Hydrogen speciation in synthetic quartz

Abstract

The dominant hydrogen impurity in synthetic quartz is molecular H_2O. 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 H_2O concentrations in the synthetic quartz crystals 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 H_2O is the dominant initial hydrogen impurity in weak quartz. The hydrolytic weakening of quartz may be caused by the transformation H_2O + Si-O-Si → 2SiOH, but this may be a transitory change with the SiOH groups recombining to form H_2O, and the average SiOH concentration remaining very low. Synthetic quartz is strengthened when the H_2O is accumulated into fluid inclusions and cannot react with the quartz framework.

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