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In this work, the crystal structure of a natural specimen of the "Messbach-type" variscite from Tooele Co. (Utah, U.S.A.) was determined, and the dehydration process leading to the transition to a new AlPO SB 4 sb phase was monitored at high temperature. Crystal structure description of variscite 2O and comparison with variscite 1O Like variscite I 1O i , the variscite I 2O i crystal structure belongs to the family of framework 3D MT structures in which octahedra (M) and tetrahedra (T) are linked by bridging O atoms ([30]). The Al-coordination change from octahedral to tetrahedral that accompanies dehydration, clearly demonstrated by the SP 27 sp Al NMR data, confirms the removal of the Al-bound H SB 2 sb O molecules as the dehydration mechanism, transforming the variscite octahedral-tetrahedral framework to a purely tetrahedral framework. Keywords: Variscite; X-ray powder diffraction; charge-flipping; nuclear magnetic resonance; polytypism; AlPO4 EN Variscite X-ray powder diffraction charge-flipping nuclear magnetic resonance polytypism AlPO4 1385 1395 11 07/05/22 20220701 NES 220701 Introduction Interest in hydrated aluminophosphates and in compounds that derive from their dehydration spans a wide spectrum of disciplines. [Extracted from the article]

Arsenate ([As.sup.5+]) substitution in the hydroxylapatite structure was examined using a combination of crystallographic and spectroscopic techniques. Samples of hydroxylapatite, the [As.sup.5+]-substituted analog (synthetic johnbaumite), and five intermediate compositions were synthesized from solution. Synchrotron X-ray diffraction data show that all samples are single-phase, confirming complete substitution. No evidence is found for lowering of symmetry below [P6.sub.3]/m. Rietveld structure refinements show progressive expansion of the unit cell with increasing As substitution, which can be accounted for primarily by an average expansion of the tetrahedral site. Sizes of Ca polyhedra show little variation as a result of As substitution. NMR results show no evidence for local clustering of P[O.sub.4] tetrahedra. EXAFS confirms that the size of As-centered tetrahedra remains constant across the solid-solution series. Keywords: Apatite, hydroxylapatite, johnbaumite, arsenic, arsenate, NMR, EXAFS

We have studied the substitutions in a natural well-crystallized carbonate-containing apatite-(CaF) (var. staffelite) using infrared (IR) and solid-state nuclear magnetic resonance (NMR) spectroscopic techniques. Our results show the presence of both A- and B-type carbonate plus a large amount of structural water (0.44 pfu). This sample also contains 0.21 pfu excess F, but only weak C-F dipolar coupling is observed indicating that a tetrahedral C[O.sub.3][F.sup.3-] complex does not occur. [sup.19]F NMR results indicate the presence of a second F environment in the apatite structure at a concentration similar to that of B-type carbonate but which does not differ from channel F in terms of coupling to [sup.31]p or [sup.1]H. Keywords: Apatite-(CaF), IR spectroscopy, NMR spectroscopy

Low-OH apatite of the compositional range [Ca.sub.4.99-5.06][(P[O.sub.4]).sub.2.98-3.00 [F.sub.0.51-0.48][Cl.sub.0.38-0.36]O[H.sub.0.14-0.12] was synthesized and characterized structurally by synchrotron-based single-crystal X-ray diffraction (XRD), and multiple nuclear magnetic resonance (NMR) spectroscopic techniques. The average structure is hexagonal with space group [P6.sub.3]/m. The presence of scattering in the single-crystal diffraction data set, which is incommensurate within the average hexagonal structure, suggests the presence of localized short-range monoclinic domains. Complex lineshapes in the [sup.31]P and [sup.19]F MAS NMR spectra are also consistent with the presence of an incommensurate phase. No evidence was detected for splitting of the Ca2 site into two distinct sites (as had been previously reported for hexagonal ternary apatites). Structure refinement and [sup.19]F {[sup.35]Cl} TRAPDOR NMR experiments verified intercolumnal neighboring of F and Cl atoms (inter-column distance of 2.62 [Angstrom]) within this low-OH- apatite suggesting that long-range neighboring ofF and Cl within the apatite anion channels is feasible. Keywords: Apatite, NMR spectroscopy, single-crystal XRD, Mars, Moon, PGE, synthesis

