The average structure of Mg[Al 2 Si 3 O 10 ], a stuffed derivative of the high-quartz structure*

Thermal induced transformations on completely Zn2+ exchanged zeolites A and Y

The thermal stability of completely Zn2+ exchanged zeolites A and Y during calcination is studied from R.T. to 1350°C. The phase formation processes are analysed by in situ X-ray diffraction experiments as well as by quenched samples analysed and quantified by Rietveld refinements. The framework structures of the exchanged zeolites A and Y are observed to collapse above 850°C. At higher temperatures the crystallisation of Zn-β-quartz, mullite, and gahnite is initiated from the amorphous phase. Above 1050°C quartz appears and Zn-β-quartz disappears. The crystallisation of willemite is only observed for ZnA, mullite is not stable above 1150°C in both systems. Further increase in temperature leads to the crystallisation of cristobalite accompanied by the disappearance of quartz. The final crystalline components at 1350°C are, for both zeolite precursors, gahnite and cristobalite. Differences between the zeolite precursors are the amounts of amorphous phase formed, indicating a suppression of the formation of a crystalline SiO2 phase in the ZnY samples.

The role of lithium in stabilizing some high-temperature silica phases*

High-quartz and keatite are two of the high-temperature silica phases. Both have negative thermal expansion and both can be stabilized by partial replacement of Si with Al and Li. Some current data are used in this paper to extend and refine the stabilization principle first outlined by Buerger. Based upon this study, it is the interstitial cations like Li that stabilize the high-temperature structure by effectively reducing the excess void space. Thus the high-temperature structure is buttressed and preserved by preventing it from collapsing into the low-temperature form at the transformation temperature. It is also found that Li can best stabilize the high-quartz and the keatite phases because of its unique characteristics. First, Li has the precise size to fit into the interstitial space with ease. Second, Li has the ideal combination of valence and coordination number so that it can best balance the local charge. Some structural data are presented to support these observations.

Influence of heat-treatment on the average structure of Mg[Al2Si3O10], a stuffed derivative of the high-quartz structure *

The quartz-like average structure of single crystals of the composition MgO · Al2O3· 3SiO2was investigated. The crystals showed in x-ray diffraction photographs diffuse superstructure reflections. These diffuse reflections were generated by heat treatment at about 1000°C and subsequent quenching of crystals showing well developed superstructure reflections in form of satellites before heating.

The Mg atoms are located within the main structure channels running parallelc. Eighty percent of the Mg atoms occupy the sites coordinated by six O atoms, the remaining Mg atoms the sites surrounded by four O atoms. This distribution of Mg atoms over all possible sites does not agree with the distribution in not heat-treated crystals, in which there was evidence that Mg atoms were only in the sixfold O coordination. The parameters of the framework atoms [(Si,Al) and O] of heat-treated and not heat-treated crystals do not show significant differences.