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The occurrence of tetrahedrally coordinated Al and B in tourmaline; an (super 11) B and (super 27) Al MAS NMR study

Aaron J. Lussier, Pedro M. Aguiar, Vladimir K. Michaelis, Scott Kroeker and Frank C. Hawthorne
The occurrence of tetrahedrally coordinated Al and B in tourmaline; an (super 11) B and (super 27) Al MAS NMR study
American Mineralogist (May 2009) 94 (5-6): 785-792


Considerable uncertainty has surrounded the occurrence of tetrahedrally coordinated Al and B at the T site in tourmaline. Although previously detected in several tourmaline specimens, the frequency of these substitutions in nature, as well as the extent to which they occur in the tourmaline structure, is not known. Using (super 11) B and (super 27) Al MAS NMR spectroscopy, we have investigated the presence of B and Al at the T site in 50 inclusion-free tourmaline specimens of low transition-metal content and different species (elbaite, "fluorelbaite," liddicoatite, dravite, uvite, olenite, and magnesiofoitite) from different localities worldwide. Chemical shifts of (super [4]) B and (super [3]) B in (super 11) B spectra, and (super [4]) Al and (super [6]) Al in (super 27) Al spectra, are well resolved, allowing detection of even small amounts of T-site constituents. In the observed spectra, (super [4]) B and (super [3]) B peaks are located at 0 and 18-20 ppm, respectively, with the greatest intensity corresponding to (super [3]) B (=3 apfu). In (super 27) Al spectra, (super [4]) Al and (super [6]) Al bands are located at 68-72 and 0 ppm, respectively, with the greater intensity corresponding to (super [6]) Al. However, inadequate separation of (super Y) Al and (super Z) Al precludes resolution of these two bands. Simulation of (super 11) B MAS NMR spectra shows that tetrahedrally and trigonally coordinated B can be readily distinguished at 14.1 T and that a (super [4]) B content of 0.0-0.5 apfu is common in tourmaline containing low amounts of paramagnetic species. (super 27) Al MAS NMR spectra show that Al is also a common constituent of the T site in tourmaline. Determination of (super [4]) Al content by peak-area integration commonly shows values of 0.0-0.5 apfu. Furthermore, the chemical shift of the (super 27) Al tetrahedral peak is sensitive to local order at the adjacent Y and Z octahedra, where (super [4]) Al- (super Y) Mg (sub 3) and (super [4]) Al- (super Y) (Al,Li) (sub 3) arrangements result in peaks located at approximately 65 and approximately 75 ppm, respectively. Both (super 11) B MAS NMR and (super 27) Al MAS NMR spectra show peak broadening as a function of transition-metal content (i.e., Mn (super 2+) +Fe (super 2+) =0.01-0.30 apfu) in the host tourmaline. In (super 11) B spectra, broadening and loss of intensity of the (super [3]) B signal ultimately obscures the signal corresponding to (super [4]) B, increasing the limit of detection of (super [4]) B in tourmaline. Our results clearly show that all combinations of Si, Al, and B: T=(Al, Si) (sub 6) , T=(B, Si) (sub 6) , T=(Al, B, Si) (sub 6) , and T=Si (sub 6) apfu, are common in natural tourmalines.

ISSN: 0003-004X
EISSN: 1945-3027
Serial Title: American Mineralogist
Serial Volume: 94
Serial Issue: 5-6
Title: The occurrence of tetrahedrally coordinated Al and B in tourmaline; an (super 11) B and (super 27) Al MAS NMR study
Affiliation: University of Manitoba, Department of Geological Sciences, Winnipeg, MB, Canada
Pages: 785-792
Published: 200905
Text Language: English
Publisher: Mineralogical Society of America, Washington, DC, United States
References: 50
Accession Number: 2009-054245
Categories: Mineralogy of silicates
Document Type: Serial
Bibliographic Level: Analytic
Illustration Description: illus. incl. 3 tables
Country of Publication: United States
Secondary Affiliation: GeoRef, Copyright 2017, American Geosciences Institute. Abstract, copyright, Mineralogical Society of America. Reference includes data from GeoScienceWorld, Alexandria, VA, United States
Update Code: 200930
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