high sanidine. In contrast with a change in the charge distribution due to the Al ordering the crystal energies do not change significantly by the replacement of potassium by sodium ions.