Clays and Clay Minerals, Vol. 56, No. 5, 520–525, 2008.
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Authors
Takahashi, Takafumi
Kanehashi, Koji
Saito, Koji
Issue Date
2008
Type
Article
Language
Keywords
Periodicals , Geology , Cis , Hydroxyl , MQMAS NMR , Na-montmorillonite , Octahedral Al , Trans
Alternative Title
First Evidence Of Multiple Octahedral Al Sites In Na-montmorillonite By 27al Multiple Quantum Mas Nmr
Abstract
The configuration of hydroxyl groups around the octahedral cations of 2:1 phyllosilicate minerals has long beenanimportan t questionincla y science. Inthe present study, 27Al multiple quantum (MQ) magic angle spinning nuclear magnetic resonance (MAS NMR) was applied to the local structural analysis of octahedral Al positions in a purified Na-montmorillonite. Three octahedral Al sites ([6]Ala, [6]Alb, an d[6]Alc) are distinguished by 27Al 5QMAS NMR, whereas these sites are not differentiated by 27Al MAS and 3QMAS NMR. The isotropic chemical shift (dcs) and the quadrupolar product (PQ) were estimated to be 5.8 ppm and 2.6 MHz for [6]Ala, 6.2 ppm and 3.0 MHz for [6]Alb, and 6.7 ppm and 3.7 MHz for [6]Alc, respectively. The three Al sites originated from geometric isomers with cis and trans structures, which have mutually different configurations of the OH groups around the central Al3+ ions. From the view point of symmetry for the OH groups, [6]Ala and [6]Alb inthe upfield regionwere assigned to cis sites, and [6]Alc inthe downfield regionwas assigned to a trans site. The occurrence of multiple Al sites implies that Na-montmorillonite used in the present study has cis-vacant structure in the octahedral sheet. This is a reasonable insight, supported by the chemical composition and the differential thermal analysis data of the Na-montmorillonite.
Description
gsccm56505-tak.pdf - 245KB
Citation
Clays and Clay Minerals, Vol. 56, No. 5, 520–525, 2008.
Publisher
The Clay Minerals Society
License
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