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

\({+}\ H_{2}O\ {+}\ B_{2}O_{3}\ =\ olenite\ {+}\ quartz\)
. 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
\((Na_{0.90}Ca_{0.11})(Fe_{1.61}Mg_{1.10}Mn_{0.01}Ti_{0.11}Zn_{0.01}Li_{0.01}Al_{0.10}{\square}_{0.05})Al_{6.00}[Si_{6.01}O_{18}]\ (B_{2.99}O_{9})(OH_{3.02}F_{0.23}O_{0.75})\ to\ (Na_{0.42}Ca_{0.31}{\square}_{0.27})(Fe_{0.14}Mg_{0.03}Mn_{0.01}Ti_{0.06}Li_{0.33}Be_{0.01}Al_{2.49})Al_{6}[Si_{4.91}Al_{0.21}B_{0.88}O_{18}](B_{3.00}O_{9})OH_{3.38}F_{0.07})\)
. 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
\(^{[Y]}Al^{3{+}}\ {+}\ ^{[T]}B^{3{+}}\ =\ ^{[T]}Si^{4{+}}\ {+}\ ^{[Y]}(Mg,\ Fe)^{2{+}}\)
, which is a modified Tschermaks' substitution,
\(^{[Y]}Al^{3{+}}\ {+}\ ^{[Y]}Li^{{+}}\ =\ 2\ ^{[Y]}(Mg,\ Fe)^{2{+}}\)
,
\(^{[X]}Ca^{2{+}}\ {+}\ ^{[X]}{\square}\ =\ 2\ ^{[X]}Na^{{+}},\ ^{[Y]}Al^{3{+}}\ {+}\ ^{[X]}{\square}\ =\ ^{[X]}Na^{{+}}\ {+}\ ^{[Y]}(Mg,\ Fe)^{2{+}}\ and\ ^{[Y]}Fe^{2{+}}\ =\ ^{[Y]}Mg^{2{+}}\)
. 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.

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