OH point defects in quartz – a review

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.

Trace metal concentrations and OH defects in quartz from Amazon River sands & and perspectives for application to the marine record

&lt;p&gt;Trace metal concentrations and associated hydrous lattice point defects (OH defects) in quartz can help reveal its host rock&amp;#8217;s crystallization history and are easily quantified using electron microprobe and infrared spectroscopy, respectively. These chemical impurities are preserved throughout the sedimentary cycle and thus lend themselves as tracers for sediment provenance analyses, particularly in settings where &amp;#8220;traditional&amp;#8221; provenance tools, e.g., thermochronology and heavy mineral analysis, are difficult due to factors like low mineral fertility and aggressive tropical weathering.&lt;/p&gt;&lt;p&gt;In this study, we apply this provenance analysis tool to detrital, sand-sized quartz grains from the Amazon River and its major tributaries, draining the Andean orogen as well as the Guiana- and Central Brazil Shields. Trace metal and OH defect concentrations from individual catchments are spread out over wide and mutually overlapping ranges of values. This means that each individual quartz grain cannot be unequivocally attributed to one catchment. However, evaluation of a statistically sound number of grains reveals that Andean quartz is, on average, richer in the trace metal aluminum (and Al-related OH defects) than quartz derived from one of the shield sources.&lt;/p&gt;&lt;p&gt;We evaluate our findings in the context of previous provenance studies on Amazon River sediments and discuss a potential future application of analyzing trace metals and OH defects in quartz in the offshore sediment record. Any past, major rearrangements in the Amazon watershed affecting the ratio of Andean vs. Shield-derived quartz grains should be detectable and our approach may therefore contribute to the reconstruction of Amazon drainage basin evolution.&lt;/p&gt;

Eclogite xenoliths document water cycling at the lithosphere-asthenosphere boundary

&lt;p&gt;The amount of water stored as OH-defects in nominally anhydrous minerals in the deep mantle is poorly constrained and its direct quantification can only be accessed by the analysis of mantle xenoliths. While the vast majority of xenoliths are peridotites and minor pyroxenites, some very rare xenoliths found in kimberlite pipes display an eclogitic mineral assemblage. We investigated three eclogite xenoliths from the 128 m.y. old Robert Victor kimberlite from South Africa that display an assemblage of garnet and omphacite with two samples showing additional kyanite, suggesting low-pressure gabbroic rock as protolith. Thermobarometry estimations based on Fe-Mg partitioning between garnet and pyroxene gives temperatures of 1100-1250 &amp;#176;C. When projected on the cratonic geotherm (Griffin &amp; O&amp;#8217;Reilly 2007) an equilibrium depth of 200-210 km is obtained, confirming that these rocks come from the lithosphere-asthenosphere boundary. Therefore these fragments might be key witnesses to understand the deep cycling of water in the mantle. &lt;br&gt;&lt;br&gt;This study focuses on the H&lt;sub&gt;2&lt;/sub&gt;O quantification in the three rock-forming minerals using Fourier transform infrared spectroscopy (FTIR). Omphacite contains 50-250 ppm H&lt;sub&gt;2&lt;/sub&gt;O, kyanite contains 40-60 ppm H&lt;sub&gt;2&lt;/sub&gt;O and garnet of only one eclogite contains 40 ppm H&lt;sub&gt;2&lt;/sub&gt;O. Garnet and omphacite with the highest OH content are enriched in Ca.&lt;br&gt;&lt;br&gt;&lt;/p&gt;&lt;p&gt;The use of advanced mapping and profiling techniques enabled the investigation of the spatial repartition of the OH component in these minerals. High-resolution mapping (5.6 &amp;#181;m) of kyanite reveals diffusive gain of OH at the rim of the crystal that is interpreted as hydration during interaction with the kimberlitic melt. The OH plateau in the core of kyanite must therefore have been acquired previously, suggesting that this is residual OH that has been transported by subduction to the lithosphere-asthenosphere boundary by a once hydrated gabbroic protolith. Our results have implications for the retention of hydrogen over long timescale at the lithosphere-asthenosphere boundary and suggest that the deep cycling of water has been running since Archean times.&lt;/p&gt;&lt;p&gt;&amp;#160;&lt;/p&gt;&lt;p&gt;Griffin, W. L., &amp; O'Reilly, S. Y. (2007). Cratonic lithospheric mantle: is anything subducted?. &lt;em&gt;Episodes,&lt;/em&gt; 30(1), 43-53.&lt;/p&gt;

Theoretical infrared spectra of OH defects in corundum (<i>α</i>-Al₂O₃)

The atomic-scale structure, relative stability and infrared spectroscopic properties of OH defects in corundum (α-Al2O3) are theoretically investigated at the density functional theory level. Comparison with experimental data makes it possible to assign most of the narrow bands observed between 3150 and 3400 cm−1 in natural and Ti- or V-doped synthetic corundum to specific defects. These defects correspond to the association of one OH group with an Al vacancy and M4+ for Al3+ substitutions in neighboring sites. The OH group is located in the large oxygen triangle forming the base of the vacant Al site. Models of interstitial proton associated with a nearby Mg2+ for Al3+ substitution are consistent with the broad band observed at 3010 cm−1 in Mg-doped corundum. Its is also suggested that two weaker OH-stretching bands observed in nominally pure synthetic corundum at 3163 and 3209 cm−1 could be associated with intrinsic defects combining an Al and an O vacancy. These results highlight the importance of defect clustering in the high-temperature incorporation of hydrogen in nominally anhydrous minerals.

Structure and theoretical infrared spectra of OH defects in quartz

The infrared spectra of natural quartz, and synthetic quartz produced in conditions relevant to natural environments, generally contain some association of OH-stretching absorption bands at 3596, 3585, 3483, 3431, 3379 and 3313 cm−1, and/or a broad band at ∼ 3400 cm−1. In this study, a series of OH-bearing defects has been theoretically investigated from first principles within the density functional theory framework. The optimized structure, infrared spectroscopic properties and relative energy of defect configurations have been determined. Comparison with experimental observations enables the identification of atomic-scale configurations related to the experimentally observed OH-stretching bands. Consistent with previous interpretations, the results confirm the assignment of the bands at 3596 and 3483 cm−1 to OH defects associated with B3+ substituting for Si4+ and to OH defects associated with Li+ cations located in the structural channels, respectively. They also confirm the assignment of the bands at 3313 and 3379 cm−1 to OH associated with the Al3+-for-Si4+ substitution and, by implication, the previously given interpretation of the 3431 cm−1 band in terms of Fermi resonance. The band at 3585 cm−1 does not appear to be related to a hydrogarnet-type defect, as has been proposed previously, but potentially corresponds to isolated OH− groups bridging two Si atoms, where the charge compensation is ensured by a nonlocal mechanism.