| Abstrakt: |
Studies of two muscovites of different iron contents, using solid-state NMR with magic-angle-spinning (MAS) combined with X-ray powder diffraction, thermal analysis and57Fe Mössbauer spectroscopy, suggest that dehydroxylation occurs by a homogeneous rather than an inhomogeneous mechanism, forming a dehydroxylate in which the aluminium is predominantly 5-coordinate. On further decomposition at about 1100° C, the tetrahedral layer and interlayer K+ form a feldspar-like phase similar to leucite (KAISi2O6), the remainder forming a spinel, which, contrary to previous suggestions, appears to contain little silicon. Further heating induces the formation of mullite (AI6Si2OP13), and, in the higher-iron sample, corundum (a-Al2O3), in addition to the feldspar-like phase. The presence of the iron impurity enhances the recrystallization reactions and promotes the conversion of mullite to corundum, which eventually becomes the sole aluminous product in the high-iron sample. In samples fired to higher temperatures, only the tetrahedral aluminium resonance is detectable by27AI NMR, probably because most of the iron is located in either the mullite or corundum phases, in which it broadens the octahedral aluminium resonance beyond detection. |
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