Xianyu Xue

Silicon Coordination and Speciation Changes in a Silicate Liquid at High Pressures

Coordination and local geometry around Si cations in silicate liquids are of primary importance in controlling the chemical and physical properties of magmas. Pressure-induced changes from fourfold to sixfold coordination of Si in silicate glass samples quenched from liquids has been detected with 29Si magic-angle spinning nuclear magnetic resonance spectrometry. Samples of Na2Si2O5 glass quenched from 8 gigapascals and 1500�C contained about 1.5 percent octahedral Si, which was demonstrably part of a homogeneous, amorphous phase. The dominant tetrahedral Si speciation in these glasses became disproportionated to a more random distribution of bridging and nonbridging oxygens with increasing pressure.

Depolymerization effect of water in aluminosilicate glasses: Direct evidence from 1H-27Al heteronuclear correlation NMR

Abstract We have applied one-dimensional (1D) 1H MAS NMR, 27Al → 1H CP MAS NMR, as well as 2D 27Al triple-quantum (3Q) MAS NMR, 27Al → 1H heteronuclear correlation (HETCOR) and high-resolution 3QMAS/HETCOR NMR techniques to KAlSi3O8 (Or), NaAlSi3O8 (Ab) and NaAlSiO4 (Ne) glasses containing 0~2 wt% H2O to shed light on the dissolution mechanisms of water in aluminosilicate melts (glasses). An Al Q3-OH group, characterized by 1H chemical shifts of 1.3-1.9 ppm, was identified for all hydrous glasses. Its abundance increases with bulk Al/Si ratio. The 27Al chemical shifts (δiAl) of this species are 64-68 ppm, larger than those of Al Q4 by 3-6 ppm. Despite this difference, it is only through 27Al → 1H HETCOR and 3QMAS/HETCOR, but not 27Al MAS or 3QMAS NMR that the peaks are resolved. This study has demonstrated that depolymerization and formation of AlOH/SiOH is a general water dissolution mechanism for polymerized aluminosilicate melts (glasses), and HETCOR NMR experiments involving 1H are the key to its revelation.

Water speciation in hydrous silicate and aluminosilicate glasses: Direct evidence from 29Si-1H and 27Al-1H double-resonance NMR

Abstract Through a combination of 1H MAS NMR, 1H → 29Si → 1H double cross-polarization (CP) MAS NMR and 27Al → 1H CP MAS NMR, different OH species [SiOH, AlOH, and (Ca,Mg)OH (free OH)] have been unambiguously identified for hydrous Ca,Mg-(alumino)silicate glasses. This confirms my earlier speciation assignments made partially on the basis of 1H chemical shift arguments. The dissolution mechanisms of water in both Al-free silicate and aluminosilicate glasses (quenched melts) are fundamentally similar. For relatively polymerized compositions, it involves dominantly the formation of TOH species (T: Si, Al) through the rupture of T-O-T linkages, in addition to molecular H2O; for more depolymerized compositions containing network-modifying cations of large field strength (e.g., Ca, Mg), free OH species are also important.

An ab initio calculation of 17O and 29Si NMR parameters for SiO2 polymorphs

Ab initio molecular orbital calculations (Hartree-Fock, HF and density functional theories, DFTs) have been carried out for SiO2 polymorphs coesite, low cristobalite, and alpha-quartz, in order to investigate the reliability of this method for predicting 29Si and 17O nuclear magnetic resonance (NMR) properties of silicates. Oxygen- and silicon-centered clusters consisting of one (1T) to three tetrahedral (3T) shells (one to four atomic shells), taken from real crystal structure, have been investigated. It is found that for reasonable predication of both the 29Si and 17O chemical shifts (deltaSi and delta(i)O), the minimum cluster is one that gives the correct second neighbors to the nucleus of interest. Both the delta(i)Si and delta(i)O have reached convergence with respect to cluster size at the OH-terminated two tetrahedral (2T) shell (three atomic shells around Si and four atomic shells around O) model. At convergence, the calculated delta(i)Si values agree well (within +/- 1 ppm) with experimental data. The calculated 17O electric field gradient (EFG)-related parameters also agree with experimental data within experimental uncertainties. The calculation also reproduces small differences in delta(i)O for O sites with similar tetrahedral connectivities, but shows deviations up to about 10 ppm in relative difference for O sites with different tetrahedral connectivities. The poor performance for the latter is mainly due to the approximations of the HF method. Our study thus suggests that the ab initio calculation method is a reliable mean for predicting 29Si and 17O NMR parameters for silicates. Such an approach should find application not only to well-ordered crystalline phases, but also to disordered materials, by combining with other techniques, such as the molecular dynamics simulation method.

