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Fluor-schorl, a new member of the tourmaline supergroup, and new data on schorl from the cotype localities

Andreas Ertl, Uwe Kolitsch, M. Darby Dyar, Hans-Peter Meyer, George R. Rossman, Darrell J. Henry, Markus Prem, Thomas Ludwig, Lutz Nasdala, Christian L. Lengauer, Ekkehart Tillmanns and Gerhard Niedermayr
Fluor-schorl, a new member of the tourmaline supergroup, and new data on schorl from the cotype localities
European Journal of Mineralogy (2015) 28 (1): 163-177


Fluor-schorl, NaFe (super 2+) (sub 3) Al (sub 6) Si (sub 6) O (sub 18) (BO (sub 3) ) (sub 3) (OH) (sub 3) F, is a new mineral species of the tourmaline supergroup from alluvial tin deposits near Steinberg, Zschorlau, Erzgebirge (Saxonian Ore Mountains), Saxony, Germany, and from pegmatites near Grasstein (area from Mittewald to Sachsenklemme), Trentino, South Tyrol, Italy. Fluor-schorl was formed as a pneumatolytic phase and in high-temperature hydrothermal veins in granitic pegmatites. Crystals are black (pale brownish to pale greyish-bluish, if <0.3 mm in diameter) with a bluish-white streak. Fluor-schorl is brittle and has a Mohs hardness of 7; it is non-fluorescent, has no observable parting and a poor/indistinct cleavage parallel to {0001}. It has a calculated density of approximately 3.23 g/cm (super 3) . In plane-polarized light, it is pleochroic, O=brown to grey-brown (Zschorlau), blue (Grasstein), E=pale grey-brown (Zschorlau), cream (Grasstein). Fluor-schorl is uniaxial negative, omega =1.660(2)-1.661(2), epsilon =1.636(2)-1.637(2). The mineral is rhombohedral, space group R3m, a=16.005(2), c=7.176(1) Aa, V=1591.9(4) Aa3 (Zschorlau), a=15.995(1), c=7.166(1) Aa, V=1587.7(9) Aa3 (Grasstein), Z=3. The eight strongest observed X-ray diffraction lines in the powder pattern [d in Aa (I)hkl] are: 2.584(100)(051), 3.469(99)(012), 2.959(83)(122), 2.044(80)(152), 4.234(40)(211), 4.005(39)(220), 6.382(37)(101), 1.454(36)(514) (Grasstein). Analyses by a combination of electron microprobe, secondary-ion mass spectrometry (SIMS), Mossbauer spectroscopic data and crystal-structure refinement result in the structural formulae (super X) (Na (sub 0.82) K (sub 0.01) Ca (sub 0.01) [] (sub 0.16) ) (super Y) (Fe (super 2+) (sub 2.30) Al (sub 0.38) Mg (sub 0.23) Li (sub 0.03) Mn (super 2+) (sub 0.02) Zn (sub 0.01) [] (sub 0.03) ) (sub Sigma 3.00) (super Z) (Al (sub 5.80) Fe (super 3+) (sub 0.10) Ti (super 4+) (sub 0.10) ) (super T) (Si (sub 5.81) Al (sub 0.19) O (sub 18) )(BO (sub 3) ) (sub 3) (super V) (OH) (sub 3) (super W) [F (sub 0.66) (OH) (sub 0.34) ] (Zschorlau) and (super X) (Na (sub 0.78) K (sub 0.01) [] (sub 0.21) ) (super Y) (Fe (super 2+) (sub 1.89) Al (sub 0.58) Fe (super 3+) (sub 0.13) Mn (super 2+) (sub 0.13) Ti (super 4+) (sub 0.02) Mg (sub 0.02) Zn (sub 0.02) [] (sub 0.21) ) (sub Sigma 3.00) (super Z) (Al (sub 5.74) Fe (super 3+) (sub 0.26) ) (super T) (Si (sub 5.90) Al (sub 0.10) O (sub 18) )(BO (sub 3) ) (sub 3) (super V) (OH) (sub 3) (super W) [F (sub 0.76) (OH) (sub 0.24) ] (Grasstein). Several additional, newly confirmed occurrences of fluor-schorl