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) ų. Analyses by a combination of electron microprobe, optical absorption spectroscopy and crystal structure refinement (R1 = 1.31%) result in the structural formula ^X(Na_0.85Ca_0.08Sr_0.06K_0.01)_S1.00 ^Y(Mg_1.68Al_0.70Fe³⁺_0.37Ti⁴⁺_0.10Fe²⁺_0.11Ca_0.03Cr³⁺_0.01)_S3.00 ^Z(Al_5.15Mg_0.80Fe³⁺_0.05)_S6.00 ^T(Si_5.82B_0.10Al_0.08O₁₈) (BO₃)₃ ^V(OH)₃ ^W[O_0.45(OH)_0.35F_0.20]. 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.

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