Dumortierite was synthesized in piston-cylinder experiments at 2.5–4.0 GPa, 650–700 °C in the Al2O3–B2O3–SiO2–H2O (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 Al7BSi3O18. 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 cm−1 comprises eight bands, of which six are located at frequencies below 3400 cm−1. 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.

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