A natural quartz, annealed first at 800°C and then irradiated with a 3.5 MeV electron beam, has been investigated by single-crystal electron paramagnetic resonance (EPR) spectroscopy at X- and W-band frequencies from 110 K to 298 K. The W-band EPR spectra allow better separation of two previously reported radiation-induced defects (D and E) and improved determinations of their spin Hamiltonian parameters. These defects have similar g tensors with the gmax axes approximately along an O—O pair and gmin axes perpendicular to the short Si—O bonds, but different 27Al hyperfine structures. Centre E is also characterized by a 29Si hyperfine structure (A/geβe = ~0.4 mT). These spin Hamiltonian parameters, together with results from density functional theory (DFT) calculations, suggest centre E to be a new variant of peroxy radicals in quartz, whereas a peroxy radical model for centre D remains tentative. Thermal stabilities and decay kinetics of centres D and E have been investigated by use of isochronal and isothermal annealing experiments on a neutron-irradiated quartz and six smoky quartz crystals in druses from a U deposit.

The [AlO4/M+]0 (where M = H, Li, Na and K) defects in α-quartz have been investigated by ab initio calculations at the density functional theory (DFT) level, using the CRYSTAL06 code, 72-atom supercells, and all-electron basis sets. Our DFT calculations yielded substantially improved results than previous cluster calculations with minimal basis sets. For example, the [AlO4/M+(a<)]0 defects with M = H, Li and Na have been shown to be more stable than their [AlO4/M+(a>)]0 structural analogues (where a> and a< denote the location of the charge-compensating ion on the long-bond and short-bond side respectively), correctly predicting the common occurrence of paramagnetic [AlO4/M+(a>)]+ centres. In addition, the [AlO4/K+]0 defects have been investigated for the first time and are shown to be stable in quartz. Moreover, our calculations confirm previous suggestions that incorporation of the [AlO4/M+]0 defects results in significant structural relaxations that extend at least to the nearest Si atoms and give Li—O and Na—O bond distances in better agreement with the experimentally obtained values. The present theoretical results on the [AlO4/M+]0 defects provide a more complete picture for the coupled Al3+—M+ substitutions and hence new insights into crystal-chemical controls on the uptake of Al in quartz.

Crystallite growth in natural agate samples has been investigated at temperatures of 350—550°C and 100 MPa pressure in the presence of water vapour. Initial crystallite coarsening is accompanied by the transformation of moganite to α-quartz that is apparently inhibited by residual moganite when the crystallite sizes reach ~50 nm. At 350—500°C the coarsening kinetics can be described by an empirical law developed to describe Zener pinning which incorporates the maximum crystallite size prior to growth inhibition: . Co = initial crystallite size, Cs = crystallite growth after time t, Cm = the maximum size achieved before inhibition and k is the rate constant that includes the activation energy which was found to be 51(±9) kJ mole—1. A more conventional isothermal growth rate law, = kt with n = 6.5, only applies at 550°C. Limited growth was obtained when small agate cubes were heated in an open furnace up to 122 d at 550°C, demonstrating that water vapour was essential for continued crystallite coarsening. The crystallite size and moganite content of agates formed under normal earth surface conditions from hosts aged 13 Ma to 3.5 Ga have also been determined. The high temperature crystallite growth rate law does not describe natural agate growth quantitatively but a qualitatively similar pattern is observed.

Concentrations of a series of trace elements of 14 quartz samples from various geological settings (hydrothermal, igneous, metamorphic) in Norway were determined by LA-ICP-MS. FTIR spectroscopy was applied to the same quartz crystals in order to determine the H concentrations and the speciation of H in the quartz lattice. A refined hypothetical charge neutrality equation is suggested, where the atomic ratio of (Al3+ + Fe3+ + B3+) to (P5+ + H+ + Li+ + Na+ + K+) should correspond to 1 for natural quartz crystals. The determined concentrations of Al, Fe, B, P, Li, K and OH- species confirm approximately the charge neutrality equation. The high H/(Li+K+P) ion ratio of igneous quartz compared to hydrothermal and metamorphic quartz, suggests that igneous quartz preferentially incorporates H as OH- in lieu of Li, K and P. The results confirm that the FTIR absorption of the OH- band at 3595 cm-1 is attributed to structural B defects in the quartz lattice. The dominating H impurity in most of the quartz samples is, however, molecular H2O. The molecular H2O is presumably related to waterbearing micro pores and not to visible fluid inclusions, because the spectra were recorded from microscopically clear crystals.