Using [sup.29]Si, [sup.1]H, and [sup.2]H magic-angle spinning (MAS) and [sup.29]Si{[sup.1]H} heteronuclear correlation (HetCor) nuclear magnetic resonance (NMR) spectroscopy the tetrahedral sheets of the 10 [Angstrom] phase are shown to contain [Q.sup.2]-type Si bonded to silanol groups that donate hydrogen bonds to interlayer [H.sub.2]O. [sup.29]Si NMR spectra of 10 [Angstrom] phase samples synthesized from oxide and from crystalline talc starting materials contain a peak near -87 ppm for [Q.sup.2] Si, in addition to the main peak at -98 ppm for the talc-like [Q.sup.3] of the tetrahedral sheet. The [sup.1]H MAS NMR spectra of the 10 [Angstrom] phase contain two distinct peaks, at chemical shifts of +7.8 and +3.2 ppm, in addition to a narrow peak near +0.9 ppm from the talc-like hydroxyl groups. [sup.29]Si{[sup.1]H} HetCor data indicate that the +7.8 ppm [sup.1]H resonance corresponds to silanol groups and that at +3.2 ppm arises from interlayer [H.sub.2]O. Comparison of the observed data with correlations of [sup.1]H NMR chemical shift and [sup.2]H quadrupolar coupling indicates that the silanol groups donate moderate hydrogen bonds to interlayer [H.sub.2]O, d(O ... O) [approximately equal to] 2.8 [Angstrom], whereas most interlayer [H.sub.2]O donate only very weak or no hydrogen bonds at ambient conditions. Our results suggest that formation of the 10 [Angstrom], phase involves formation of vacancies, which allow favorable hydrogen bond interaction between interlayer [H.sub.2]O and the normally hydrophobic talc-like 2:1 layers. Keywords: NMR, l0 [Angstrom] phase, talc, silanol, hydrogen bond

We show that the signal observed in [sup.13]C{[sup.1]H]} cross-polarization magic-angle-spinning (CP/MAS) NMR spectra of synthetic calcite precipitated at room temperature arises from bicarbonate ion. Although this peak occurs at the same chemical shift as for the bulk carbonate signal, its observation by CP/MAS indicates that it corresponds to carbonate associated with H. The variation in the CP/MAS intensity with contact time shows oscillations characteristic of C-H pairs separated by 1.9(1) [Angstrom]. and remote from other H, consistent with bicarbonate. [sup.13]C{[sup.1]H} heteronuclear correlation spectra indicate that the hydrogen in the bicarbonate groups gives a relatively narrow [sup.1]H NMR signal at +7.4 ppm. A peak at this chemical shift is also observed in direct-observe [sup.1]H MAS NMR spectra of the synthetic sample, and also in natural biogenic and abiogenic calcite. This [sup.1]H chemical shift indicates a moderate hydrogen bonding interaction d(OH ... O) [approximately equal to] 1.85 A, which suggests significant structural relaxation occurs near the bicarbonate. Keywords: Calcite, NMR spectroscopy, bicarbonate, trona, nahcolite, hydrogen bond

Trivalent aluminum ions are important in natural bodies of water, but the structure of their coordination shell is a complex unsolved problem. In strong acid (pH < 3.0), [Al.sup.III] exists almost entirely as the octahedra[ Al[([H.sub.2]O).sub.6.sup.3+] ion, whereas in basic conditions (pH > 7), a tetrahedral Al[(OH).sub.4.sup.-] structure prevails. In the biochemically and geochemically critical pH range of 4.3 to 7.0, the ion structures are less clear. Other hydrolytic species, such as AlOH[(aq).sup.2+], exist and are traditionally assumed to be hexacoordinate. We show, however, that the kinetics of proton and water exchange on aqueous [Al.sup.III], coupled with Car-Parrinello simulations, support a five-coordinate Al[([H.sub.2]O).sub.4]O[H.sup.2+] ion as the predominant form of AlOH[(aq).sup.2+] under ambient conditions. This result contrasts [Al.sup.III] with other trivalent metal aqua ions, for which there is no evidence for stable pentacoordinate hydrolysis products., Aluminum is the third most abundant element in Earth's crust, after oxygen and silicon, and its chemistry in water is central to geochemistry, environmental science, and medicine (1, 2). In [...]

A variety of nonequilibrium phenomena, including glass formation, metastable crystallization, and polymorphism in the undercooled liquid, is displayed by the yttria-alumina (Y2O3-Al2O3; YA) pseudo-binary system. The formation mechanism and coordination of polyamorphic YA glasses is discussed, in the context of the structural and thermodynamic properties.

