Andreas Ertl
Mineralogisch-Petrographische
Abt., Naturhistorisches Museum, Burgring 7,
1010 Vienna, Austria
Institut
für Mineralogie und Kristallographie,
Geozentrum, Universität Wien, Althanstrasse 14, 1090 Vienna, Austria
M.
Darby Dyar
Department of Geography and Geology, Mount Holyoke
College, South Hadley, Massachusetts 01075, U.S.A.
Hans-Peter
Meyer
Institut für Geowissenschaften, Universität Heidelberg, Im Neuenheimer
Feld 236, 69120 Heidelberg, Germany
George R. Rossman
Division of Geological and Planetary Sciences, California Institute of
Technology
Pasadena,
California 91125, USA
Darrell J. Henry
Department of Geology
and Geophysics, Louisiana State University, Baton Rouge, Louisiana
70803, U.S.A.
Markus Prem
Institut für
Mineralogie und Kristallographie,
Geozentrum, Universität Wien, Althanstrasse 14, 1090 Vienna, Austria
Thomas
Ludwig
Institut für
Geowissenschaften, Universität Heidelberg, Im Neuenheimer
Feld 236, 69120 Heidelberg, Germany
Lutz Nasdala, Christian
L Lengauer,
Ekkehart Tillmanns
Institut
für Mineralogie und Kristallographie,
Geozentrum, Universität Wien, Althanstrasse 14, 1090 Vienna, Austria
ABSTRACT
Fluor-schorl, NaFe2+3Al6Si6O18(BO3)3(OH)3F, is a new mineral species of the tourmaline group from alluvial tin deposits near Steinberg, Zschorlau, Erzgebirge (Saxony 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 ~3.23 g/cm3. 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, ω = 1.660(2)-1.661(2), ε = 1.636(2)-1.637(2). The mineral is rhombohedral, space group R3m, a = 16.005(2), c = 7.176(1) Å, V = 1591.9(4) Å3 (Zschorlau), a = 15.995(1), c = 7.166(1) Å, V = 1587.7(9) Å3 (Grasstein), Z = 3. The strongest eight observed X-ray diffraction lines in the powder pattern [d in Å(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, SIMS, Mössbauer spectroscopic data and crystal structure refinement result in the structural formulae
X(Na0.82K0.01Ca0.01[]0.16) Y(Fe2+2.30Al0.38Mg0.23Li0.03Mn2+0.02Zn0.01[]0.03)S3.00 Z(Al5.80Fe3+0.10Ti4+0.10) T(Si5.81Al0.19O18) (BO3)3 V(OH)3 W[F0.66(OH)0.34] (Zschorlau)
and X(Na0.78K0.01[]˙0.21) Y(Fe2+1.89Al0.58Fe3+0.13Mn2+0.13Ti4+0.02Mg0.02Zn0.02[]˙0.21)S3.00 Z(Al5.74Fe3+0.26) T(Si5.90Al0.10O18) (BO3)3 V(OH)3 W[F0.76(OH)0.24] (Grasstein).
Several additional, newly confirmed occurrences of fluor-schorl are reported. Fluor-schorl, ideally NaFe2+3Al6Si6O18(BO3)3(OH)3F, is related to end-member schorl, ideally NaFe2+3Al6Si6O18(BO3)3(OH)3(OH), by the substution F ® (OH). For comparison purposes, the chemical compositions and refined crystal structures were also determined for several schorl samples from cotype localities for schorl (alluvial tin deposits and tin mines, including Zschorlau) in the Erzgebirge. The unit-cell parameters of schorl from these localities are variable, a = 15.90-16.00(2), c = 7.160-7.178(1) Å, corresponding to structural formulae ranging from
~X(Na0.5[]0.5) Y(Fe2+1.8Al0.9Mg0.2[]0.1) Z(Al5.8Fe3+0.1Ti4+0.1) T(Si5.7Al0.3O18) (BO3)3 V(OH)3 W[(OH)0.9F0.1]
to ~X(Na0.7[]0.3) Y(Fe2+2.1Al0.7Mg0.1[]0.1) Z(Al5.9Fe3+0.1) T(Si5.8Al0.2O18) (BO3)3 V(OH)3 W[(OH)0.6F0.4].
The investigated tourmalines from the Erzgebirge
show that there exists
a fluor-schorl
– schorl solid solution. 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 (r2
= 0.92). A strong positive
correlation was found to exist between the F content and the <Y–O> distance (r2
= 0.93). This
correlation indicates that Fe2+-rich tourmalines
from the
investigated localities clearly tend to have a F-rich or -dominant
composition.
A further strong positive correlation 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.
last revised 29-May-2015