Structural models previously proposed for metakaolinite are examined in light of the most recent published experimental data and new information obtained by solid-state highresolution 29Si and 27AI NMR. A new model for metakaolinite is proposed, consisting of anhydrous regions of distorted AI-0 tetrahedra containing randomly distributed isolated residual hydroxyls associated with A1-0 configurations of regular octahedral and tetrahedpal symmetry. Such a structure, which can readily be formed from kaolinite by the removal of hydroxyls in certain sequences, accounts for the lack of a well-defined X-ray pattern and the persistence of =lo% residual hydroxyls in metakaolinite and is consistent with the most recent density data, bond lengths, and the new 29Si and z7AI NMR data.

Solid-state 27AI and 25Mg MAS NMR and X-ray powder diffraction in combination with thermal analysis indicates that synthetic hydrotalcite decomposes thermally in at least three weight-loss steps. The first step, at 250-280 "C, principally involves the loss of the interlayer water, but the appearance of some tetrahedral Al in the 27AI NMR spectra suggests that this is accompanied by the onset of Al dehydroxytation. The second step, at 300-400 "C, is accompanied by a rapid increase in the proportion of tetrahedral Al to a maximum at 400 "C, indicating full Al dehydroxylation of the Al regions. The third step, above 400 "C, represents loss of interlayer carbonate and dehydroxylation of Mg and is accompanied by the appearance of poorly crystalline MgO in both the X-ray and 25Mg NMR spectra. In the range ca. 400-900 "C, we suggest that the phase assemblage consists of intergrown microdomains of poorly crystalline MgO, possibly containing substituent AI3+ and vacancies, and a phase with NMR characteristics similar to spinel-based aluminas such as y-Al ,O,.By 1200 "C, migration of Al from the MgO phase and of Mg into the spinel has occurred, resulting in the appearance of crystalline pure MgO and MgAI,O, with an inversion parameter of ca. 0.3.

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Abstract--Solid-state nuclear magnetic resonance (NMR) spectroscopy, thermal analysis, and X-ray powder diffraction data on the tubular, hydrous aluminosilicate imogolite were found to be fully consistent with a previously proposed crystal structure consisting of a rolled-up, 6-coordinate A1-O(OH) sheet, bonded to isolated orthosilicate groups. The calculated 29Si chemical shift of this structure agreed with the observed shift within 3 ppm. Thermal dehydroxylation of the A1--O(OH) sheet produced predominantly NMR-transparent 5-coordinate A1, but a few 4-and 6-coordinate sites and some residual hydroxyl groups may also have formed, as shown by NMR spectroscopy. Changes in the 29Si NMR spectrum on dehydroxylation suggest a condensation of the orthosilicate groups, but steric considerations rule out bonding between adjacent silicons. To account for these observations, an alternative mechanism to orthosilicate condensation has been proposed, involving the fracture and unrolling of the tubes, followed by the condensation of fragments to form a layer structure. The layer structure has a calculated z9Si chemical shift of -95.6 ppm, in good agreement with the observed value of -93 ppm.

Samples of gibbsite, boehmite, pseudoboehmite and bayerite, calcined at temperatures up to 1000"C, were studied using 27AI MAS NMR with spinning speeds up to 15 kHz in magnetic fields of 14.1, 11.8 and 7 T (156, 130 and 78 MHz 27A1 frequencies). The high magnetic fields reduce the effects of the second order quadrupolar interactions, giving much better resolution of the peaks than previous studies at 7 T and aUowing quantitation of the different species. The samples generally show only octahedral and tetrahedral species, with the latter increasing with calcination temperature. The exceptions are p-alumina and ;~-alumina which show a peak at 33 ppm (in the 130 MHz spectra) with about 20% and 7% respectively of the total intensity. The peaks show large upfield tails anda dependence on magnetic field which differs from that expected fora single electric field gradient. Spectra were simulated assuming a random distribution of electric field gradients, and the dependence of the speetra on the field strength, quadrupolar parameters and the range of electric field gradients is illustrated and empirical equations derived. The method provides a good fit to the observed spectra at all 3 field strengths. These materials show considerable disorder which can be categorized by the parameter used to describe the range of electric field gradients.