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

Abstract

At the margins of a plagioclase-quartz pegmatite near Stoffhutte (Koralpe, Austroalpine realm, Austrian Alps), a new solid solution series within the tourmaline group (XY 3 Z 6 [T 6 O 18 ](BO 3 ) 3 V 3 W) has been found, namely between magnesian schorl 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 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 Si contents X Mg values as well as increasing B, Al Li contents. Over a distance of less than five centimetres, tourmaline composition determined by secondary ion microprobe (B, Li, Be, H) electron microprobe (remaining elements, total Fe as Fe 2+ ) varies continuously from \((Na_{0.90}Ca_{0.11})(Fe_{1.61}Mg_{1.10}Mn_{0.01}Ti_{0.11}Zn_{0.01}Li_{0.01}Al_{0.10}{\square}_{0.05})Al_{6.00}[Si_{6.01}O_{18}] (B_{2.99}O_{9})(OH_{3.02}F_{0.23}O_{0.75}) (Na_{0.42}Ca_{0.31}{\square}_{0.27})(Fe_{0.14}Mg_{0.03}Mn_{0.01}Ti_{0.06}Li_{0.33}Be_{0.01}Al_{2.49})Al_{6}[Si_{4.91}Al_{0.21}B_{0.88}O_{18}](B_{3.00}O_{9})OH_{3.38}F_{0.07})\) . In contrast to the proton-deficient ideal olenite, the olenitic tourmaline from Stoffhutte 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 Tschermaks9 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 feldspar have considerable B (0.42–0.55 wt% 71–134 ppm, respectively), Be (49–140 ppm 79–169 ppm, respectively) Li contents (6–9 ppm 9–126 ppm, respectively). The apparent partition coefficient for total B between tourmaline muscovite (D B = c tur / c ms ) varies between 6.5 16.8, but the values for tetrahedrally coordinated B (D B(T) ) range from 0 to 13.0.