In present day research on the chemical composition of minerals (both major elements and trace elements) a number of different analytical techniques are available. These include techniques such as electron microscopy (both scanning and transmission as well as microprobe) with energy-dispersive or wavelength-dispersive X-ray spectrometry, XRF spec- trometry, X-ray photoelectron spectroscopy (XPS), inductively coupled plasma atomic emission spectroscopy (ICP- AES), also known as inductively coupled plasma-optical emission spectroscopy (ICP-OES) and inductively coupled plasma-mass spectrometry (ICP-MS).
2.2.1 Electron microscopy
An electron microscope is a microscope that utilizes a beam of accelerated electrons as a source of illumination. Since the wavelength of an electron can be up to 100,000 times shorter than that of visible light photons, electron microscopes have a higher resolving power than light microscopes and can reveal the structure of much smaller objects. For exam- ple, a scanning transmission electron microscope (TEM) has attained better than 50 pm resolution in annular dark-field imaging mode and magnifications of up to about 10,000,0003, while most light microscopes are limited by diffraction to about 200 nm resolution and useful magnifications below 20003. Electron microscopes can be applied to study the ultrastructure of a wide range of biological and inorganic specimens including microorganisms, cells, large molecules, biopsy samples, metals, and crystals. Industrially, electron microscopes are often used for quality control and failure analysis. Modern electron microscopes produce electron micrographs using specialized digital cameras and frame grab- bers to capture the images.
2.2.1.1 Transmission electron microscope
The original form of the electron microscope, the TEM, applies a high-voltage electron beam to illuminate the specimen and create an image. The electron beam is produced by an electron gun, normally containing a tungsten filament cath- ode as the electron source. The electron beam is accelerated by an anode typically at 1100 keV (40400 keV) with respect to the cathode, focused by electrostatic and electromagnetic lenses, and transmitted through the specimen which is in part transparent to electrons and in part scatters them out of the beam. After it appears from the specimen, the elec- tron beam carries information about the structure of the specimen that is magnified by the objective lens system of the microscope. The spatial variation in this information (the “image”) can be viewed directly by projecting the magnified electron image onto a fluorescent viewing screen coated with a phosphor or scintillator material such as zinc sulfide.
On the other hand, the image can be photographically recorded by exposing a photographic film or plate directly to the electron beam, or a high-resolution phosphor may be coupled by means of a lens optical system or a fiber optic light guide to the sensor of a digital camera. The image detected by the digital camera may be displayed on a monitor or computer.
The resolution of TEMs is limited principally by spherical aberration, but a new generation of hardware correctors can decrease spherical aberration to increase the resolution in high-resolution transmission electron microscopy (HRTEM) to below 0.5 A˚ (50 pm), permitting magnifications above 50 million times. The ability of HRTEM to deter- mine the positions of atoms within materials is useful for nanotechnologies research and development. TEMs are fre- quently used in electron diffraction mode. The advantages of electron diffraction over X-ray diffraction are that the specimen does not have to be a single crystal or even a polycrystalline powder, and also that the Fourier transform reconstruction of the object’s magnified structure occurs physically and therefore circumvents the need for solving the phase problem faced by the X-ray crystallographers after obtaining their X-ray diffraction patterns.
The most important disadvantage of the TEM is the requirement of extremely thin sections of the specimens, typi- cally about 100 nm. Producing these thin sections for biological and materials specimens can technically be very chal- lenging. Solid material, such as those of rocks and minerals, thin sections can be produced by applying a focused ion beam.
2.2.1.2 Scanning electron microscope
The scanning electron microscope (SEM) produces images by probing the surface of a specimen with a focused electron beam that is scanned across a rectangular area of the specimen (raster scanning). When the electron beam interacts with the specimen, it loses energy by several different mechanisms. The lost energy is transformed into alternative forms such as heat, emission of low-energy secondary electrons and high-energy backscattered electrons, light emission
(cathodoluminescence), or X-ray emission, all of which create signals providing information about the properties of the specimen surface, such as its topography and composition. The SEM image displays in a sense a map of the varying intensity of any of these signals into the image in a position corresponding to the position of the beam on the specimen when the signal was generated.
