Aluminium coordination and structural disorder in halloysite and kaolinite by 27Al NMR spectroscopy

Clay Minerals - Tập 29 Số 3 - Trang 305-312 - 1994
Roger H. Newman1, C.W. Childs1,2, G. Jock Churchman3,1
1Industrial Research Ltd., PO Box 31-310, Lower Hutt, New Zealand
2Landcare Research, Private Bag 3127, Hamilton, New Zealand
3Division of Soils, CSIRO, Private Bag No. 2, Glen Osmond, SA 5064, Australia

Tóm tắt

AbstractIt has been suggested that interlayer water in halloysite is due to the presence of hydrated cations that balance the negative layer charge produced by Al for Si substitution. To find evidence of 4-coordinate Al (Al(IV)), we investigated six halloysites and two kaolinites using ‘high-field’ and ‘medium-field’ solid-state 27Al MAS NMR spectroscopy. We found Al(IV) in both kaolinite and five halloysite samples, but the contents are all <1% and provide no basis for distinguishing between kaolinite and halloysite. Therefore, the presence of interlayer water in halloysite cannot be attributed to Al for Si substitution. There are, however, signals, tentatively assigned to A1(V), present in the kaolinite spectra but not in the halloysite spectra. The shapes of the low-frequency ‘tails’ of Al(VI) signals in medium-field NMR vary from sample to sample. We interpret this variation in terms of a ‘crystallinity index’. Disorder in kaolinite appears to be primarily the result of Al-vacancy displacements in the octahedral sheet. The NMR crystallinity indices correlate with those from IRS and DTA but not with those from XRD.

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Tài liệu tham khảo

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Clay Minerals

Tập 29 Số 3

305-312

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