Original paper

Complete solid solution between magnesian schorl and lithian excess-boron olenite in a pegmatite from the Koralpe (eastern Alps, Austria)

Kalt, Angelika; Schreyer, Werner; Ludwig, Thomas; Prowatke, Stefan; Bernhardt, Heinz-Jürgen; Ertl, Andreas

Abstract

At the margins of a plagioclase-quartz pegmatite near Stoffhütte (Koralpe, Austroalpine realm, Austrian Alps), a new solid solution series within the tourmaline group (XY3Z6[T6O18](BO3)3V3W) has been found, namely between magnesian schorl and lithian excess-boron olenite. At the pegmatite contact with mylonitic garnet-biotite schists, a tourmalinite band developed. Massive tourmaline cores are Mg-rich schorl with normal boron contents of 3.0 cations per formula unit that most likely formed by a reaction of melt with garnet. Within the pegmatite, only olenitic crystals were found. Within a transition zone, the Mg-rich schorl crystals become zoned and develop olenitic rims of increasing size, most likely formed from a fluid via the reaction plagioclase + H2O + B2O3 = olenite + quartz. The major compositional changes are decreasing Fe, Mg and Si contents and XMg values as well as increasing B, Al and Li contents. Over a distance of less than five centimetres, tourmaline composition determined by secondary ion microprobe (B, Li, Be, H) and electron microprobe (remaining elements, total Fe as Fe2+) varies continuously from (Na0.90Ca0.11)(Fe1.61Mg1.10Mn0.01Ti0.11Zn0.01Li0.01Al0.10[]0.05)Al6.00[Si6.01O18](B2.99O9)(OH3.02F0.23O0.75) to (Na0.42Ca0.31[]0.27)(Fe0.14Mg0.03Mn0.01Ti0.06Li0.33Be0.01Al2.49)Al6[Si4.91Al0.21B0.88O18](B3.00O9(OH3.38F0.07). In contrast to the proton-deficient ideal olenite, the olenitic tourmaline from Stoffhütte contains 3.0-3.4 hydrogens per formula. The main substitutions are most likely [Y]Al3+ + [T]B3+ = [T]Si4+ + [Y](Mg, Fe)2+, which is a modified Tschermaks' substitution, [Y]Al3+ + [Y]Li+ = 2 [Y](Mg, Fe)2+, [X]Ca2+ + [X][] = 2 [X]Na+, [Y]Al3+ + [X][] = [X]Na+ + [Y](Mg, Fe)2+ and [Y]Fe2+ = [Y]Mg2+. Coexisting muscovite and feldspar have considerable B (0.42-0.55 wt% and 71- 134 ppm, respectively), Be (49-140 ppm and 79-169 ppm, respectively) and Li contents (6-9 ppm and 9-126 ppm, respectively). The apparent partition coefficient for total B between tourmaline and muscovite (DB = ctur/cms) varies between 6.5 and 16.8, but the values for tetrahedrally coordinated B (DB(T)) range from 0 to 13.0.

Keywords

schorloleniteexcess boronion microprobekoralpe