Generally, the image resolution of an SEM is lower than that of a TEM. Nevertheless, because the SEM images the surface of a sample rather than its interior, the electrons do not have to travel through the sample. This reduces the need for extensive sample preparation to thin the specimen to electron transparency. The SEM can image bulk samples lim- ited in size only by the size of its stage and still be moved, including a height less than the working distance being used, often 4 mm for high-resolution images. The SEM also has a great depth of field and so can produce images that are good representations of the three-dimensional surface shape of the sample, for example, crystals of minerals.
Another advantage of SEMs comes with environmental SEMs that can produce images of good quality and resolution with hydrated samples or in low, rather than high, vacuum or under chamber gases.
2.2.1.3 Sample preparation
Embedding materials (for electron microprobe analysis)—after embedding in resin, the specimen is usually ground and polished to a mirror-like finish using ultra-fine abrasives. The polishing process must be performed carefully to mini- mize scratches and other polishing artifacts that reduce image quality.
Ion beam milling (TEM)—thins samples until they are transparent to electrons by firing ions (typically argon) at the sample’s surface from an angle and sputtering material from the surface. A subclass of this is focused ion beam milling, where gallium ions are used to produce an electron transparent membrane in a specific region of the sample. Ion beam milling may also be used for cross section polishing prior to SEM analysis of materials that are difficult to prepare using mechanical polishing.
Conductive coating (SEM and electron microprobe)—an ultrathin coating of electrically conducting material, depos- ited either by high vacuum evaporation or by low vacuum sputter coating of the sample. This is done to prevent the accumulation of static electric fields at the specimen due to the electron irradiation required during imaging. The coat- ing materials include gold, gold/palladium, platinum, tungsten, graphite, etc.
2.2.1.4 Electron microprobe
An EMP, also known as an electron probe microanalyzer (EPMA) or electron microprobe analyzer (EMPA), is an ana- lytical technique based on the principles of the SEM used to nondestructively determine the chemical composition of small volumes of solid materials. It works similarly to an SEM: the sample is bombarded with an electron beam, emit- ting X-rays at wavelengths characteristic to the elements being analyzed. This allows the concentrations of elements present within small sample volumes (typically 1030 cubic micrometers or less) to be determined. The concentrations of elements from beryllium to plutonium can be measured at levels as low as 100 parts per million (ppm). Recent mod- els of EPMAs can accurately measure elemental concentrations of approximately 10 ppm.
Low-energy electrons are produced from a tungsten filament, a lanthanum hexaboride crystal cathode or a field emission electron source and accelerated by a positively biased anode plate to 330,000 electron volts (keV). The anode plate has central aperture and electrons that pass through it are collimated and focused by a series of magnetic lenses and apertures. The resulting electron beam (approximately 5 nm to 10μm diameter) may be scanned across the sample or used in spot mode to produce excitation of various effects in the sample. Among these effects are: phonon excitation (heat), cathodoluminescence (visible light fluorescence), continuum X-ray radiation (Bremsstrahlung), char- acteristic X-ray radiation, secondary electrons (plasmon production), backscattered electron production, and Auger elec- tron production.
When the beam electrons (and scattered electrons from the sample) interact with bound electrons in the innermost electron shells of the atoms of the various elements in the sample, they can scatter the bound electrons from the electron shell producing a vacancy in that shell (ionization of the atom). This vacancy is unstable and must be filled by an elec- tron from either a higher energy bound shell in the atom (producing another vacancy which is in turn filled by electrons from yet higher energy bound shells) or by unbound electrons of low energy. The difference in binding energy (BE) between the electron shell in which the vacancy was produced and the shell from which the electron comes to fill the vacancy is emitted as a photon. The energy of the photon is in the X-ray region of the electromagnetic spectrum. As the electron structure of each element is unique, the series of X-ray line energies produced by vacancies in the innermost shells is characteristic of that element, although lines from different elements may overlap (interference). As the inner- most shells are involved, the X-ray line energies are generally not affected by chemical effects produced by bonding
between elements in compounds except in low atomic number (Z) elements (B, C, N, O, and F for Kαand Al to Cl for Kβ) where line energies may be shifted as a result of the involvement of the electron shell from which vacancies are filled in chemical bonding.
The characteristic X-rays are used for chemical analysis. Specific X-ray wavelengths or energies are selected and counted, either by wavelength-dispersive X-ray spectroscopy (WDS) or energy-dispersive X-ray spectroscopy (EDS).
