Fluor-schorl, a new member of the tourmaline supergroup 

and additional data about schorl from the area of the cotype localities

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) ų (Zschorlau), a = 15.995(1), c = 7.166(1) Å, V = 1587.7(9) ų (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(Na_0.82K_0.01Ca_0.01[]_0.16) ^Y(Fe²⁺_2.30Al_0.38Mg_0.23Li_0.03Mn²⁺_0.02Zn_0.01[]_0.03)_S3.00 ^Z(Al_5.80Fe³⁺_0.10Ti⁴⁺_0.10) ^T(Si_5.81Al_0.19O₁₈) (BO₃)₃ ^V(OH)₃ ^W[F_0.66(OH)_0.34] (Zschorlau) 

and ^X(Na_0.78K_0.01[]ÿ_0.21) ^Y(Fe²⁺_1.89Al_0.58Fe³⁺_0.13Mn²⁺_0.13Ti⁴⁺_0.02Mg_0.02Zn_0.02[]ÿ_0.21)_S3.00 ^Z(Al_5.74Fe³⁺_0.26) ^T(Si_5.90Al_0.10O₁₈) (BO₃)₃ ^V(OH)₃ ^W[F_0.76(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(Na_0.5[]_0.5) ^Y(Fe²⁺_1.8Al_0.9Mg_0.2[]_0.1) ^Z(Al_5.8Fe³⁺_0.1Ti⁴⁺_0.1) ^T(Si_5.7Al_0.3O₁₈) (BO₃)₃ ^V(OH)₃ ^W[(OH)_0.9F_0.1]

to ~^X(Na_0.7[]_0.3) ^Y(Fe²⁺_2.1Al_0.7Mg_0.1[]_0.1) ^Z(Al_5.9Fe³⁺_0.1) ^T(Si_5.8Al_0.2O₁₈) (BO₃)₃ ^V(OH)₃ ^W[(OH)_0.6F_0.4]. 

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.