OH defect contents in quartz in a granitic system at 1–5 kbar

Abstract

Quartz is able to incorporate trace elements (e.g., H, Li, Al, B) depending on the formation conditions (P, T, and chemical system). Consequently, quartz can be used as a tracer for petrogenetic information of silicic plutonic bodies. In this experimental study, we provide the first data set on the OH defect incorporation in quartz from granites over a pressure/temperature range realistic for the emplacement of granitic melts in the upper crust. Piston cylinder and internally heated pressure vessel synthesis experiments were performed in a water-saturated granitic system at 1–5 kbar and 700–950 °C. Crystals from successful runs were analysed by secondary ion mass spectrometry (SIMS) and Fourier transform infrared (FTIR) spectroscopy, and their homogeneity was verified by FTIR imaging. IR absorption bands can be assigned to specific OH defects and analysed qualitatively and quantitatively and reveal that (1) the AlOH band triplet at 3310, 3378 and 3430 cm−1 is the dominating absorption feature in all spectra, (2) no simple trend of total OH defect incorporation with pressure can be observed, (3) the LiOH defect band at 3470–3480 cm−1 increases strongly in a narrow pressure interval from 4 kbar (220 µg/g H2O) to 4.5 kbar (500 µg/g H2O), and declines equally strong towards 5 kbar (180 µg/g H2O). Proton incorporation is charge balanced according to the equation H+ + A+ + P5+ = M3+ + B3+, with A+ = alkali ions and M3+ = trivalent metal ions.

Structure and theoretical infrared spectra of OH defects in quartz

“…1) is indicated by the dotted line. c = 5.41 Å; Levien et al, 1980), which is consistent with previous theoretical determinations at the DFT-GGA level (Hamann, 1996;Rosa et al, 2005;Méheut et al, 2007).…”

“…The second class of OH-bearing defect that could occur in the quartz-water system results from the hydrolysis of a Si-O-Si bridge and dissociation of a water molecule forming two silanol (Si-OH) groups, i.e., generically Si-O-Si + H 2 O = Si-OH + HO-Si (Brunner et al, 1961;Griggs and Blacic, 1965). The optimization of the "Griggs defect" leads to a structure almost identical to that previously determined by Rosa et al (2005), with an asymmetric configuration involving the formation of two fivefold coordinated Si atoms and two OH bonds with different lengths (Griggs_1 model; Table 1; Fig. 5a).…”

“…4; Table 1), the shortest OH group points outside the tetrahedral site formed by the four oxygens, whereas the three other OH groups point along the edges of the tetrahedron. A closed triangular configuration of these three OH groups, as determined by Rosa et al (2005), has not been found to be stable. The (4H) ×…”

“…The lowest defect concentration of periodic models is limited by the cell size that can be treated within an acceptable computational time. For OH defects in quartz, Rosa et al (2005) found that a cell containing 36 atoms was large enough to minimize defectdefect interactions. The present use of a 2×2×2 supercell is thus expected to provide an accurate description of the local structure and vibrational properties of OH defects.…”

“…Despite experimental constraints, the precise configuration of the OH-bearing structural defects in quartz remains elusive. Previous theoretical studies using quantum-mechanical modeling approaches have proposed stable models of OHbearing defects in quartz (e.g., Lin et al, 1994;McConnell et al, 1995;Rosa et al, 2005), mostly focusing on their thermodynamic stability. Quantum-mechanical modeling tools can also be used to investigate the vibrational spectroscopic properties of models of OH defects that can be tested against experimental observations.…”