X-ray powder diffraction and NA-23, AL-27, AND SI-29 MAS-NMR investigation of nepheline-kalsilite crystalline solutions

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Abstract
  • Specimens of both natural (Monte Somma) and synthetic Na-rich nepheline have been used to synthesize two ion-exchange series, ranging in composition to pure kalsilite. Excess Si in the natural and synthetic series was 5.2 and 1.7 mol%, respectively. The associated vacancies (VAC) in the alkali sites were found to play an important role in the crystal chemical behavior of these minerals. Room temperature X-ray powder diffraction measurements revealed the presence of three phases in each series: nepheline from sodic compositions to 0.64 (+/- 0.01) mole fraction K + VAC, tetrakalsilite at 0.72 (+/- 0.02) K + VAC, and kalsilite above 0.80 K + VAC, with a narrow miscibility gap between K-rich nepheline and tetrakalsilite. Increases of a, c, and volume were linear with K content within each structural region. However, for nepheline the increases accelerated beyond compositions at which K (or K + VAC) was forced to occupy Na structural positions. For the more vacancy-rich natural series this occurred at lower K contents, suggesting preference of vacancies for the larger alkali position. The overall effect of square Si exchange in nepheline, in fact, was found to resemble that of large-ion substitution in the alkali site. Both the nepheline to tetrakalsilite and tetrakalsilite to kalsilite transitions were accompanied by apparent discontinuities in the c unit-cell dimension. Unit-cell volume was discontinuous for a hypothetical nepheline to kalsilite transition. Magic-angle-spinning (MAS) NMR data of Na-23, Al-27, and Si-29 for part of the natural series suggest that the nepheline structure is distorted on both a local (unit-cell) and long-range scale with the addition of K. Similarly, the kalsilite structure is distorted with increasing Na content. Stoichiometric nepheline (K + VAC = 0.25) is observed to have the least distorted tetrahedral sites as calculated from the Al-27 quadrupolar coupling constant. Lastly, the Si-29 and Al-27 NMR suggest complete ordering between the Si and Al tetrahedral sites, thus preserving the Al avoidance rule.
TitleX-ray powder diffraction and NA-23, AL-27, AND SI-29 MAS-NMR investigation of nepheline-kalsilite crystalline solutions
CreatorHovis, Guy L.
Clare, A.
Roux, J.
Spearing, D. R.
Stebbins, J. F.
PublisherAmerican Mineralogist
Academic DepartmentGeology & Environmental Geosciences
DivisionNatural Sciences
OrganizationLafayette College
Date Issued1992
Date Available2013-02-06T20:58:34Z
TypeArticle
LanguageEnglish
Bibliographic CitationHovis, G. L., et al. (1992) "X-ray powder diffraction and NA-23, AL-27, AND SI-29 MAS-NMR investigation of nepheline-kalsilite crystalline solutions." American Mineralogist 77 (1/2): 19-29.
Standard IdentifierHandle 10385/1092
Permalinkhttp://hdl.handle.net/10385/1092
Rights Statement In Copyright In Copyright
Rights HoldersMineralogical Society of America
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