High-resolution /sup 28/Si NMR spectroscopy of 2:1 layer silicates: correlations among chemical shift, structural distortions, and chemical variations

Journal Article · · Am. Mineral.; (United States)

OSTI ID:6996855

Weiss, Jr, C A; Altaner, S P; Kirkpatrick, R J

With increasing /sup IV/Al substitution and increasing total layer charge, the /sup 29/Si NMR chemical shifts for specific types of Q/sup 3/(nAl) sites in 2:1 layer silicates become progressively deshielded (less negative). This deshielding is related to both structural distortions caused by mismatch of the tetrahedral and octahedral sheets and compositional variations in the octahedral sheet. One measure of structural distortion in 2:1 layer silicates, the amount of tetrahedral rotation within the crystallographic a-b plane, is linearly correlated with /sup 29/Si chemical shift. As noted by previous authors, /sup 29/Si chemical shifts in trioctahedral phyllosilicates are systematically more negative (more shielded) than /sup 29/Si chemical shifts in analogous dioctahedral structures. This difference is apparently related to a greater total electronegativity of the octahedral cations coordinated to the apical oxygen in trioctahedral phases (three Mg/sup 2 +/ cations) as compared to dioctahedral phases (two Al/sup 3 +/ cations). One-for-one substitution of Mg for Al in dioctahedral phases and Li for Mg in trioctahedral phases causes deshielding at Si, apparently owing to a lower average electronegativity of the cations coordinated to the apical oxygen. Different interlayer cations cause only small changes in the /sup 29/Si chemical shift. These correlations should be important in the structural analysis of smectite, vermiculite, illite, and mixed-layer clay minerals and provide information for these phases that is not easily obtainable from X-ray diffraction.

• Research Organization: Univ. of Illinois, Urbana (USA)
• OSTI ID: 6996855
• Journal Information: Am. Mineral.; (United States), Vol. 72:9-10
• Country of Publication: United States
• Language: English

Related Subjects

58 GEOSCIENCES
SILICATE MINERALS
CHEMICAL COMPOSITION
SILICATES
CRYSTAL STRUCTURE
STRUCTURAL CHEMICAL ANALYSIS
CATIONS
CHEMICAL SHIFT
CLAYS
CRYSTALLOGRAPHY
ION EXCHANGE
LATTICE PARAMETERS
MASS SPECTROSCOPY
NUCLEAR MAGNETIC RESONANCE
CHARGED PARTICLES
IONS
MAGNETIC RESONANCE
MINERALS
OXYGEN COMPOUNDS
RESONANCE
SILICON COMPOUNDS
SPECTROSCOPY
580300* - Mineralogy
Petrology
& Rock Mechanics- (-1989)