Roger D. Aines
Division of Geological and Planetary Sciences, 170-25,
California Institute of Technology,
Pasadena, California 91125, U.S.A.
Stephen H. Kirby
U.S. Geological Survey
Menlo Park, California 94025, U.S.A.
George R. Rossman
Division of Geological and Planetary Sciences, 170-25,
California Institute of Technology,
Pasadena, California 91125, U.S.A.
The dominant hydrogen impurity in synthetic quartz is molecular H2O.
H-OH groups also occur, but there is no direct evidence for the
hydrolysis of Si-O-Si bonds to yield Si-OH HO-Si groups. Molecular H2O
concentrations in the synthetic quartz crtystals studied range from
less than 10 to 3,300 ppm (H/Si), and decrease smoothly by up to an
order of magnitude with distance away from the seed. OH-
concentrations range from 96 to 715 ppm, and rise smoothly with
distance away from the seed by up to a factor of three. The observed OH-
is probably all associated with cationic impurities, as in natural
quartz. Molecular H2O is the dominant initial hydrogen
impurity in weak quartz. The hydrolytic weakening of quartz may be
caused by the transformation H2O + Si-O-Si -> 2SiOH, but
this may be a transitory change with the SiOH groups recombining to
form H2O, and the average SiOH concentration remaining very
low. Synthetic quartz is strengthened when the H2O is
accumulated into fluid inclusions and cannot react with the quartz
framework.