Aluminum avoidance rule

Using the more advanced quantum chemical computational methods it is now possible to determine the fundamental electronic properties of zeolite structural units. The quantum chemical basis of Loewenstein s "aluminum avoidance" rule is explored, and the topological features of energy expectation value functionals within an abstract "nuclear charge space" model yield quick estimates for energy relations for zeolite structural units. 

In the following we shall briefly review some of the recent applications of computational quantum chemistry to zeolites, in particular, some studies on the quantum chemical origin of Loewenstein s aluminum avoidance rule, and on the role of counter ions in stabilizing various structural units in zeolite lattices. These calculations are often extremely time consuming, nevertheless, the scope of their application is continuously expanding. 

If energy values are available for the single atom Er and for one of the cluster models, e.g. for system asa of the alternating Al, Si, Al arrangements then result (23) gives both upper and lower energy bounds for system aas, that system is not favoured according to Loewenstein s aluminum avoidance rule. 

The formula Dim is that of an idealized Al-celadonite, Bio is that of an idealized phlogopite, and Chi is that of an idealized clinochlore. (Dim + tk) is, of course, the idealized muscovite formula (Bio + tk) is that of an idealized eastonite, and (Chi + tk) is that of an idealized corundophyllite. The last two are taken as the extreme tk+ limits on the basis of the aluminum avoidance rule. The choice of the clinochlore formula as the tk- limit makes good crystallo-chemical sense for 14-Aigstrdn chlorites, the ones found in pelitic schists. The original chlorite structure described by Pauling (1930) was, in fact, that of a clinochlore found in a blackwall skarn in close proximity to a lizardite-bearing serpentinite near Chester, Vermont, U.S.A. The idealized lizardite formula may be taken... 

De Jong, B. H. W. S., Schramm, C. S., and Paraxial, V. E. (1983) Polymerization of Silicate and Aluminate Tetrahedra in Glasses, Melts, and Aqueous Solutions IV. Aluminum Coordination in Glasses and Aqueous Solutions and Comments on the Aluminum Avoidance Rule, Geochim. Cosmochim. Acta, 47, 1223-1236. 

Loewensteins aluminum avoidance rule is, therefore, satisfied because each Si is surrounded by four A1 tetrahedra and vice versa. 

As a general rule, samples should be collected in decontaminated flasks and even those considered metal-free should be pre-washed. Glass recipients should be avoided. A good procedure for cleaning plastic containers is keeping them at least 24 h in a 10% (v/v) ethanolic nitric acid solution. Aqueous solutions do not leach out the aluminum well, because water does not wet plastic surfaces better contact occurs when an alcoholic solution is used. It can be seen that all alcoholic solutions were more efficient than the aqueous solutions. Table 10 shows the aluminum extracted from polyethylene by the action of some washing solutions. Just before use, the containers should be abundantly washed with ultrapure water. The best option is to use the flasks just after rinsing them, however if they must be dried, they should not be placed in an oven, even with the open side on tissue paper (paper contains aluminum). The best way is let them dry under a laminar flow. The proper heat and air movement inside the hood will help to rapidly dry the flasks. 

Low temperatures can also affect materials by thermal contraction. The thermal expansion coefficient is a function of temperature. For many materials, which are cooled down from room to cryogenic temperature, more than 90 % of the total contraction experienced will have already taken place at 77 K. Rule-of-thumb figures of thermal contraction are 0.3 % in iron-based alloys, 0.4 % in aluminum, or over 1 % in many plastics [43]. Cryogenic vessels or piping systems must account for this contraction to avoid large thermal stresses. 

---