## 1 Introduction
### Basic SEM Operation
Fluorescent X-rays are a subset of characteristic X-rays, resulting from the filling up of the inner shell by outer shell electrons (Figure 2). Characteristic X-rays are emitted from the vacancy left by an ejected inner shell electron, while fluorescent X-rays are produced when the outer shell electron fills the vacancy. Fluorescent X-rays contribute valuable elemental information in addition to that provided by the main characteristic X-rays. They also help in studying the background radiation in the X-ray spectrum [11] (Figure 1). The signals of electron and X-ray production are collected by various detectors present in the specimen chamber of the SEM. A monitor is fed with the signals from each detector, and a rectangular pattern of parallel scanning lines is synchronized with the electron beam [11]. Field emission gun SEM (FEG-SEM) can produce high-resolution secondary electron images owing to the intense electron beam and is capable of achieving subnanometer to nanometer-scale resolution [49]. It is important to understand the meaning of the magnification value in SEM images. In general, the magnification value provides information about the size ratio between the actual and enlarged images of the specimen. SEM produces high-resolution images for visualizing material surfaces. During the imaging process, the level of enlargement applied  
Figure 2: Various levels of electron penetration through the sample surface. to the specimen is given by the magnified value of the SEM image. This magnification is usually expressed as a numerical value (such as 1000\(\times\) or 10,000\(\times\)) visualizing how many times larger the imaged structures or features are shown compared to the actual sample. One of the fundamental features of SEM is the ability to control magnification. The level of magnification can be adjusted to provide an overview of the specimen's surface structure or to focus on any specific structural details. Figure 3 shows ilmeinte (FeTiO\({}_{3}\)) micrographs with various magnification levels, aimed at analyzing the existing (a) cracks, (b)arrows, and (c) particle size and shape in the sample [50]. Particle sizing by SEM is not necessarily representative of bulk samples, as sampling bias and fixation in adhesive tape can affect the observable size distribution, but it can be considered at least semiquantitative and preferably reported as a range, such as 50-200 \(\upmu\)m, as reported in [50]. SEM can also be used to infer mechanistic phenomena, for example, the cracks and veins of ilmeinte were used to suggest that these features are gained from particle-particle collisions as it is transported along the seabed. That is, SEM can be used to make qualitative descriptions of mineral alterations in terms of whether they result from chemical, physical, or even biological processes. Figure 3 also exemplifies the use of magnification levels on the identification of mineral phases and crystals within particles, as studied for the identification of rare earth elements (REEs) in carbonatite ore [8]. REEs are largely speciated as monazite ((REE)PO\({}_{4}\)), and SEM images enable identification of the crystal structures, wherein larger colloform, acicular, and massive crystals, which are relatively large, are identifiable in lower magnification images, and the presence of submicrometer-sized crystals requires higher magnification to elucidate their associated porosity and crystal intergrowth [8]. Such morphological and mineral assemblage information is critical in mineral processing to predict the behavior of the mineral upon comminution and hydrometallurgical processing.