ALBERT

Description: Dumortierite was synthesized in piston-cylinder experiments at 2.5–4.0 GPa, 650–700°C in the Al2 O3 –B2 O3 –SiO2 –H2 O (ABSH) system. Electron-microprobe (EMP) analyses reveal significant boron-excess (up to 0.26[4] B per formula unit, pfu) and silicon-deficiency relative to the ideal anhydrous dumortierite stoichiometry Al7 BSi3 O18. The EMP data in conjunction with results from single-crystal Raman spectroscopy and powder X-ray diffraction provide evidence that silicon at the tetrahedral site is replaced by excess boron via the substitution[4] Si↔ [4] B + H. The Raman spectrum of synthetic dumortierite in the frequency region 2000–4000 cm1 comprises eight bands, of which six are located at frequencies below 3400 cm1. This points to strong hydrogen bonding, most likely O2–H...O7 and O7–H...O2, arising from a high number of octahedral vacancies at the Al1 site and substitution of trivalent Al3+ and B3+ for Si4+ at Si1 and Si2 sites, causing decreasing acceptor–donor distances and lower incident valence at the acceptor oxygen. Contrary to the synthetic high-pressure ABSH-dumortierite, magnesiodumortierite from the Dora-Maira Massif, which is assumed to have formed at similar conditions (2.5–3.0 GPa, 700°C), does not show any B-excess. Tourmaline shows an analogous behaviour in that magnesium-rich (e.g., dravitic) tourmaline formed at high pressure shows no or only minor amounts of tetrahedral boron, whereas natural aluminum-rich tourmaline and synthetic olenitic tourmaline formed at high pressures can incorporate significant amounts of tetrahedral boron. Two mechanisms might account for this discrepancy: (i) Structural avoidance of[6] Mg– O– [4] R3+ configurations in magnesiodumortierite due to charge deficieny at the oxygens O2 and O7 and strong local distortion of M1 due to decreased O2 O7 bond length, and/or (ii) decreasing fluid mobility of boron in Al-rich systems at high pressures.