Minerals and Materials
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Microsc. Microanal. 14, 184–204, 2008 / doi: 10.1017 S143192760808029X Microscopy AND Microanalysis © MICROSCOPY SOCIETY OF AMERICA 2008 Luminescence Database I—Minerals and Materials Colin M. MacRae* and Nicholas C. Wilson CSIRO Minerals, Microbeam Laboratory, Bayview Avenue, Clayton, Victoria 3168, Australia Abstract: A luminescence database for minerals and materials has been complied from the literature, the aim being to create a resource that will aid in the analysis of luminescence spectral of ionic species in minerals and materials. The database is based on a range of excitation techniques and records both major and minor lines, and their activators. The luminescence techniques included in the database are cathodoluminescence, ion luminescence, and photoluminescence. When combined with other traditional X-ray measurements collected on the same region, use of the luminescence database will give additional insight into the chemistry of minerals and materials. Key words: luminescence, cathodoluminescence, photoluminescence, ion, minerals INTRODUCTION cite, dolomite, magnesite, witherite!, or silicates ~albite, feldspar, quartz, zeolites, kaolinite, forsterite, zircon, garnet, titanite, thorite, willemite!. The presence of luminescence, Minerals and materials can luminescence when they are in many cases, allows rapid identification of the different exposed to an electron, X-ray, ion, or photon beam. Lumi- mineral constituents using cathodoluminescence micros- nescence is generally associated with light in the ultraviolet copy. This is particularly important if samples consist of / ~UV! to infrared ~IR! region and can exhibit both broad fine-grained material and or of minerals with similar opti- and narrow band spectra. From the spectra it is possible to cal or crystallographic properties. These grains can then be identify both the activators responsible for the lumines- further characterized by electron microprobe or optical cence and their charge states. A large number of research microscopy. groups routinely employ luminescence analysis as a key Furthermore, many of the phases occurring in ceramics macro- and micro-characterization techniques in the study ~Hagni & Karakus, 1989!, glasses, refractory materials ~Kara- of minerals and materials. kus, 2005!, and biomaterials show distinct luminescence For many years the microanalyst has had available properties allowing a rapid identification of phase distribu- KLM lines for identifying peaks in X-ray spectra; however, tion and transformations. Luminescence spectroscopy is no such tool has been available for luminescence generated particularly important in the characterization of materials by electrons, light, protons, or ions. To address this problem that contain significant proportions of noncrystalline com- a luminescence database of lines has been compiled that ponents, multiple phases, or low concentrations of mineral contains over 1,000 lines or bands from over 70 minerals phases. and synthetic materials. In this article the luminescence database is described, and in a subsequent article software tools and web access will be described. It is the authors’ LUMINESCENCE FUNDAMENTALS intention to make the database easily accessible and provide a procedure for external users to add new lines and spectra Characteristic X-ray lines result from core level transitions, from minerals and materials. while the generation mechanism for luminescence is more A number of minerals have distinguishing lumines- complex. Characteristic X-rays are largely unaffected by cence properties. These include: diamond, sulphides ~chalco- bonding as the core orbitals do not take part and therefore a cite, sphalerite!, oxides ~periclase, corundum, cassiterite!, particular elemental transition is independent of the host halides ~fluorite, halite!, sulphates ~anhydrite, alunite!, wol- lattice. However, the luminescence emission is sensitive to framates ~scheelite!, phosphates ~apatite!, carbonates ~cal- material composition and structure of the host lattice, be- cause it originates from effects such as conduction to va- Received November 27, 2007; Accepted January 2, 2008 lence transitions and phonon modes ~Marfunin, 1979!. This *Corresponding author. E-mail: [email protected] makes luminescence sensitive to subtle effects such as trace Luminescence Database 185 level dopants and their valence, the host lattice, and quench- of luminescent minerals and synthetic materials ~Pagel et al., ing ions. This sensitivity results in luminescence providing 2000; MacRae & Miller, 2003!. extremely useful characterization information, but its inter- pretation is more difficult than that of characteristic X-rays. Generation of Luminescence Cathodoluminescence can