instrumental techniques applied to mineralogy and geochemistry

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TEM in Geology. Basics and applications Fernando Nieto García Departamento de Mineralogía y Petrología and IACT, Universidad de Granada-CSIC, 18002 Granada, Spain Introduction In general a microscope is capable of producing an enlarged image of an object through a combination of lenses. The fundamental lens is the objective lens, which produces a diffraction pattern of the object in its back-focal plane. If these diffracted beams are focused and magnified again by additional lenses, we finally obtain the enlarged image of the object. This is the normal means of operation of an optical microscope (Fig. 1). The resolution of the microscope depends on the wavelength of the radiation used. Therefore, the resolution of an optical microscope is limited to textural relationships between crystalline objects, but it is unable to provide information about the atomic structure of these crystalline objects. In theory, X-rays and electrons have wavelengths small enough to produce such information. Consequently, these two types of radiation are usually employed to study the crystalline structure of matter. Nevertheless, no lenses exist for X-rays; therefore, they cannot be focused to produce an image and X-rays microscopes do not exist. An X-ray “image” can only be generated by crystallographers by calculating the intensity of the diffracted beams; however, in the electron microscope, the Fourier transform of the diffracted beams is physically carried out by the electromagnetic lenses and a real image can be obtained. The geometry and optical paths of the rays in optical and electron microscopes are exactly equivalent (Fig. 1). A significant difference between these two types of microscopes is that electromagnetic lenses can continuously change their focal length, and thus their magnification, in contrast to glass lenses. Some consequences are easily deduced, but perhaps the fundamental one is the possibility of bringing to the image plane the back focal plane of the objective lens—that is, the diffraction pattern instead of the
22 Fernando Nieto intermediate image (Fig. 1). In modern microscopes, the change from image to diffraction can be accomplished instantly by pressing a button. When electrons interact with matter, other signals (in addition to various types of electrons) such as electromagnetic waves are produced. Analytically, X-rays are the most important, as their wavelength contains the fingerprint of the chemical elements present in the sample. This is the basis of all the modern in-situ analytical techniques. Although electron microprobe and scanning electron microscopes with EDX are the best-known examples of such techniques, this possibility is also present in the transmission electron microscope (TEM). FIGURE 1. Schematic of the column of a transmission electron microscope (left) and optical path of the rays valid for both electron and optical microscopes (right). Two different field strengths of the projector lenses allow the production of two different ray paths after the first intermediate image. Thus, it is possible to bring to the image plane the back focal plane of the objective lens (diffraction pattern) or the intermediate image to obtain diffraction or an image, respectively. Modified from Buseck (1992) and Putnis (1993).
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