Natural and synthetic (pure and doped) zircon (ZrSiO4) have been studied with a variety of spectroscopic techniques. These techniques are based on different physical phenomena, for instance transitions between spin states of nuclei and electrons, energetic transitions of valence electrons, intra-molecular vibrations, or vibrations of atoms and molecular units in the lattice. All of the diverse spectroscopic techniques, however, have in common that they probe energy differences between a ground and excited states, mostly upon interaction of the mineral with incident radiation. Such interactions are not only determined by the excited elementary particles or molecules themselves but depend greatly on their local environments (i.e. number, type, valence and geometrical arrangement of neighboring atoms). Spectroscopic techniques are thus sensitive to the local structure and provide information on the short-range order.

Most research on zircon crystals using spectroscopic techniques was done to study their “real structures,” that is the characterization of deviations from “perfect” zircon. Such features include the incorporation of non-formula elements, structural defects and the presence of inclusions and other impurities. Correspondingly, most of the spectroscopic investigations can be assigned to two major groups. The first group represents studies done to characterize the structural position and local environment of non-formula elements when incorporated in the zircon lattice, and accompanying effects on physical properties. The second group comprises studies subjected to the real structures of “metamict” zircon samples, i.e., changes of the zircon structure caused by the impact of self-irradiation and upon recovery from radiation damage (Ewing et al., this volume).

It is most obvious that a spectroscopic bulk or point analysis will first of all yield a spectrum (i.e. a plot of the intensity of the respective physical parameter versus wavelength, frequency or wavenumber), and this is what is used in most studies. In addition, image generation based on …

Spectroscopic methods applied to zircon

Experimentation and interpretation of cathodoluminescence (CL) microscopy and spectroscopy applied to the microcharacterization of material minerals are reviewed. The origins of the intrinsic (host lattice) and extrinsic (impurities) luminescence emissions in crystals are briefly discussed. Merits and limitations of the available techniques are illustrated. CL emission changes as a function of the incident electron dose are illustrated for the case of natural quartz and sphalerite (ZnS) crystals. These effects are discussed in terms of the development of bulk charging, production of heat, diffusion of impurities, and creation of lattice defects induced by the incident ionizing particles. Although CL emission is mostly extrinsic in origin there is no general rule for identifying the nature of impurities from the CL emission spectra of minerals. However there is potential for using CL spectroscopy for trace element analysis as presented for the case of minerals containing rare-earth luminescent ions. The CL emission is a signature of the crystal-chemistry properties of minerals and hence contains potential genetic information. Some of the applications of CL emissions in the geosciences are summarized.

Cathodoluminescence applied to the microcharacterization of mineral materials : a present status in experimentation and interpretation

Cathodoluminescence (CL) measurements made on natural zircon crystals are reported for the temperature range 45-300 K. Different combinations of excitation conditions used at various temperatures, coupled with information on the decay lifetimes of the emission features provided by modulation of the electron beam, enable identification of excited state transitions occurring within rare-earth impurities. A broad-band feature at 385 nm is noted, in common with other luminescence studies using different forms of excitation, and is attributed to an intrinsic decay process.

https://iopscience.iop.org/article/10.1088/0953-8984/4/25/015/pdf

Cathodoluminescence of natural zircons

Radiation effects of X-rays in zircon (ZrSiO4) single crystals were studied at RT, by analysing the kinetics and spectrum of the thermoluminescence between 20 degrees and 420 degrees C. The results indicate that some of the electrons produced by the irradiation are captured by Dy3+ impurities apparently substituting for Zr4+ ions, while others are trapped at Si4+ lattice ions. The hole traps include O2- lattice ions near Si3+ or Dy2+ electron centres, as well as other unidentified impurities or point defects. A TL peak at 77.5 degrees C which consists of a broad emission band around 385 nm typical of the SiO44- group is ascribed to the recombination of an electron and a hole trapped at adjacent Si4+ and O2- ions respectively. Its measured activation energy (0.89 eV) is in agreement with the theoretical value (0.87 eV). A series of TL peaks having activation energies of 0.82-1.60 eV and showing the spectral lines of Dy3+ are attributed to electron-hole recombinations at the RE site. The main TL peak at 109 degrees C (E=1.18 eV), which contains both the 385 nm band and the Dy3+ lines, might involve a recombination process which excites both the SiO44- group and the RE ion.

