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Synthetic tourmaline (olenite) with excess boron replacing silicon in the tetrahedral site; I, Synthesis conditions, chemical and spectroscopic evidence

Werner Schreyer, Ulrich Wodara, Bernd Marler, Peter A. van Aken, Friedrich Seifert and Jean-Louis Robert
Synthetic tourmaline (olenite) with excess boron replacing silicon in the tetrahedral site; I, Synthesis conditions, chemical and spectroscopic evidence
European Journal of Mineralogy (June 2000) 12 (3): 529-541

Abstract

One selected composition within the system Na (sub 2) O-Al (sub 2) O (sub 3) -B (sub 2) O (sub 3) -SiO (sub 2) -H (sub 2) O (NABSH) was studied with the aim of synthesizing the new tourmaline end member olenite with the theoretical formula NaAl (sub 3) Al (sub 6) [Si (sub 6) O (sub 18) ] (BO (sub 3) ) (sub 3) O (sub 3) (OH). The starting material consisted of a gel with the anhydrous composition 0.625Na (sub 2) O.4.5Al (sub 2) O (sub 3) .6SiO (sub 2) , but with 100 % excess B (sub 2) O (sub 3) added over that of the above formula, and a surplus of water to aid crystallization. Run conditions ranged from 4 to 50 kbar, 400-900 degrees C, but a tourmaline phase could only be obtained at or above 10 kbar, with yields between 80 and 100 % when ignoring amorphous quench products from the coexisting fluids. The synthetic olenites exhibit much smaller cell parameters than those reported for natural olenites, and indeed the smallest ones ever measured for any tourmaline phase. Analytical data on an olenite prepared at 25 kbar, 600 degrees C, show excess boron and water relative to the theoretical formula, coupled with deficiencies in Si, Al, and Na. Spectroscopic investigations (MAS NMR, EELS, IR) prove - directly or indirectly - that boron occurs not only in trigonal coordination, but is also located in the tetrahedral ring site. Thus, a provisional structural formula is derived as (Na (sub 0.65) (sub 0.35) ) (Al (sub 2.72) (sub 0.28) ) (Al (sub 5.42) Si (sub 0.58) ) [Si (sub 3.73) B (sub 2.27) O (sub 18) ] (BO (sub 3) ) (sub 3) (OH) (sub 3.87) O (sub 0.13) . In this synthetic olenite the OH-content is close to the maximum of 4.0 p.f.u.; boron replaces tetrahedral Si according to BHSi (sub -1) , with this substitution cancelling the proton deficiency of the theoretical olenite formula. Because the sum (B+Si) exceeds 9.0, some Si seems to replace octahedral Al. Nevertheless, octahedral vacancies remain. There are indications that the above tourmaline composition is not unique for the system studied, but that a range of olenite solid solutions exists as a function of starting material and run conditions, possibly extending to the ideal olenite formula. Excess-boron tourmalines are probably confined to very Al-rich (or M (super 3+) -rich) compositions which - for stoichiometric reasons - should have proton deficiencies, but these may be compensated by the BHSi (sub -1) substitution.


ISSN: 0935-1221
Serial Title: European Journal of Mineralogy
Serial Volume: 12
Serial Issue: 3
Title: Synthetic tourmaline (olenite) with excess boron replacing silicon in the tetrahedral site; I, Synthesis conditions, chemical and spectroscopic evidence
Affiliation: Ruhr-Universitaet, Institut fuer Mineralogie, Bochum, Federal Republic of Germany
Pages: 529-541
Published: 200006
Text Language: English
Publisher: Schweizerbart'sche Verlagsbuchhandlung (Naegele u. Obermiller), Stuttgart, Federal Republic of Germany
References: 33
Accession Number: 2000-049397
Categories: Mineralogy of silicates
Document Type: Serial
Bibliographic Level: Analytic
Illustration Description: illus. incl. 2 tables
Secondary Affiliation: T.U. Darmstadt, DEU, Federal Republic of GermanyUniversitaet Bayreuth, DEU, Federal Republic of Germany
Country of Publication: Germany
Secondary Affiliation: GeoRef, Copyright 2017, American Geosciences Institute.
Update Code: 200016
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