**1. Introduction**

Rare earth element (REE) ore deposits occur in a wide variety of geological contexts and are hosted by a considerable diversity of host rocks [1]. Igneous host rocks appear mainly carbonatites and peralkaline plutonic rocks as nepheline syenites [2–5]. Although pegmatites may also contain significant amounts of REE-bearing minerals [6], they are usually too small in volume to be of economic significance. REE mineralisation also occurs in hydrothermal veins and stockworks [7,8]. Recent and fossile mineral placers can be sedimentary REE mineral deposits. Significant enrichment of REE is also possible by chemical weathering with recoverable REE concentrations occurring in lateritic clays formed at the expense of magmatic REE deposits. Some of the largest known REE deposits owe their origin to a sequence of natural enrichment processes. Primary igneous occurrences in alkali syenites and carbonatites as well as sedimentary heavy mineral accumulations and weathering crusts underwent metamorphic and hydrothermal overprint in depth, followed by weathering. Due to their possible complex geological evolution, the deportment of the elements in such REE mineral deposits are often ambiguous and the subject of scientific discussions.

REE ores are not only marked by geological complexity, but REE ore mineralogy is also very complex, with di fferent minerals having complex chemical and crystallographic properties. More than 200 REE minerals are known [9]. Actually, in the largest REE mineral deposit of Bayan Obo in Mongolia one can distinguish three ore types with Fe-REE-, dolomite-REE- and silicate-REE-ores. The economic REE minerals are bastnaesite and monazite [10]. Other important REE mineral deposits are associated with carbonatite intrusions, as Mountain Pass (USA) and Mount Weld (Australia). The ore minerals in these deposits are bastnaesite, allanite (part of the epidote mineral group), monazite, apatite and pyrochlor. In lateritic clay deposits, REE released during chemical weathering of igneous host rocks may occur adsorbed in clay minerals [11–14].

The beneficiation of REE ores poses major technological challenges [15–18]. These may be understood—and then overcome—by applying modern and quantitative analytical methods that yield not only chemical but also mineralogical and microfabric data. Bulk chemical analysis of ore and processing products by X-ray fluorescence (XRF) and inductively coupled plasma mass spectrometry (ICP-MS) will readily provide elemental concentrations, especially of the REE [19]. X-ray di ffraction (XRD) analysis, in turn, allows the identification of minerals, but is a fflicted with considerable uncertainty and error when mineral modal abundance is below ~1 wt %. However, both analytical methods require powdered samples at grainsizes <2 μm and provide no tangible information on particle and mineral grain sizes, particle compositions, mineral intergrowths and liberation. However, such particle-related parameters are essential during the beneficiation of REE ores. Therefore, non-destructive, element sensitive methods based on scanning electron microscopy (SEM) with energy dispersive spectroscopy (EDS) and involving automated backscattered electron (BSE) image analysis, referred to as automated mineralogy, are widely applied [20–23]. Automated mineralogical studies of REE ores are confronted with the following mineralogical challenges:


Mineral phases are defined first by the crystallographic parameters (structure) and second by the chemical composition [24]. For the REE-bearing minerals as outlined above, there arises the consequence that even when crystallographic parameters by XRD are available it is often very hard to define a correct mineral name based on energy-dispersive X-ray (EDX) spectra and element compositions. Also, the conventional method of labelling EDX spectra from REE-bearing minerals by mineral names is severely hampered by the fact that the X-ray energy peaks and sub-peaks of LREE and HREE display considerable overlap and interference along the keV scale, which cannot be resolved by an analysis with the EDX, even when interference corrections are applied. This requires dedicated routines during the application of automated SEM methods [25,26]. Our study introduces an approach of applying generic labelling of a list of EDX reference spectra obtained from REE-bearing minerals in nepheline syenites and carbonatites, which is followed by a distinct mineral grouping. This allows robust classification and the extraction of mineralogical data from automated scanning electron microscopy-mineral liberation analysis (SEM-MLA) measurements. Many of the problems of the nomenclature and assignment of the REE-bearing minerals are thus avoided.