Electron and hole centres produced in zircon by X-irradiation at room temperature

Cathodoluminescence (CL) microscopy and spectroscopy combined with SHRIMP ion probe measurements were carried out on detrital zircons from the Cretaceous Weferlingen quartz sand (Germany) to distinguish and characterize different zircon populations. Investigations by CL microscopy, SEM-CL and BSE imaging show that there are three main types of zircons (general grain sizes of 100-200 mu m): (1) apparently weakly zoned, rounded grains with relict cores, (2) well rounded fragments of optically more or less homogeneous zircon grains showing CL zoning predominantly parallel to the z-axis, and (3) idiomorphic to slightly rounded zircon grains typically showing oscillatory euhedral CL zoning. A fourth type of low abundance is characterized by well-rounded grain fragments with an irregular internal structure showing bright yellow CL. High-resolution CL spectroscopic analyses reveal that blue CL is mainly caused by an intrinsic emission band centered near 430 nm. Dy (super 3+) is the dominant activator element in all zircons, whereas Sm (super 3+) , Tb (super 3+) , Nd (super 3+) have minor importance. Yellow CL (emission band between 500 and 700 nm) is probably caused by electron defects localized on the [SiO 4 ] groups (e.g. related to oxygen vacancies) or activation by Yb (super 2+) generated by radiation. Variations of the integral SEM-CL intensity are mainly controlled by the intensity of the broad bands and the Dy (super 3+) peaks. SHRIMP analysis provides in situ high-resolution U-Pb dating of single zircon grains and confirms different ages for the evaluated different zircon types. The measurements show that the U-Pb ages of the zircons from Weferlingen scatter over a wide range (340 to 1750 Ma), backing up earlier conclusions that the quartz sand from Weferlingen is quite heterogeneous in terms of provenance.

High resolution cathodoluminescence combined with SHRIMP ion probe measurements of detrital zircons

The 365 nm emission band observed at 100, 165, 205, 260, and 325 K in the thermoluminescence of undoped zircons, synthesized with the hydrothermal method, and which are X-irradiated at 77 K, is attributed to the SiO lattice groups. It is established that a paramagnetic trap centre is associated to most TL peaks. These traps are fairly well described by the Curie model: an electron is trapped in the field of two positive charges, one substituted to silicon, the other to a neighbouring oxygen.



La bande d'emission situee autour de 365 nm et apparaissant a 100, 165, 205, 260 et 325 K dans la thermoluminescence de zircons non dopes, synthetises par la methode hydrothermale, et ayant subi une irradiation X a 77 K, est attribuee aux groupements du reseau SiO. Il est montre qu'a un pic de thermoluminescence est en general associe un centre de piegeage paramagnetique, et que ces derniers sont assez bien representes par le modele de Curie: un electron est piege dans le champ de deux charges positives, l'une substituee a un silicium, l'autre a un oxygene voisin.

Trapping and emission centres in X-irradiated zircon. II. Contribution of the SiO44− groups

After X irradiation at 77 K, hydroxylated zircons show thermoluminescence (TL) peaks at 115 and 160 K, with an intense emission band around 285 nm. By correlating EPR and TL observations on hydroxylated zircons, it is shown that X-irradiation can dissociate OH− groups into O− and H0, and suggested that dissociation into OH and e− also occurs. It is proposed that recombination of O− and H0 at 115 K, and of OH and e− at 160 K, lead to the same excited state (OH−)*, which explains the similar 285 nm emission for both TL peaks.



Les courbes de thermoluminescence (TL) obtenues, apres irradiation X a 77 K, sur des zircons hydroxyles, revelent la presence de pics a 115 et 160 K, dont les spectres d'emission comportent une intense bande d'emission maximale a 285 nm. La confrontation des observations en RPE et TL montre que l'irradiation X peut decomposer le groupement OH− en O− et H0, et suggere qu'une dissociation en OH et e− peut aussi exister. Nous montrons que la recombinaison de O− et H0 a 115 K, et de OH et e− a 160 K, conduit a un měme etat excite (OH−)*, dont la desexcitation explique l'emission a 285 nm.

A. Deville

Papers

Trapping and emission centres in X-irradiated zircon. II. Contribution of the SiO44− groups

Trapping and emission centres in X-irradiated zircon. I. Contribution of the OH- ion