Dense hydrous magnesium silicates, phase D, and superhydrous B: New structural constraints from one- and two-dimensional 29Si and 1H NMR

Abstract To gain new structural insights into phase D and superhydrous B, two phases of potential mantle water reservoir, we have applied a range of one- (1D) and two-dimensional (2D) 1H and 29Si NMR techniques, as well as Raman spectroscopy, to samples synthesized at 24 GPa and 900~1100 °C. These data have revealed that phase D is characterized by disordered and varying local structures around both Si and H. The 29Si NMR spectra of phase D contain a nearly symmetric, broad peak near -177.7 ppm, attributable to octahedral Si with local structural disorder. The high-resolution 1H CRAMPS spectra of phase D contain a main broad peak near 12.6 ppm with shoulders near 10 and 7 ppm, suggesting a distribution of hydrogen bonding distances. For superhydrous B, our comprehensive 2D 1H and 29Si NMR results have clearly revealed that it contains dissimilar hydrogen (H1-H2) pairs and one tetrahedral Si site, consistent with space group Pnn2.

Cation order and hydrogen bonding of high-pressure phases in the Al2O3-SiO2-H2O system : An NMR and raman study

Abstract Topaz-OH, phase egg, and δ-AlOOH are hydrous phases in the Al2O3-SiO2-H2O system that have been found to be stable at successively higher pressures up to those corresponding to the lower mantle, and thus they may be important water reservoirs in the deep mantle. We have applied 1H, 29Si, and 27Al nuclear magnetic resonance (NMR) and Raman spectroscopy to shed new light on the structure of these phases. 29Si and 27Al NMR results clearly revealed that the Si-Al distribution in phase egg is partially disordered. The presence of structural disorder in topaz-OH was also confirmed. 1H NMR and Raman data are both consistent with strong, but asymmetric hydrogen bonding in δ-AlOOH and phase egg, and a range of hydrogen bonding distances in topaz-OH. The observed structural disorder and hydrogen bonding could be responsible for the high upper temperature stability limits (1500~1700 °C) of phase egg and topaz-OH, and are also relevant to the incorporation mechanisms of water in nominally anhydrous stishovite.

Phase relations in Na2O–SiO2 and K2Si4O9 systems up to 14 GPa and 29Si NMR study of the new high-pressure phases: implications to the structure of high-pressure silicate glasses

Abstract Preliminary studies have been made of phase relations in the K 2 Si 4 O 9 , Na 2 Si 2 O 5 , Na 2 Si 3 O 7 and Na 2 Si 4 O 9 systems up to 14 GPa. Several high-pressure sodium silicate phases have been observed for the first time and were characterized by means of powder X-ray diffraction and 29 Si MAS NMR techniques. In the K 2 Si 4 O 9 system, the wadeite (K 2 ZrSi 3 O 9 )-type phase was found to be stable and melt congruently at least up to 12 GPa. In the Na 2 Si 2 O 5 system, phase C, a Na 2 Si 2 O 5 polymorph previously reported at 10–40 MPa, was observed at 2.5 GPa. However, at 5–6 GPa, a previously unknown phase ( e -Na 2 Si 2 O 5 ) appeared. This phase was replaced by yet another new phase ( ζ -Na 2 Si 2 O 5 ) at 8–10 GPa. In the Na 2 Si 3 O 7 system, a new high-pressure Na 2 Si 3 O 7 phase was detected at about 10 GPa. In the Na 2 Si 4 O 9 system, a new Na 2 Si 4 O 9 phase appeared at 6–8 GPa and it decomposed to stishovite (SiO 2 ) plus the high-pressure Na 2 Si 3 O 7 phase at ≥10 GPa. The 29 Si MAS NMR spectra revealed that the e -Na 2 Si 2 O 5 phase contains only tetrahedral Si sites, whereas the ζ -Na 2 Si 2 O 5 , Na 2 Si 3 O 7 and Na 2 Si 4 O 9 phases contain both tetrahedral and octahedral Si sites. Recently, Fleet and Henderson [Fleet, M., Henderson, G.S., 1995a. Epsilon sodium disilicate: A high-pressure layer structure [Na 2 Si 2 O 5 ]. J. Solid State Chem., 119: 400–404; Fleet, M., Henderson, G.S., 1995b. Sodium trisilicate: A new high-pressure silicate structure (Na 2 Si[Si 2 O 7 ]). Phys. Chem. Min., 22: 383–386.] and Fleet [Fleet, M., 1996. Sodium tetrasilicate: A complex high-pressure framework silicate (Na 6 Si 3 [Si 9 O 27 ]). Am. Mineral., 81: 1105–1110.] have determined the structures of the e -Na 2 Si 2 O 5 , Na 2 Si 3 O 7 and Na 2 Si 4 O 9 phases. Subsolidus phase transformations with pressure for these alkali silicate systems can be described in terms of reduction of Si–O–Si angles at lower pressures and formation of octahedral Si through conversion of nonbridging oxygens to bridging oxygens at higher pressures. Similar structural changes might be expected for alkali silicate glasses (and melts) within this pressure range.

