Czochralski growth of single-crystal gehlenite (Ca 2Al 2SiO 7)

Single-crystal boules of gehlenite (Ca 2Al 2SiO 7) were grown from even- and off-stoichiometry melts under differing conditions of oxygen fugacity (ƒ O 2). Growth was accomplished by the Czochralski method at 1600°C and 10 5 Pa (1 atm) total pressure, using inductively heated Ir or Pt-20%Rh crucibles. The supra melt gas ambients included air ( ƒ O 2 = 0.2 × 10 4 Pa ), Ar (10 Pa), Ar-50%CO 2-2%H 2 (10 -2 Pa), and Ar-4%H 2-1.5%H 2O (10 -4 Pa). Colorless, transparent material up to 8 mm diam. by 15 mm long was obtained from the evenly stoichiometric melt composition under Ar at growth rates of 1.5 mm/h or less. Growth at rates exceeding 2 mm/h or growth at a high ƒ O 2 (e.g., air), led to the formation of bubbles and elongated voids or inclusions, predominantly in the core regions of boules. Optical, X-ray diffraction, and electron-induced X-ray flourescence data are included.