#### **2. Approach and Analytical Methods**

At the TU Bergakademie Freiberg/Saxony a scanning electron microscope FEI Quanta 600 (FEI, Hillsboro, OR, USA) equipped with a field emission gun (FEG) as electron source, two Bruker energy dispersive X-ray (EDX) SDD detectors (Bruker Quantax 200 with two Dual XFlash 5030 EDX detectors, (Bruker, Berlin, Germany), and backscattered electron (BSE) and SE detectors is applied for Mineral Liberation Analysis [20]. For the measurements presented here, the commercial MLA 2.9 software package (JKTech, Brisbane, Australia) has been used [21].

The analytical routine proposed here commences with the survey of a sample by a BSE image, labelled as a frame, at 25 kV acceleration voltage and 10 nA beam current. The instrument-specific working distance was at 12 mm. REE-bearing mineral grains with high average atomic numbers and molecular masses appear in light grey in the BSE image compared to gangue minerals as quartz and feldspar with darker grey colour (Figure 1a–c). The calibration of the BSE greyscale with contrast and brightness was performed with gold reference. After automated image analysis (Figure 1d–f), the electron beam is directed into the barycentres of contiguous mineral grains characterised by their BSE grey values, and a single EDX spectrum is obtained (XBSE measurement mode, [21]). In the case of thin sections or polished blocks of ore, the image analyser performs only the particle segmentation within a frame, and a grid of single EDX spectra is gained from each contiguous domain with distinct BSE grey values (GXMAP measurement mode). Each EDX spectrum is normalised by the counting rates (cts/s)N of the coupled EDX detectors and plotted against the keV scale (Figure 1g–i). These EDX spectra have characteristic peaks at distinct positions in the keV scale and distinct relative cts/s allowing identification of major elements present, giving a semiquantitative indication of their concentrations. Both measurement routines allow a later examination of both geometrical and mineralogical particle and grain parameters, as various size and shape parameters, mineral locking and mineral liberation [21]. The classification of the measured EDX spectra from the sample is performed by a comparison to a list of labelled reference EDX spectra. The list of reference spectra can incorporate up to 250 different reference EDX spectra which were gained under similar measurement conditions from the analysed samples and/or from related reference samples.

**Figure 1.** (**<sup>a</sup>**–**<sup>c</sup>**) Particles of grinded rare earth element (REE) carbonatite ore in backscattered electron images (BSE). The REE minerals monazite (Mnz), bastnaesite (Bas), and the REE-Ca-F minerals (synchysite) are in light grey to white; the REE-Nb- and Nb-bearing minerals (Nb-min) are also in light grey; the gangue minerals as ankerite (Ank), dolomite (Dol) and fluorite (Flu) are dark grey. (**d**–**f**) REE carbonatite ore particles after classification of automated scanning electron microscopy (SEM) measurement in the mineral liberation analysis (MLA)-XBSE routine. Classification with list of generically labelled energy-dispersive X-ray (EDX) spectra and corresponding grouping of the spectra (see text). REE minerals in the particles display only poor and partial liberation. (**g**–**i**) EDX spectra of some REE minerals in keV versus normalised counting rate (cts/s)N. Positions of maxima in the spectra are labelled by the corresponding elements. The generic labelling of the spectra in REE-Si-Ca-F-P is according to a quantitative EDX element analysis of the corresponding REE mineral grain, shown on the right side.

#### **3. Energy-Dispersive X-ray (EDX) Spectra of Rare Earth Element (REE)-Bearing Minerals**

As outlined above, REE-bearing minerals have rather complex chemical compositions [24]. This mineralogical and chemical diversity is also evident from the currently most important economic REE bearing minerals (Table 1). The REE phosphate monazite (LREE,Y,Th,Si,Ca)PO4 shows highly variable concentrations of the LREE elements La, Ce, Nd in solid solutions toward cheralite (REE,Ca,Th)(P,Si)O4, huttonite (ThSiO4) and the xenotime group of minerals (Y,HREE)PO4 involving coupled substitution of Y, Ca, Si, Th and P [24]. Hydrated species such as rhabdophane (REE)PO4(H2O) and florencite REE)Al3(PO4)2(OH)6 occur as well. In the britholite group (REE,Ca,Th)5(SiO4,PO4)3(OH,F) variable REE and Y contents occur together with Ca, Si, P and F. The synchysite group Ca,REE(CO3)2F contains

