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X-site vacant Al-tourmaline; a new synthetic end-member

Ulrich Wodara and Werner Schreyer
X-site vacant Al-tourmaline; a new synthetic end-member (in Experimental mineralogy, petrology and geochemistry, Anonymous)
European Journal of Mineralogy (June 2001) 13 (3): 521-532

Abstract

As a missing link of the tourmaline classification, an X-site vacant analogue of olenite was synthesized from three different starting compositions in the system Al (sub 2) O (sub 3) -B (sub 2) O (sub 3) -SiO (sub 2) -H (sub 2) O (ABSH) over the PT-range 4-40 kbar, 450-700 degrees C, however never as a single phase product; dumortierite with either quartz or with jeremejevite-OH were the coexisting solids. The starting materials containing 100% excess B (sub 2) O (sub 3) produced an Al-tourmaline with exceptionally low cell parameters similar to those of synthetic excess-boron olenite. Electron microprobe analyses indeed show a surplus of boron over the classical 3.0 per formula unit, which is linked with a deficiency in silicon. There is a negative linear correlation between the cell volume a of Al-rich tourmalines and their tetrahedral boron contents. A preliminary structural formula of the synthetic Al-tourmaline obtained at 20 kbar, 600 degrees C is: ([] (sub 0.96) Na (sub 0.04) )Al (sub 3) (Al (sub 5.83) Si (sub 0.19) ) [Si (sub 4.49) B (sub 1.51) O (sub 18) ](BO (sub 3) ) (sub 3) (OH) (sub 3.21) O (sub 0.79) . The small Na impurity is probably due to contamination from the NaCl pressure cell, octahedral Si is hypothetical. If the excess Si is considered to be due to analytical error, other hypothetical formulae exhibiting octahedral vacancies result. As none of the cations in the various structural sites of the tourmaline exceed 50% of the theoretical value, the following formula can be defined as idealized Al-tourmaline end-member: [] Al (sub 3) Al (sub 6) [Si (sub 6) O (sub 18) ](BO (sub 3) ) (sub 3) O (sub 2) (OH) (sub 2) . In nature, such X-site vacant Al-rich tourmalines would be expected to occur in Al-rich and (Mg,Fe)-, Ca- and alkali-free products of hydrothermal systems or of wall-rock alterations, or in their metamorphic equivalents such as dumortierite quartzites. An unusually Al-rich, complex tourmaline from a hydrothermal system in Nevada contains as much as 56 mol % of the above end-member together with schorl, foitite and rossmanite components.


ISSN: 0935-1221
EISSN: 1617-4011
Serial Title: European Journal of Mineralogy
Serial Volume: 13
Serial Issue: 3
Title: X-site vacant Al-tourmaline; a new synthetic end-member
Title: Experimental mineralogy, petrology and geochemistry
Author(s): Wodara, UlrichSchreyer, Werner
Author(s): Anonymous
Affiliation: Ruhr-Universitaet, Institut fuer Geologie, Mineralogie und Geophysik, Bochum, Federal Republic of Germany
Pages: 521-532
Published: 200106
Text Language: English
Publisher: Schweizerbart'sche Verlagsbuchhandlung (Naegele u. Obermiller), Stuttgart, Federal Republic of Germany
Meeting name: Eighth international symposium on Experimental mineralogy, petrology and geochemistry
Meeting location: Bergamo, ITA, Italy
Meeting date: 20000416April 16-19, 2000
References: 35
Accession Number: 2001-055197
Categories: Mineralogy of silicates
Document Type: Serial Conference document
Bibliographic Level: Analytic
Illustration Description: illus. incl. 3 tables
Country of Publication: Germany
Secondary Affiliation: GeoRef, Copyright 2017, American Geosciences Institute.
Update Code: 200117
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