Hydroxyl defect concentrations of quartz phenocryst fragments from various rhyolitic pyroclastic density current deposits from the Bükk Foreland Volcanic Area, Hungary, were determined by using micro-FTIR spectrometry. In addition trace-element analysis and SEM cathodoluminescence imaging were performed on the same crystals. Hydroxyl defect-content (expressed in water equivalent) of volcanic quartz ranges from 0.9 ± 0.1 to 2.8 ± 0.4 wt. ppm, which is lower than those in quartz of plutonic (granitic), metamorphic and hydrothermal origin. The incorporation of hydroxyl defect is mainly due to H+ + Al3+ substitutions into Si-tetrahedral vacancies of quartz. Furthermore, the presence of molecular water probably in (nano-)inclusions was proven. The post-eruptive diffusive loss of hydroxyl defects during cooling seems to be the main factor causing the very low concentration of hydroxyl defects. This may be also manifested in the almost homogeneous distribution of hydroxyl defects regardless of the considerable zonation in Al-content, although complete diffusive loss of hydroxyl defects was possibly hindered by the cooling effect of phreatomagmatism (interaction of excess water with magma during eruption) and deposition in a shallow submarine environment.
Moreover, a reasonable linear correlation is observed between the integrated area of Si–O bands (between 2110 and 1440 cm−1) and sample thickness up to ~300 μm by studying unoriented quartz phenocryst fragments. This may enable the quantitative analysis of hydroxyl defects in separated quartz crystals without the need for preparing oriented thin sections.