Structural Characterization of Moganite-Type AlPO4 by NMR and Powder X-ray Diffraction

Structural characterization of a new high-pressure AlPO(4) phase synthesized at 5 GPa and 1500 °C is reported. The phase is monoclinic (P2/a) with a = 8.7437(1) Å, b = 4.8584(1) Å, c = 10.8600(2) Å, β = 90.124(1)° (Z = 6). (31)P MAS NMR and two-dimensional (2D) (27)Al triple-quantum (3Q) MAS NMR revealed that it contains two tetrahedral P sites of 1:2 abundance ratio, and two tetrahedral Al sites with 1:2 ratio. 2D (31)P dipolar-recoupled double-quantum (DQ) MAS NMR and (27)Al → (31)P dipolar-based (through-space) and J coupling-based (through-bond) 3Q-heteronuclear correlation (HETCOR) experiments provided direct information on the linkages of these sites. The crystal structure was solved and refined from synchrotron powder X-ray diffraction data utilizing the information from NMR. The phase is isostructural to moganite, a rare SiO(2) polymorph, and its structure can be derived from the latter via an ordered replacement of tetrahedral Si sites by Al and P. The NMR parameters of the phase were also calculated by first-principles method, which are consistent with those observed. Contrary to the other moganite phases known to date (i.e., SiO(2) and PON), moganite-AlPO(4) has a higher-pressure stability field than the corresponding quartz phase. This is the first moganite-type phase found in the ABX(4) system.

Si-Al distribution in high-pressure CaAl4Si2O11 phase: A 29Si and 27Al NMR study

Abstract High-resolution 29Si and 27Al NMR techniques have been applied to resolve the Si-Al distribution and coordination in the high-pressure CaAl4Si2O11 (CAS) phase, a potentially important mineral in subducted crustal materials in the deep mantle that has a unique hexagonal ferrite structure containing two octahedral (M1; M2) and one trigonal bipyramidal sites. The 29Si MAS NMR spectra of the CAS phase synthesized at 20 GPa and 1400~1600°C show two broad, asymmetric peaks near -92.7 and -182.7 ppm with an intensity ratio of 1:3, suggesting that 1/4 of the Si are in tetrahedral coordination and 3/4 in octahedral coordination. Therefore, the trigonal bipyramidal and M1 octahedral sites are each occupied by equal proportions of Si and Al, and the former are effectively half-occupied face-sharing tetrahedra (at least for Si). The 27Al MAS and 3Q MAS NMR spectra contain only one unresolved peak typical of octahedral Al with a range of quadrupolar coupling constants

Structures of two new high-pressure forms of AlPO4 by X-ray powder diffraction and NMR spectroscopy

The crystal structures of two new high-pressure AlPO(4) phases are reported. One phase synthesized at 6 GPa and 1523 K is triclinic (P1) whilst the other phase synthesized at 7 GPa and 1773 K is monoclinic (P2(1)/c). (31)P MAS (magic-angle spinning) NMR suggests three tetrahedral P sites with equal abundance in both phases. (27)Al 3Q MAS NMR spectra provided evidence for two octahedral sites and one five-coordinated Al site in each phase. The crystal structures were solved using an ab initio structure determination technique from synchrotron powder X-ray diffraction data utilizing the local structural information from NMR, and were further refined by the Rietveld method. Both phases contain doubly bent chains made of six edge-shared Al polyhedra (including five-coordinated Al), which are joined by PO(4) tetrahedra. The P1 phase is isostructural with FeVO(4) and AlVO(4). The two phases differ in the packing manner of the chains. This study has demonstrated that the combined application of ab initio structure determination via X-ray powder diffraction and solid-state NMR spectroscopy is a powerful approach to the rapid solution of complex inorganic crystal structures.