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Synthetic and natural ammonium-bearing tourmaline

Bernd Wunder, Eleanor Berryman, Birgit Plessen, Dieter Rhede, Monika Koch-Mueller and Wilhelm Heinrich
Synthetic and natural ammonium-bearing tourmaline
American Mineralogist (January 2015) 100 (1): 250-256

Abstract

Due to the similar ionic radius of K (super +) and NH (sub 4) (super +) , K-silicates can incorporate a significant amount of NH4. As tourmaline is able to accommodate K in its crystal structure at high and ultrahigh pressure, we test if this also holds true for NH4. Piston-cylinder experiments in the system (NH (sub 4) ) (sub 2) O-MgO-SiO (sub 2) -Al (sub 2) O (sub 3) -B (sub 2) O (sub 3) -H (sub 2) O at 4.0 GPa, 700 degrees C, with B2O3 and NH4OH in excess produce an assemblage of tourmaline, phengite, and coesite. The tourmaline crystals are up to 10X40 mu m in size. EMP analyses indicate that the tourmalines contain 0.22 (+ or -0.03) wt% (NH (sub 4) ) (sub 2) O and are solid solutions mainly along the magnesio-foitite and "NH (sub 4) -dravite" join with the average structural formula (super X) [(NH (sub 4) ) (sub 0.08(1)) [] (sub 0.92(1)) ] (super Y) [Mg (sub 2.28(8)) Al (super 0.72(8)) ] (super Z) [Al (sub 5.93(6)) Si (sub 0.07(6)) ] (super T) [Si (sub 6.00(5)) O (sub 18) ](BO (sub 3) ) (sub 3) (OH) (sub 4) . NH (sub 4) incorporation is confirmed by characteristic <N-H> stretching and bending modes in the IR-spectra of single crystals on synthetic tourmaline. Further evidence is the increased unit-cell parameters of the tourmaline [a=15.9214(9) Aa, c=7.1423(5) Aa, V=1567.9(2) Aa (super 3) ] relative to pure magnesio-foitite. Incorporation of NH (sub 4) in natural tourmaline was tested in a tourmaline-bearing mica schists from a high-P/low-T (>1.2 GPa/550 degrees C) metasedimentary unit of the Erzgebirge, Germany, rich in NH (sub 4) . The NH (sub 4) -concentrations in the three main NH (sub 4) -bearing phases are: biotite ( approximately 1400 ppm)>phengite ( approximately 700 ppm)>tourmaline ( approximately 500 ppm). This indicates that tourmaline can act as important carrier of nitrogen between the crust and the deep Earth, which has important implications for a better understanding of the large-scale light element cycle.


ISSN: 0003-004X
EISSN: 1945-3027
Coden: AMMIAY
Serial Title: American Mineralogist
Serial Volume: 100
Serial Issue: 1
Title: Synthetic and natural ammonium-bearing tourmaline
Affiliation: GeoForschungsZentrum Potsdam, Potsdam, Germany
Pages: 250-256
Published: 201501
Text Language: English
Publisher: Mineralogical Society of America, Washington, DC, United States
References: 53
Accession Number: 2016-016658
Categories: Mineralogy of silicates
Document Type: Serial
Bibliographic Level: Analytic
Illustration Description: illus. incl. 3 tables
N50°10'00" - N51°00'00", E12°00'00" - E14°10'00"
Country of Publication: United States
Secondary Affiliation: GeoRef, Copyright 2017, American Geosciences Institute.
Update Code: 201609
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