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X-site vacant Al-tourmaline

Institut für Geologie, Mineralogie und Geophysik, Ruhr-Universität, D-44780 Bochum, Germany

^* e-mail: werner.schreyer{at}ruhr-uni-bochum.de

This paper was presented at the EMPG VIII meeting in Bergamo, Italy (April 2000)

As a missing link of the tourmaline classification, an X-site vacant analogue of olenite was synthesized from three different starting compositions in the system Al₂O₃-B₂O₃-SiO₂-H₂O (ABSH) over the PT-range 4–40 kbar, 450–700 °C, however never as a single phase product; dumortierite with either quartz or with jeremejevite-OH were the coexisting solids. The starting materials containing 100% excess B₂O₃ produced an Al-tourmaline with exceptionally low cell parameters similar to those of synthetic excess-boron olenite. Electron microprobe analyses indeed show a surplus of boron over the classical 3.0 per formula unit, which is linked with a deficiency in silicon. There is a negative linear correlation between the cell volume a of Al-rich tourmalines and their tetrahedral boron contents. A preliminary structural formula of the synthetic Al-tourmaline obtained at 20 kbar, 600 °C is: (_0.96 Na_0.004) Al₃ (Al_5.83 Si _0.19) [Si_4.49 B _1.51 O₁₈] (BO₃)₃ (OH)_3.21 O_0.79.

The small Na impurity is probably due to contamination from the NaCl pressure cell, octahedral Si is hypothetical. If the excess Si is considered to be due to analytical error, other hypothetical formulae exhibiting octahedral vacancies result. As none of the cations in the various structural sites of the tourmaline exceed 50% of the theoretical value, the following formula can be defined as idealized Al-tourmaline end-member: Al₃ Al₆ [Si₆O₁₈] (BO₃)₃ O₂ (OH)₂. In nature, such X-site vacant Al-rich tourmalines would be expected to occur in Al-rich and (Mg, Fe)-, Ca- and alkali-free products of hydrothermal systems or of wall-rock alterations, or in their metamorphic equivalents such as dumortierite quartzites. An unusually Al-rich, complex tourmaline from a hydrothermal system in Nevada contains as much as 56 mol % of the above end-member together with schörl, foitite and rossmanite components.

Key-words: Al-tourmaline with tetrahedral boron, new tourmaline end-member, hydrothermal synthesis, system Al₂O₃-B₂O₃-SiO₂-H₂O.

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