Research has been conducted on the fluoroaluminate complexes. The results of the NMR study of these complexes in aqueous solution indicate that each substitution of a bound water molecule by a fluoride anion will affect the exchange rate of bound water with free water.

Site-isolated metal clusters of propylidyne were studied for reactivity.

Author(s): Brian L. Phillips [1]; William H. Casey (corresponding author) [2, 3]; Magnus Karlsson [2, 4] The kinetic stability of oxide surfaces affects a broad range of physical phenomena, including [...]

Substitution reaction studies on aluminum (hydr)oxide dissolution finds that hydroxide or fluoride ion substitution into the inner coordination sphere of the chemical greatly weakens water molecule-complex bonding. Findings however, show that there are differences in calculated and actual activation energies suggesting that a significant contributing factor to the required activation energy is the interaction between hydration waters and second-sphere water molecules.

An 17O nuclear magnetic resonance study was conducted to examine the rate of exchange of water molecules from bulk solvent to the inner coordination sphere between hydrated aluminum complexes and bulk aqueous solution. It was observed that the lability of other inner-sphere oxygens are increased by a factor of almost equal to the 10(super 4) after the deprotonation of an inner-sphere water in Al(H2O)(sub 6)(super 3+)(aq). Findings also showed that the hydroxyl group has a labilizing effect on the other inner-sphere hydration waters in the complex.

We present 29Si MAS-NMR spectroscopic data for a series of synthetic feldspar samples along the join Ca[Al.sub.2][Si.sub.2][O.sub.8]-Sr[Al.sub.2][Si.sub.2][O.sub.8], from which the composition dependence and coupling of order parameters describing Si-Al order and the triclinic-monoclinic displacive transition were determined. Spectra of Sr[Al.sub.2][Si.sub.2][O.sub.8] contain narrow peaks for the two crystallographic Si sites, plus additional peaks for Si having three and two Al nearest neighbors, indicating the presence of approximately 0.14 Al-O-Al linkages per formula unit and a value of [Sigma] = 0.93 for the short-range order parameter. For the triclinic feldspar samples, short-range Si-Al order increases continuously with Sr content from [Sigma] = 0.89(3) for Ca[Al.sub.2][Si.sub.2][O.sub.8] to 0.97(1) for [Sr.sub.0.80][Ca.sub.0.20][Al.sub.2][Si.sub.2][O.sub.8] but then decreases discontinuously to 0.93(2) for the monoclinic samples, [Sr.sub.0.85][Ca.sub.0.15][Al.sub.2][Si.sub.2][O.sub.8] to Sr[Al.sub.2][Si.sub.2][O.sub.8]. The variation of peak positions with composition is consistent with a structural phase transition near [Sr.sub.0.85][Ca.sub.0.15] [Al.sub.2][Si.sub.2][O.sub.8] from [Mathematical Expression Omitted] to I2/c. The order parameter for this displacive transition is reflected by the chemical shift of the T1mz crystallographic site, and its composition dependence gives an order-parameter critical exponent of [Beta] = 0.49(2), indicating classical second-order behavior.

27Al and 29Si magic angle spinning NMR experiments help analyze the Si-Al distribution in a natural leucite sample, its Rb- and Cs- exchanged derivatives and analcime. NMR data help derive the populations of tetrahedral cation clusters and use of few equations helps determine configuration entropy (Sc) due to Si-Al disorder from these populations. Si-Al disorder induces a Sc of 6.6 J/(mol.K) in cubic leucite, while Sc in analcime varies from 3.1 to 8.1 J/(mol.K). NMR data suggest a linear correlation between bridging-bond angle and 27Al chemical shifts of leucite samples.

A study was conducted on the cation order in three polycrystalline MgSiO3-rich garnet. All Al was assumed to be octahedrally coordinated based on spectrum analysis. The three samples exhibited 29Si peaks within the chemical shift range of -68 to -90 ppm which were attributed to (4)Si. Examination of the octadhedral sites also indicated the presence of little shot-range octahedral Mg,Al,Si order in the tyhree samples.

A study was conducted on the Al,Si order occuring in anorthite using nuclear magnetic resonance spectroscopy. Results revealed the presence of Al-O-Al linkages in the synthetic anorthite specimens. The average number of linkages per formula unit was subsequently determined. Changes in the number of Al-O-Al linkages with short annealing time were also observed and were found to be associated with the total surface area reduction of the antiphase domains.