Cathodoluminescence (CL) spectroscopy in an SEM has been used to investigate the point defect structure of clear natural α-quartz at 295 K. Cathodoluminescence processes and experimental factors that influence the CL spectra from α-quartz are investigated. Electron irradiation may induce changes in the average local crystal micro structure of quartz, locally transforming it to a less dense, amorphized state. The observed CL emissions are identified with a range of native and impurity defect centres including: interstitial molecular O at 0.968 eV; a charge-compensated substitutional Fe3+ impurity centre at —1.65 eV; a non-bridging oxygen hole centre (NBOHC) at 1.9 eV; an NBOHC with OH~ precursor at 1.95 eV; an NBOHC with a non-bridging impurity (e.g. Li+, Na+ or K+) precursor at — 1.9 eV; a radiative recombination of the self-trapped exciton (STE) associated with an E' centre in electron-irradiation-amorphized quartz at 2.3 eV; a radiative recombination of the STE associated with the E1' centre in α-quartz at 2.7 eV; a charge-compensated-substitutional Al3+ impurity centre at 3.3 eV; and a neutral relaxed O vacancy at 4.3 eV. In addition, unresolved contributions from O-deficient defects in electron-irradiation-amorphized SiO2 are likely at —2.7 and 4.3 eV.

Quartz grain surfaces from the Quaternary sand deposits of northeastern Tunisia were examined by scanning electron microscopy. Superficial textures on the quartz grain surface are commonly well preserved and are characteristic of specific formation conditions, allowing the reconstruction of the transport and sedimentation chronology and environment of individual grains.

In the northeastern coastal part of Tunisia (Cap Bon peninsula), the quartz grain exoscopy analysis involves a complex sedimentary history during the Quaternary. The Quaternary dune samples provided a mixture of sub-angular and rounded quartz morphotypes with well preserved mechanical impacts on the grain surfaces. The investigated quartz grains revealed reasonably distinct textural combinations and are characterized firstly, by mechanical features, and secondly, by chemical processes. In all the materials examined, differing environmental evolutions, marine, aeolian and pedologic were detected.

Oxygen isotope measurements using SIMS and laser-fluorination methods confirm the presence of concentric and sector zoning in low-temperature (200°C to <400°C) hydrothermal quartz from Alpine veins. While concentric zoning is most readily explained by changes in the chemical composition of the fluid or temperature of crystallization, the reasons for sector zoning are more difficult to explain. Relative enrichment in 18O for crystallographically different sectors of quartz corresponds to m >r >z. Sector zoning is, however, largely limited to the exterior zones of crystals and/or to crystals with large Al (>1000 ppm) and trace element contents, probably formed at temperatures <250°C. Differences in δ18O between the prismatic (m) relative to the rhombohedral (r and z) growth sectors of up to 2% can be explained by a combination of a face-related crystallographic and/or a growth rate control. In contrast, isotopic sector zoning of up to about 1.5% amongst the different rhombohedral faces increases in parallel with the trace element content and is likely to represent disequilibrium growth. This is indicated by non-systematic, up to 2%, differences within single growth zones and the irregular, larger or smaller, δ18O values (of several permil) of the exterior compared to the inner zones of the same crystals. Disequilibrium growth may be related to the large trace element content incorporated into the growing quartz at lower temperatures (<250°C) and/or be related to fluid-vapour separation, allowing crystal growth from both a vapour as well as a liquid phase.

Authigenic quartz overgrowths and hydrothermal quartz crystals from locations in Oman and Switzerland have been investigated with SIMS, EPMA, SEM-CL and SEM-CC. All techniques reveal similar zonation patterns with SEM-CL having the best resolution followed by SEM-CC, EPMA and finally SIMS. The observed zonations reflect chemical and/or physical changes during growth in the precipitation environment or disequilibrium precipitation at the crystal surface (i.e. sectoral and intrasectoral zonation). Based on the total Al content, two types of authigenic quartz are distinguishable. When the Al concentration is <500 μg g–1 the predominant CL emission is at ~630 nm; in such quartzes, SEM-CL and SEM-CC are directly correlated, and signal intensities drop as a function of increasing Al concentration. In contrast, authigenic quartz with Al concentrations between 500 μg g–1 and 1000 μg g–1 has CL emission maxima at both ~630 nm and ~380—400 nm, at which point the panchromatic SEM-CL and SEM-CC intensities become decoupled.