are reported. Fluor-schorl, ideally NaFe (super 2+) (sub 3) Al (sub 6) Si (sub 6) O (sub 18) (BO (sub 3) ) (sub 3) (OH) (sub 3) F, is related to end-member schorl by the substrution F-->(OH). The chemical compositions and refined crystal structures of several schorl samples from cotype localities for schorl (alluvial tin deposits and tin mines in the Erzgebirge, including Zschorlau) are also reported. The unit-cell parameters of schorl from these localities are slightly variable, a=15.98-15.99, c=7.15-7.16 Aa, corresponding to structural formulae ranging from approximately (super X) (Na (sub 0.5) [] (sub 0.5) ) (super Y) (Fe (super 2+) (sub 1.8) Al (sub 0.9) Mg (sub 0.2) [] (sub 0.1) ) (super Z) (Al (sub 5.8) Fe (super 3+) (sub 0.1) Ti (super 4+) (sub 0.1) ) (super T) (Si (sub 5.7) Al (sub 0.3) O (sub 18) )(BO (sub 3) ) (sub 3) V(OH)3 (super W) [(OH) (sub 0.9) F (sub 0.1) ] to approximately (super X) (Na (sub 0.7) [] (sub 0.3) ) (super Y) (Fe (super 2+) (sub 2.1) Al (sub 0.7) Mg (sub 0.1) [] (sub 0.1) ) (super Z) (Al (sub 5.9) Fe (super 3+) (sub 0.1) ) (super T) (Si (sub 5.8) Al (sub 0.2) O (sub 18) ) (BO (sub 3) ) (sub 3) (super V) (OH) (sub 3) (super W) [(OH) (sub 0.6) F (sub 0.4) ]. The investigated tourmalines from the Erzgebirge show that there exists a complete fluor-schorl-schorl solid-solution series. For all studied tourmaline samples, a distinct inverse correlation was observed between the X-O2 distance (which reflects the mean ionic radius of the X-site occupants) and the F content (r (super 2) =0.92). A strong positive correlation was found to exist between the F content and the Y-Or (super 2) distance (r (super 2) =0.93). This correlation indicates that Fe (super 2+) -rich tourmalines from the investigated localities clearly tend to have a F-rich or F-dominant composition. A further strong positive correlation (r (super 2) =0.82) exists between the refined F content and the Y-W (F,OH) distance, and the latter may be used to quickly estimate the F content.

ISSN: 0935-1221
EISSN: 1617-4011
Serial Title: European Journal of Mineralogy
Serial Volume: 28
Serial Issue: 1
Title: Fluor-schorl, a new member of the tourmaline supergroup, and new data on schorl from the cotype localities
Affiliation: Naturhistorisches Museum, Mineralogisch-Petrographische Abteilung, Vienna, Austria
Pages: 163-177
Published: 2015
Text Language: English
Publisher: Schweizerbart'sche Verlagsbuchhandlung (Naegele u. Obermiller), Stuttgart, Germany
References: 58
Accession Number: 2016-016546
Categories: Mineralogy of silicates
Document Type: Serial
Bibliographic Level: Analytic
Illustration Description: illus. incl. 6 tables
N50°34'00" - N50°34'00", E12°39'00" - E12°39'00"
Secondary Affiliation: Universitaet Wien, AUT, AustriaMount Holyoke College, USA, United StatesUniversitaet Heidelberg, DEU, GermanyCalifornia Institute of Technology, USA, United StatesLouisiana State University, USA, United States
Country of Publication: Germany
Secondary Affiliation: GeoRef, Copyright 2017, American Geosciences Institute.
Update Code: 201609
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