@article{WunderKutzschbachHosseetal.2018,
  author    = {Wunder, Bernd and Kutzschbach, Martin and Hosse, Luisa and Wilke, Franziska Daniela Helena and Schertl, Hans-Peter and Chopin, Christian},
  title     = {Synthetic B-[4]-bearing dumortierite and natural B-[4]-free magnesiodumortierite from the Dora-Maira Massif},
  series = {European journal of mineralogy},
  volume    = {30},
  journal   = {European journal of mineralogy},
  number    = {3},
  publisher = {Schweizerbart},
  address   = {Stuttgart},
  issn      = {0935-1221},
  doi       = {10.1127/ejm/2018/0030-2742},
  pages     = {471 -- 483},
  year      = {2018},
  abstract  = {Dumortierite was synthesized in piston-cylinder experiments at 2.5-4.0 GPa, 650-700 degrees C in the Al2O3 -B2O3-SiO2-H2O (ABSH) system. Electron-microprobe (EMP) analyses reveal significant boron-excess (up to 0.26 B-[4] 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 Si-[4] <--> B-[4] + 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 center dot center dot center dot O7 and O7-H center dot center dot center dot O2, arising from a high number of octahedral vacancies at the All 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 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 Mg-[6]-(OR3+)-R-[4] 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.},
  language  = {en}
}
@article{SchertlO'Brien2013,
  author    = {Schertl, Hans-Peter and O'Brien, Patrick},
  title     = {Continental crust at mantle depths - key minerals and microstructures},
  series = {Elements : an international magazine of mineralogy, geochemistry, and petrology},
  volume    = {9},
  journal   = {Elements : an international magazine of mineralogy, geochemistry, and petrology},
  number    = {4},
  publisher = {Mineralogical Society of America},
  address   = {Chantilly},
  issn      = {1811-5209},
  doi       = {10.2113/gselements.9.4.261},
  pages     = {261 -- 266},
  year      = {2013},
  abstract  = {Finding evidence for ultrahigh-pressure (UHP) metamorphism in crustal rocks is far from straightforward. The index minerals coesite and diamond are incredibly inconspicuous and are therefore difficult to use as UHP prospecting tools. Consequently, petrographers rely on recognizing subtle breakdown microstructures that result from pressure release during the return to the surface of the once deeply buried rock. Similarly, many other UHP minerals are first suspected on the basis of typical reaction or exsolution microstructures. Thus, the painstaking use of microscopic techniques has been fundamental to the tremendous advances in characterizing, quantifying, and understanding macroscopic-scale, deep continental subduction, rapid exhumation, and mountain-building processes.},
  language  = {en}
}