OH-point defects in quartz in B- and Li-bearing systems and their application to pegmatites

Abstract

OH incorporation in quartz in Al-, B- and Li-bearing systems (granitic systems containing tourmaline or spodumene) was studied experimentally in order to investigate the effect of pressure, temperature and chemical impurities on the generation of OH-defects. High-pressure experiments were carried out at pressures between 5 and 25 kbar and temperatures between 800 and 900 °C, and OH-contents in quartz were calculated from IR absorption spectra measured on oriented quartz crystals. IR absorption features were assigned to impurity substitutions, such as AlOH (3,420, 3,379 and 3,315 cm⁻¹) and BOH (3,595 cm⁻¹), LiOH (3,483 cm⁻¹), and hydrogarnet substitution (4H)_Si defects (3,583 cm⁻¹). Results indicate a negative correlation of incorporated Al-specific OH-defect content versus pressure (630 ± 130 wt ppm H₂O at 5 kbar to 102 ± 6 wt ppm H₂O at 25 kbar), but no clear correlation of B-specific OH-defects with pressure. In runs initially containing spodumene, virtually OH-free quartzes were observed at pressures ≥10 kbar, where impurity cations compensate each other forming an anhydrous eucryptite-defect component. In contrast, at 5 kbar, both Li- ad Al-specific OH-defects are observed (corresponding to 470 ± 75 wt ppm H₂O). Results from this study may therefore be used to monitor formation conditions of quartz in terms of pressure and trace metal saturation of the crystallizing petrological system. IR spectra obtained from natural quartz grains from a tourmaline-bearing pegmatite exhibit B- and Al-related OH-bands. The B-related OH-band is also exhibited in quartz from a tourmaline + spodumene-bearing pegmatite. Li- and Al-related OH-bands, however, are subordinate or not observed at all in the spodumene-bearing system, which suggests that OH-vibrations do not reflect absolute Li-contents in quartz due to efficient coupled substitution involving Al. Data from experimental runs and natural specimens indicate that the B-related OH-band can be used as a rough proxy for B-contents in quartz, confirming the previously postulated charge balance equation [H⁺] = [B³⁺] + [Al³⁺] − [Li⁺] − [K⁺] − [P⁵⁺].