Quartz is one of the most abundant minerals in the Earth’s crust and the most important silica mineral, occurring in large amounts in igneous, metamorphic and sedimentary rocks. The mineral is widely used as a raw material in several industrial applications. Because of its chemical composition (SiO2) and its specific properties, quartz can be used both as a bulk product (e.g. quartz sands in the glass or foundry industry) and as a high-tech material (e.g. piezo or optical quartz).

Dependent on the specific conditions of either natural or synthetic formation, quartz can display typomorphic properties. Variations in crystal shape, specific micro-structure, trace element or isotope compositions, characteristic spectroscopic properties, etc. may be controlled by the genesis of the quartz involved. Accordingly, the defect structure of quartz is a fingerprint of its conditions of formation. A knowledge of the interrelation between quartz genesis and the specific properties developed at that time can be used both for the reconstruction of geological processes and for specific technical applications. Selected examples in the present study give an overview of how to analyse and use the specific information inherent in the mineral quartz.

Agates from the Bighorn district in Montana (USA), the so-called Dryhead area, and their adjacent host rocks have been examined in the present study. Analyses by XRD, polarizing microscopy, LA-ICP-MS, cathodoluminescence (CL), SEM and of oxygen isotopes were performed to obtain information surrounding the genesis of this agate type.

Investigations of the agate microstructure by polarizing microscopy and CL showed that chalcedony layers and macrocrystalline quartz crystals may have formed by crystallization from the same silica source by a process of self-organization. High defect densities and internal structures (e.g. sector zoning) of quartz indicate that crystallization went rapidly under non-equilibrium conditions. Most trace-element contents in macrocrystalline quartz are less than in chalcedony due to a process of ‘selfpurification’, which also caused the formation of Fe oxide inclusions and spherules.

Although the agates formed in sedimentary host rocks, analytical data indicate participation of hydrothermal fluids during agate formation. Trace elements (REE distribution patterns, U contents up to 70 ppm) and CL features of agate (transient blue CL), as well as associated minerals (fluorite, REE carbonates) point to the influence of hydrothermal processes on the genesis of the Dryhead agates. However, formation temperatures <120°C were calculated from O-isotope compositions between 28.9‰ (quartz) and 32.2‰ (chalcedony).

This study documents the textures and chemical evolution of igneous quartz (Qz) in granite from the Oslo Rift (Norway) during the magmatic-hydrothermal transition. Contrary to the other major igneous phases, primary quartz is well preserved. SEM-CL imaging documents four types of quartz (Qz1— Qz4). Qz1: bright primary magmatic quartz that grew under H2O-undersaturated conditions and developed a conspicuous sector zoning. Qz2: light grey luminescent secondary quartz that surrounds Qz1 and altered Qz1 in a ‘non-destructive’ process. Qz3: is usually darker than Qz2 and intersects Qz1 and Qz2. It is formed by dissolution/recrystallization processes involving saline deuteric fluids. Qz4: found in narrow cracks and patches of black quartz intersecting all the other types. EPMA in situ analyses of the different quartz generations confirm that the intensity of luminescence of quartz is positively correlated with the Ti content of the quartz. Aluminium and K are mostly incorporated in quartz in the form of [AlO4/K+]0 centre defects. In the Drammen granite, the Ti and Al contents of Qz1 averages 200 ppm and 80 ppm respectively. Titanium in Qz1 varies from 50 to 95 ppm in the peralkaline granite known as ekerite, whereas Al is irregular and ranges between 100 ppm and values below the limit of detection (LODAl at 2σ = 14 ppm). In all samples, Qz2 and Qz3 are strongly depleted in Ti and Al compared to Qz1. Either the Ti content in Qz2 is falling gradually towards Qz1 or more abruptly, whereas it is sharp from Qz3 towards Qz1 and Qz2. Potassium is variable in all four quartz types and samples, and ranges from values below the detection limit (LODk, at 2σ = 8 ppm) to 120 ppm. Grains in Qz4, being only 1–2µm wide, could not be resolved with the EPMA beam. In all granites, quartz crystallized from haplogranitic melts at P ~1.5 kbar and T = 700—750°C. SEM-CL and EPMA studies of igneous Oslo Rift quartz illustrate vividly the complex chemical and physical processes that characterize the magmatic-hydrothermal transition in shallow granitic systems and show that the chemistry of primary aqueous fluids is strongly modified from its primary igneous composition before eventually being expelled from the granitic system and perhaps incorporated in ore-forming hydrothermal convection systems.