Ca, F and C together with REE. Parisite, with more F and C but less Ca can be considered there as a subspecies. Bastnaesite REE(CO3)F is a hydrated halogene-bearing carbonate mineral with variable REE, Y, F, C concentrations. In contrast, fluocerite (REE)F3 is a simple fluor-bearing mineral (Table 1). A systematic search for REE-bearing minerals in available databases, such as *MinIdent* [27,28] and websites (http://webmineral.com/chem/Chem; http://rru ff.info.ima) by using the mineral chemical compositions leads to long and desperately confusing list of mineral names. This complexity of mineral compositions and mineral names renders correct identification of individual REE minerals di fficult. This pertains in particular to automated mineralogy studies where mineral identification is based on EDX spectra with minute X-ray counts, as ~10,000 cts exemplified in this study. Even when pertinent expert knowledge upon the corresponding mineral groups is available, an assignment of the EDX spectra to mineral names remains biased. To overcome this particular problem of reference EDX spectra denomination for SEM-based image analysis measurements, the following workflow is proposed. Although the proposed workflow is based here on the MLA 2.9 or MLA 3.1 software versions, it is easily transferable to similar instrumental and software platforms.


**Table 1.** List of REE-bearing minerals and their assignment to mineral groups as applied in the presented MLA studies of REE ores. The given mineral compositions (in wt %; density Dens.) are not representative and refer to the analysed REE ores and/or are partly taken from databases (e.g., www.webmineral.com).


The SEM-MLA measurements of various REE ores will provide a list of reference EDX spectra. Given a possible maximum of 250 reference EDX spectra for a measurement classification, then ~50 spectra should be sufficient for REE-bearing minerals. The mineral xenotime will be readily distinguished by the abundance of Y and will not require more than ~5 spectra [32]. The reference spectra list further encloses minerals with Nb (~15 spectra) and also minerals with both Y and Nb (~5 spectra). The other spectra in the list concern gangue minerals, i.e. feldspars (~15 spectra for K-feldspar, albite, plagioclase), quartz (5 spectra), carbonate minerals (~20 spectra for calcite, dolomite, ankerite, siderite), fluorite (~5 spectra), as well as accessory minerals such as Ti-bearing minerals (~15 spectra for rutile, ilmenite, titanite), apatite (5 spectra) and zircon (3 spectra).

The classification of an SEM-MLA measurement against the labelled reference EDX spectra list provides the proportion in area% of the grains that are classified by a distinct spectrum. However, the evaluation of area proportions dedicated to up to 250 EDX spectra is at best unmanageable. Therefore, the area proportions of several EDX spectra have to be integrated or summarised into groups. An educated grouping of spectra is usually sufficient to address important issues of the mineral processing [32]. A re-grouping of existing data is possible at any time without performing a new measurement. Depending on the MLA software version, a mineral formula, an exact or approximated element composition, and a specific weight can be assigned to each reference EDX spectrum (MLA 3.1 version) or to a group of spectra (MLA 2.9 version). The REE ores analysed in this study were grouped as follows (Table 1):


A further group of EDX spectra includes Y-HREE and P-rich xenotime and associated Nb-Ta minerals. Dependent on the mineralogy of the deposit, particular attention during spectra grouping is recommended to minerals that contain Y and Nb, as well as Nb and REE, such as the aeschynite mineral group. For the presented case studies, the REE-Nb group and the Nb-Y groups were established. For the discrimination of spectra from Y and Nb the secondary lines have to be considered. After a tentative grouping of the EDX spectra from REE-bearing minerals from an ore sample, the corresponding area proportions should be examined. In the following, the convenience and suitability of the classification workflow introduced above is presented in some case studies dealing with automated SEM of complex REE ores and process samples from three di fferent deposits.
