Detailed characterization of a Sr-rich, high-pressure tourmaline from 

the Kreuzeck Mountains, Eastern Alps, Austria

Ertl A1, Topa D2, Giester G1, Rossman GR3, Tillmanns E1, Konzett J4
 
1 Institut für Mineralogie und Kristallographie           2Mineralogisch-Petrographische Abt.
               Universität Wien                                              Naturhistorisches Museum
       1090 Vienna, Austria                                               1010 Vienna Austria

       
Division of Geological and Planetary Science              4Institut für Mineralogie und Planetologie
California Institute of Technology                                        Universität Innsbruck
     Pasadena, CA  91125-2500, USA                                         6020 Innsbruck, Austria


ABSTRACT

A detailed investigation was conducted on high-pressure tourmaline from an eclogite, which occurs in the Kreuzeck Mountains, Eastern Alps, Austria. Tourmaline from this locality contains the highest amount of Sr2+ (up to 0.68 wt% SrO) known to date. This tourmaline (space group R3m) exhibits lattice parameters a = 15.944(1), c = 7.202(1) Å, V = 1585.5(3) Å3. Analyses by a combination of electron microprobe, optical absorption spectroscopy and crystal structure refinement (R1 = 1.31%) result in the structural formula X(Na0.85Ca0.08Sr0.06K0.01)S1.00 Y(Mg1.68Al0.70Fe3+0.37Ti4+0.10Fe2+0.11Ca0.03Cr3+0.01)S3.00 Z(Al5.15Mg0.80Fe3+0.05)S6.00 T(Si5.82B0.10Al0.08O18) (BO3)3 V(OH)3 W[O0.45(OH)0.35F0.20]. The T site contains mainly Si and additionally small amounts of B and Al. Optical absorption spectroscopy was used to estimate the percentage of Fe2+. For this purpose the band near 1120 nm was used, because it is the band, which is best separated from other bands. We conclude that 79±2% of the total iron in this tourmaline is in the 3+ oxidation state. It can be presumed that this high-pressure tourmaline crystallized during oxidizing conditions. A rim near zone contains 0.6 wt% Cr2O3, 0.5 wt% PbO2, 0.2 wt% NiO and 0.1 wt% V2O3. Only a small F content was found by structure refinement. There is no evidence that the investigated tourmaline zones exhibit any significant vacancies at the X site. The composition of this high-pressure tourmaline is close to (an Fe3+-bearing) oxy-dravite. We discuss the possibility, that the original boron source for the tourmaline crystallization in the eclogite, the tourmaline-bearing pegmatites, were influenced by a Sr-enriched marble.

Polarized sample colors
A 0.55 mm wide crystal of the Kreuzeck Mtn tourmaline in linerally polarized light

Spectrum

Optical Spectrum of the Krekkuzeck Mtns oxy-dravite