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Acicular zoned tourmaline (magnesio-foitite to foitite) from a quartz vein near Tisovec, Slovakia; the relationship between crystal chemistry and acicular habit

Peter Bacik, Andreas Ertl, Martin Stevko, Gerald Giester and Peter Seckar
Acicular zoned tourmaline (magnesio-foitite to foitite) from a quartz vein near Tisovec, Slovakia; the relationship between crystal chemistry and acicular habit
The Canadian Mineralogist (March 2015) 53 (2): 221-234

Abstract

Distinctly chemically zoned tourmaline was found in a quartz vein near Tisovec, Slovak Ore Mountains, Slovakia. The tourmaline forms radial aggregates of light grey to green thin prismatic to acicular crystals growing in cracks in the host rock. The root zone of the aggregates has dravitic to oxy-dravitic compositions, shifting to magnesio-foitite in the middle parts of the crystals and to foititic compositions in the outer parts of the aggregates. The optimized formula, considering the single-crystal structure refinement (SREF) and the chemistry of the magnesio-foititic zone, is approximately (super X) ([] (sub 0.5) Na (sub 0.4) Ca (sub 0.1) ) (super Y) (Mg (sub 1.1) A (sub l1.0) Fe (super 2+) (sub 0.8) Fe (super 3+) (sub 0.1) ) (super Z) (Al (sub 5.7) Fe (super 3+) (sub 0.3) ) (Si (sub 5.9) Al (sub 0.1) O (sub 18) ) (BO (sub 3) ) (sub 3) (OH)3 [O (sub 0.6) (OH) (sub 0.3) F (sub 0.1) ], with lattice parameters (SREF) a 15.929(2) Aa, c 7.163(1) Aa, and V 1574.0(7) Aa3. SREF data as well as a detailed study of bond lengths indicate a dominancy of Al (super 3+) at the Z site substituted by a small proportion of Fe (super 3+) , while Fe (super 2+) and Mg (super 2+) essentially occupy the Y site along with a significant amount of Al. The chemical composition is controlled by the substitutions FeMg (sub -1) , Al[](Fe,Mg) (sub -1) Na (sub -1) , and AlO(Fe,Mg) (sub -1) (OH) (sub -1) . The AlO(Fe,Mg) (sub -1) (OH) (sub -1) substitution is dominant in the transition from oxy-dravite to magnesio-foitite, and the Al[](Fe,Mg) (sub -1) Na (sub -1) substitution prevails in the magnesio-foitite and foitite compositions. Both proton- and alkali-deficient substitutions produce the enrichment in Al which can be linked to the forming of the acicular habit. Aluminum strongly prefers the Z-site, whose octahedra form a network of chains parallel to c. Consequently, polymerization of the ZO (sub 6) octahedral network will dominate growth in the a direction. Therefore, the chemical composition, mainly Al enrichment, which is common for most of the acicular to fibrous tourmalines, can be an important factor promoting preferential growth in the c direction. The rate and time for crystallization, which depend mostly on the temperature and cooling rate, are also factors which should be considered.


ISSN: 0008-4476
EISSN: 1499-1276
Coden: CAMIA6
Serial Title: The Canadian Mineralogist
Serial Volume: 53
Serial Issue: 2
Title: Acicular zoned tourmaline (magnesio-foitite to foitite) from a quartz vein near Tisovec, Slovakia; the relationship between crystal chemistry and acicular habit
Affiliation: Comenius University in Bratislava, Department of Mineralogy and Petrology, Bratislava, Slovakia
Pages: 221-234
Published: 201503
Text Language: English
Publisher: Mineralogical Association of Canada, Ottawa, ON, Canada
References: 43
Accession Number: 2016-005403
Categories: Mineralogy of silicates
Document Type: Serial
Bibliographic Level: Analytic
Illustration Description: illus. incl. 5 tables, sketch map
N48°25'60" - N48°55'60", E19°06'00" - E21°19'60"
Secondary Affiliation: Naturhistorisches Museum, AUT, AustriaUniversitt Wien, AUT, Austria
Country of Publication: Canada
Secondary Affiliation: GeoRef, Copyright 2017, American Geosciences Institute.
Update Code: 201603
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