Synthetic (super [4]) B-bearing dumortierite and natural (super [4]) B-free magnesiodumortierite from the Dora-Maira Massif; differences in boron coordination in response to ultrahigh pressure

Dumortierite was synthesized in piston-cylinder experiments at 2.5-4.0 GPa, 650-700 degrees C in the Al (sub 2) O (sub 3) -B (sub 2) O (sub 3) -SiO (sub 2) -H (sub 2) O (ABSH) system. Electron-microprobe (EMP) analyses reveal significant boron-excess (up to 0.26 (super [4]) B per formula unit, pfu) and silicon-deficiency relative to the ideal anhydrous dumortierite stoichiometry Al (sub 7) BSi (sub 3) O (sub 18) . 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 (super [4]) Si <--> (super [4]) B + H. The Raman spectrum of synthetic dumortierite in the frequency region 2000-4000 cm (super -1) comprises eight bands, of which six are located at frequencies below 3400 cm (super -1) . This points to strong hydrogen bonding, most likely O (sub 2) -H...O (sub 7) and O (sub 7) -H...O (sub 2) , arising from a high number of octahedral vacancies at the Al1 site and substitution of trivalent Al (super 3+) and B (super 3+) for Si (super 4+) at Si (super 1) and Si (super 2) 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 degrees 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 (super [6]) Mg-O- (super [4]) R (super 3+) configurations in magnesiodumortierite due to charge deficiency at the oxygens O (sub 2) and O (sub 7) and strong local distortion of M1 due to decreased O (sub 2) -O (sub 7) bond length, and/or (ii) decreasing fluid mobility of boron in Al-rich systems at high pressures.