WDS uses Bragg diffraction from crystals to select X-ray wavelengths of interest and direct them to gas-flow or sealed proportional detectors. In contrast, EDS uses a solid-state semiconductor detector to accumulate X-rays of all wave- lengths produced from the sample. While EDS yields more information and typically requires a much shorter counting time, WDS is normally a more exact technique with lower detection limits due to its superior X-ray peak resolution and better peak to background ratio.
The chemical composition is determined by comparing the intensities of characteristic X-rays from the sample mate- rial with intensities from known composition (standards). Counts from the sample must be corrected for matrix effects (such as depth of production of the X-rays, absorption, and secondary fluorescence) to yield quantitative chemical com- positions. The resulting chemical information is gathered in textural context. Variations in chemical composition within a material (zoning), such as a mineral grain or metal, can be readily determined.
2.2.1.5 Limitations
WDS is useful for higher atomic numbers; therefore WDS cannot determine elements below number 5 (boron). This limitation sets restrictions to WDS when analyzing geologically important elements such as H, Li, and Be.
Notwithstanding the improved spectral resolution of elemental peaks, some peaks exhibit significant overlaps that result in analytical challenges (e.g., VKα and TiKβ). WDS analyses are not able to distinguish among the valence states of elements (e.g., Fe21vs. Fe31) such that this information must be obtained by other techniques (e.g., Mo¨ssbauer spec- troscopy or electron energy loss spectroscopy). The multiple masses of an element (i.e., isotopes) cannot be determined by WDS, but rather are most commonly obtained with a mass spectrometer.
This technique is most commonly used by mineralogists and petrologists. Most rocks are aggregates of small min- eral grains. These grains may retain chemical information related to their formation and subsequent alteration. This information may provide information regarding geologic processes such as crystallization, lithification, volcanism, metamorphism, orogenic events (mountain building), and plate tectonics. The change in elemental composition from the center (also known as core) to the edge (or rim) of a mineral grain or crystal can provide information about the his- tory of the crystal’s formation, including the temperature, pressure, and chemistry of the surrounding medium. Quartz crystals, for example, incorporate a small, but measurable amount of titanium into their structure as a function of tem- perature, pressure, and the amount of titanium available in their environment. Changes in these parameters are recorded by titanium as the crystal grows. This technique is also used for the study of extra-terrestrial rocks (i.e., meteorites) and provides chemical data that are vital to understanding the evolution of the planets, asteroids, and comets.
2.2.2 X-ray fluorescence
XRF is the emission of characteristic “secondary” (or fluorescent) X-rays from a material that has been excited by being bombarded with high-energy X-rays or gamma rays. The phenomenon is widely used for elemental analysis and chemi- cal analysis, particularly in the investigation of metals, glass, ceramics, and building materials, and for research in geo- chemistry, forensic science, archeology, and art objects such as paintings and murals. When materials are bombarded with short-wavelength X-rays or gamma rays, ionization of their component atoms can take place. Ionization consists of the ejection of one or more electrons from the atom and may occur if the atom is exposed to radiation with an energy greater than its ionization energy. X-rays and gamma rays can be energetic enough to expel tightly held electrons from the inner orbitals of the atom. The ejection of such an electron makes the electronic structure of the atom unstable, and electrons in higher orbitals “fall” into the lower orbital to fill the hole left behind. During this process, energy is released in the form of a photon, the energy of which is equal to the energy difference of the two-electron orbitals involved. Thus the material emits radiation, which has energy characteristic of the atoms present. The term fluorescence is applied to phenomena in which the absorption of radiation of a specific energy results in the reemission of radiation of different energy (usually lower).
In order to excite the atoms, a source of radiation is required, with enough energy to expel tightly held inner elec- trons. Conventional X-ray generators are most frequently used, as their output can readily be “tuned” for this technique, and since higher power can be deployed relative to other techniques. However, gamma-ray sources can be utilized
without the need for an elaborate power supply, permitting an easier use in small portable instruments (e.g., useful dur- ing geological field work). When the energy source is a synchrotron or the X-rays are focused by an optic like a polyca- pillary, the X-ray beam can be very small and very intense. As a result, atomic information on the submicrometer scale can be obtained. X-ray generators in the range between 20 and 60 kV are used, which allow excitation of a broad range of atoms. The continuous spectrum consists of “Bremsstrahlung” radiation: radiation produced when high-energy elec- trons passing through the tube are progressively decelerated by the material of the tube anode (the “target”). In energy dispersive analysis, the fluorescent X-rays emitted by the material sample are directed into a solid-state detector which produces a “continuous” distribution of pulses, the voltages of which are proportional to the incoming photon energies.