be observed with a variety of Dopant Ions electron beam instruments ~Petrov, 1996!. One simple type Minerals and materials often contain optically active dop- of instrument is the electron beam flood gun that mounts ants ions. Generally there are considered to be three types of directly onto the optical stage of a petrographic microscope dopant ions that influence and determine the net emission ~Marshall, 1988!. Another approach is to mount a spectrom- of a particular mineral. They are referred to as activators, eter onto a scanning electron microscope. This typically sensitizers, or quenchers. Activators produce emission by collects light with a mirror that is usually on a retractable releasing the absorbed energy as photons. The most com- arm, and the light is measured using a grating-type spec- mon activators are transition metal ions such as Cr3ϩ, trometer ~Katona et al., 2004; Vernon-Parry et al., 2005!.A Mn2ϩ,Mn4ϩ,Sn2ϩ,Pb2ϩ,Fe3ϩ ~Gotze, 2002!, and rare earth more recent approach is to integrate cathodoluminescence elements ~Marfunin, 1979!. Sensitizers are ions that work in capture into an electron probe microanalyzer using the combination with an activator by absorbing the energy and existing collection optics and to employ an optic fiber subsequently transferring the energy to the activator. Quench- coupled to a CCD spectrometer to measure the lumines- ers trap part or all of the absorbed energy resulting in cence ~MacRae et al., 2001, 2005; Edwards et al., 2003!.All nonradiative decay of the energy. As a result, quenchers these collection systems measure cathodoluminescence with tend to eliminate the emission of light from minerals. The differing lateral and spectroscopic resolutions. They each presence of the quencher causes new closely spaced energy have their own benefits and virtues. levels to be set up, and the electron can easily return to the Photoluminescence spectra can be induced by mono- ground state with the emission of a succession of low- chromatic light in the band of the ion absorption or by energy photons ~IR! or by losing energy to the lattice as heat means of UV radiation with a spectrum necessary to excite ~Marshall, 1988!. An example of a well-recognized quencher the ion absorption bands. Ultraviolet radiation is necessary ion in minerals is Fe2ϩ. to excite the ion up to energy levels lying above the emission level, which is usually in the visible to near IR region Types of Emission ~Marfunin, 1979!. Luminescence emission is generally grouped into two types: Other Effects intrinsic and extrinsic. Intrinsic luminescence is native to host materials and involves band-to-band recombination of Luminescence lifetime or decay time can provide informa- electron and hole pairs. Intrinsic luminescence emission tion on the nature of the center and is a measure of the may also be associated with lattice defects ~anion vacancies! transition probability from the emitting state. The transi- within the minerals or material. This type of luminescence tion probability is unique for each center and therefore can is also referred to as “defect center” luminescence. The be used to differentiate centers. Time-resolved luminescence second type of emission, referred to as extrinsic, is the most spectra have been studied using pulsed or chopped electron common form of luminescence. Extrinsic emission is attrib- or laser-induced excitation and lock in amplifiers coupled uted to the presence of trace element impurities, transition to spectrometers to measure the decay spectra ~Gorton metal, and rare earth ions. This type of luminescence is et al., 1997; Pagel et al., 2000; Edwards et al., 2003; Merano referred to as an “impurity center.” et al., 2005!. The emission process involves an electronic transition Polarized luminescence has been observed with cathod- from an excited state ~Ee! to a lower energy level or ground oluminescence, and this can enhance differentiation of state ~Eg!. When an electron beam interacts with a surface minerals and materials and offers information about site and produces light, this process is known as cathodolumi- symmetries of the luminescence centers ~Gorz et al., 1970; nescence ~CL!. Similarly, an ion beam interacting with a Chandrasekhar & White, 1992!. The database does not surface and producing light is often referred to as ion indicate whether minerals or materials have polarized emis- luminescence ~IL!. Emission occurring when a photon beam sion. Only a few studies have considered this effect; for interacts with a surface is referred to as photoluminescence example, information on polarization has been studied in ~PL!, and for X-ray beams the process is referred to as silicates ~Bhalla & White, 1970, 1972; Gorz et al., 1970; roentgenoluminescence. Cathodo- and photo-luminescence Chandrasekhar & White, 1992;