Silica lattice, impurities

Fio. 18. Spin-lattice relaxation time Ti for silica gel (SG) and silica-alumina (SA) versus paramagnetic impurity content N. The straight line corresponds to the relation Ti oc (irS). 

Another interesting natural example of colour centres lies in the colour of smoky quartz and amethyst. These semi-precious stones are basically crystals of silica, Si02, with some impurities present. In the case of smoky quartz, the silica contains a little aluminium impurity. The substitutes for the Si " in the lattice, and the electrical... 

Aluminium oxide exists in many crystalline modifications, usually designated by Greek letters, some with hexagonal and some with cubic lattices (cf. refs. 11 and 24). The best known and mostly used forms are a- and 7-alumina but practical catalysts are seldom pure crystallographic specimens. This makes the surface chemistry of aluminas rather complicated. Moreover, the catalytic activity of alumina depends very much on impurities. Small amounts of sodium (0.08—0.65%) poison the active centres for isomerisation but do not affect dehydration of alcohols [10]. On the other hand, traces of sulphates and silica may increase the number of strong acidic sites and change the activity pattern. 

Bands at 3745 and 3690 cm 1 are also present. The 3745 cm-1 band has usually been attributed to silica impurities occluded within the zeolite framework or to Si—OH groups terminating the lattice. The 3690 cm 1 band is due to undissociated water since its intensity is dependent upon the degree of hydration. 

High silicious HZSM-5 zeolites were synthesized from silica glass, and intracrystalline diffusivities of eight kinds of hydrocarbons within these zeolites in the temperature range of 373-773 K were directly measured by the constant volume method. A method was developed for predicting the diffusivity on the basis of the thermodynamics of an impure metal atom diffusing among metal atoms which form crystal lattices. Its validity was experimentally examined. 

Silica O crystallizes from lithium silicate glasses during devitrification at low temperature. It has a crystal lattice similar to quartz and may simply be high quartz" stabilized below 573 C, the normal transition temperature, to low quartz (53, 70) by inclusions of metal ion impurities. The only way pure material can be obtained is by neutron bombardment of quartz. 

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