Abstract
Abstract. Research results of the past 6 decades on the incorporation of OH point defects in quartz are summarised and evaluated in terms of their application to natural samples and processes, and a link between experimental petrology, natural archives, and model calculations is made. A strong focus is put on recent studies on quartz as a rock-forming mineral, as a geochemical and/or petrological tracer, and as a tool for provenance analysis in sediments and sedimentary rocks. The most relevant defects for natural specimens are generated by coupled substitution involving mono- and trivalent cations, the most prominent being Li+, Al3+, and B3+. OH incorporation is rather a function of the availability of trace metals and water than of pressure and temperature, though temperature indirectly influences the incorporation by the solubility of trace metals in the fluid. Pressure has a negative influence on the formation of OH defects, so the most pure quartzes are probably formed in the deep crust close to the quartz/coesite transition. Natural quartz grains from the Earth's crust have on average 10 wt ppm (weight parts per million) water (5 wt ppm median), but grains with OH defect contents corresponding to up 250 wt ppm water have been discovered in sedimentary archives, matching the concentration of quartz from high-pressure experiments <4 kbar under water-saturated conditions in granitic systems. A rough division into three classes is suggested: (1) grains with pristine igneous and/or hydrothermal origin, (2) mildly thermally annealed grains, and (3) strongly dehydrated grains. While samples derived from the currently exposed Scandinavian Shield are dominated by the third class, considerable contributions of the first two classes are found in the younger rock systems in Central Europe. OH defect contents may be used to estimate mixing ratios for sediments with different sources, provided that a sufficiently large data set exists and that the different sources can be clearly distinguished by their OH inventory. Furthermore, metamorphic overprint leads to a higher degree of equilibration of OH defects between individual grains and may thus be used as a geothermometer. Finally, OH defect retention in quartz allows for estimating timescales of volcanic processes.
Highlights
Quartz is formed under most conditions realised within the Earth’s crust, ranging from the Earth’s surface down to the base of the continents
This review summarises early findings on “water” in quartz and its influence on physical properties, followed by a review of theoretical and experimental considerations on OH point defects, and reports recent data on OH point defects in common quartz as a rock-forming mineral from igneous rocks, metamorphic rocks, and sedimentary archives
Since molecular water gives rise to broad isotropic absorption bands and hydrous defects are nearly perfectly polarised ||no (Brunner et al, 1961), a distinction between both fundamentally different groups of species was made early by only taking into account the sharp polarised absorption bands (Bambauer et al, 1962) or by the separate quantification of point defects and molecular water (Müller and Koch-Müller, 2009; Biró et al, 2016). Another strategy to overcome the contribution of non-defect OH is a revised protocol for the treatment of IR absorption spectra (Stalder and Konzett, 2012), in which all measurements are performed on oriented sections ||c for both crystallographic directions (E||no and E||ne) on the same spot
Summary
Quartz is formed under most conditions realised within the Earth’s crust, ranging from the Earth’s surface down to the base of the continents. Depending on crystallisationrelevant physical and chemical parameters (pressure, stress, temperature, temperature gradients, time, presence and composition of melts and fluids) and geological circumstances (cracks and voids in the surrounding rock), the resulting habit and size of quartz crystals is very diverse, ranging from microcrystalline fibrous chalcedony to decimetre- and metresized euhedral crystals. Both crystal size extremes are not representative of the main portion of quartz as a rockforming mineral (12 vol % of the Earth’s crust; Ronov and Yaroshevski, 1969), they are very prominent in our mind and important to human history because they were used since prehistoric times as precious stones (such as agate, amethyst, and rock crystal) or for tools and weapons (such as flint and rock crystal). As will be outlined in this study, molecular water follows different incorporation laws than OH point defects, and, the wealth of studies on fluid inclusions in quartz, as well as on OH species (molecular water and OH point defects) in microcrystalline and/or amorphous silica such as opal and chalcedony (Langer and Flörke, 1974; Frondel, 1982; Adams et al, 1991; Chauviré et al, 2017) and moganite (Flörke et al, 1984; Heaney and Post, 1992; Hardgrove and Rogers, 2013), are not considered in this review
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Free) 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
