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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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Borate minerals composed of [Bφ 3 ] triangles and/or [Bφ 4 ] tetrahedra (φ = O or OH) commonly exhibit complex polymerizations to form diverse polyanion groups. High-resolution solid-state magic angle spinning (MAS) 11 B and 25 Mg NMR spectroscopy at moderate to ultrahigh magnetic fields (9.4, 14.1, and 21.1 T) allows for very accurate NMR parameters to be obtained for the borate dimorphs, inderite, and kurnakovite, [MgB 3 O 3 (OH) 5 ·5H 2 O]. Improved agreement between experimental results and ab initio density functional theory (DFT) calculations using Full Potential Linear Augmented Plane Wave (FP LAPW) with WIEN2k validates the geometry optimization procedures for these minerals and permits refinements of the hydrogen positions relative to previous X-ray diffraction crystal structures. In particular, the optimized structures lead to significant improvements in the positions of the H atoms, suggesting that H atoms have significant effects on the 11 B and 25 Mg NMR parameters in inderite and kurnakovite. This study shows that combined high-resolution NMR spectroscopy and ab initio theoretical modeling provides an alternative method for the refinement of crystal structures, especially H positions.

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