The thermal reactions of muscovite studied by high-resolution solid-state 29-Si and 27-Al NMR

• Published in: Journal of materials science 1987-07, Vol.22 (7), p.2645-2654
• Main Authors: MACKENZIE, K. J. D, BROWN, I. W. M, CARDILE, C. M, MEINHOLD, R. H
• Format: Article
• Language: English
• Subjects: Earth sciences; Earth, ocean, space; Exact sciences and technology; Mineralogy; Silicates
• ISSN: 0022-2461; 1573-4803
• DOI: 10.1007/BF01082158

Studies of two muscovites of different Fe contents, using solid-state NMR with magic-angle-spinning (MAS) combined with X-ray powder diffraction, thermal analysis and exp 57 Fe Mossbauer spectroscopy, suggest that dehydroxylation occurs by a homogeneous rather than an inhomogeneous mechanism, forming a dehydroxylate in which the Al is predominantly five-coordinate. On further decomposition at approx 1100 deg C, the tetrahedral layer and interlayer K exp + form a feldspar-like phase similar to leucite (KAlSi sub 2 O sub 6 ), the remainder forming a spinel, which, contrary to previous suggestions, appears to contain little Si. Further heating induces the formation of mullite (Al sub 6 Si sub 2 O sub 13 ), and, in the higher-Fe sample, corundum ( alpha -Al sub 2 O sub 3 ), in addition to the feldspar-like phase. The presence of the Fe impurity enhances the recrystallization reactions and promotes the conversion of mullite to corundum, which eventually becomes the sole aluminous product in the high-Fe sample. In samples fired to higher temperatures, only the tetrahedral Al resonance is detectable by exp 27 Al NMR, probably because most of the Fe is located in either the mullite or corundum phases, in which it broadens the octahedral Al resonance beyond detection. 32 ref.--AA