This signal is processed by a multichannel analyzer which produces an accumulating digital spectrum that can be pro- cessed to obtain analytical data. In wavelength dispersive analysis, the fluorescent X-rays emitted by the material sam- ple are directed into a diffraction grating monochromator. The diffraction grating used is usually a single crystal. By varying the angle of incidence and take-off (θ) on the crystal, a single X-ray wavelength (λ) can be selected. The wave- length obtained is given by Bragg’s law:
nλ52dsinð ịθ
wheredis the spacing of atomic layers parallel to the crystal surface andn is a positive integer. In energy dispersive analysis, dispersion and detection are a single operation, as already mentioned above. Proportional counters or various types of solid-state detectors [e.g., PIN diode, Si(Li), Ge(Li), silicon drift detector (SDD)] are used. They are all based on the same detection principle: an incoming X-ray photon ionizes many detector atoms with the amount of charge pro- duced being proportional to the energy of the incoming photon. The charge is then collected, and the process repeats itself for the next photon. Detector speed is clearly critical, as all charge carriers measured must come from the same photon to measure the photon energy correctly (peak length discrimination is used to eliminate events that seem to have been produced by two X-ray photons arriving almost simultaneously). The spectrum is then built up by dividing the energy spectrum into discrete bins and counting the number of pulses registered within each energy bin. Energy disper- sive X-ray fluorescence detector types vary in resolution, speed, and the means of cooling (a low number of free charge carriers is critical in the solid-state detectors): proportional counters with resolutions of several hundred eV cover the low end of the performance spectrum, followed by PIN diode detectors, while the Si(Li), Ge(Li) and SDD occupy the high end of the performance scale. In wavelength dispersive analysis, the single-wavelength radiation produced by the monochromator is passed into a photomultiplier, a detector comparable to a Geiger counter, which counts individual photons as they pass through. The counter is a chamber containing a gas that is ionized by X-ray photons. A central electrode is charged at (typically)11700 V with respect to the conducting chamber walls, and each photon triggers a pulse-like cascade of current across this field. The signal is amplified and transformed into an accumulating digital count. These counts are then processed to obtain analytical data.
The fluorescence process is inefficient, and the secondary radiation is significantly weaker than the primary radia- tion. Additionally, the secondary radiation from the lighter elements is of relatively low energy (long wavelength), has low penetrating power, and is severely attenuated if the beam passes through air for any distance. As a result of this, for high-performance analysis, the path from tube to sample to detector is maintained under vacuum (around 10 Pa residual pressure). This means in practice that most of the working parts of the instrument must be located inside a large vacuum chamber. The difficulties of maintaining moving parts in vacuum, and of rapidly introducing and withdrawing the sam- ple without losing vacuum, form major challenges for the design of the instrument. For less demanding applications, or when the sample is damaged by a vacuum (e.g., a volatile sample), a helium-swept X-ray chamber can be substituted, with some loss of low-Z(Z5atomic number) intensities.
The use of a primary X-ray beam to excite fluorescent radiation from the sample was first proposed by Glocker and Schreiber in 1928. These days, the method is generally applied as a nondestructive analytical tool, and as a process con- trol technique in many extractive and processing industries. In principle, the lightest element that can be analyzed is beryllium (Z54), but due to instrumental limitations and low X-ray yields for the light elements, it is often difficult to quantify elements lighter than sodium (Z511), unless background corrections and very comprehensive interelement corrections are made.
2.2.2.1 Energy dispersive spectrometry
In energy dispersive spectrometers (EDX or EDS), the detector allows the determination of the energy of the photon when it is detected. Detectors historically have been based on silicon semiconductors, in the form of lithium-drifted sili- con crystals, or high-purity silicon wafers. Si(Li) detectors contain essentially a 35 mm thick silicon junction type