X-ray Microanalysis Phase Map on Rare Earth Minerals with a Conventional and an Annular Silicon Drift Detector
Rare earth elements (REEs) have become important components in over 200 products across an extensive range of applications, causing the demand for these elements keeping rising and the rare earth industry keeping flourishing [1]. The Nechalacho rare earth deposit, located in the Northwest Territories in Canada, is one of the most important REE sources in the world [2]. The aim of this work is to use one high spatial resolution and high count rate scanning electron microscope/x-ray energy dispersive spectrometry (SEM/EDS) system to characterize rare earth minerals (REMs) in this deposit using phase maps. The Xray microanalysis was achieved by a conventional silicon drift detector (SDD) with 60 mm collecting area located on the side of specimen and an annular SDD with a same collecting area inserted below the objective lens. The position of the annular detector provides the shortest working distance and the largest solid angle, allowing the X-ray microanalysis with a count rate as high as 2,000,000 cps and short acquisition time. Because of the high count rate, the relatively low accelerating voltage can be used, providing a high spatial resolution, making the characterisation of small features in micron-scale possible. The traditional EDS elemental map shows distribution of elements, but it cannot provide enough information for phase identification. The problem is more obvious in geological materials, where the samples have numerous micro phases with unknown composition. In order to overcome that, we develop a phase map method using the traditional EDS map and the f-ratio method [3]. The method was applied to acquire the compositional information of REM and to show their distribution.
X-Ray Imaging with a Silicon Drift Detector Energy Dispersive Spectrometer
The scanning electron microscope (SEM) was primary developed for imaging applications. With the introduction of the Si(Li) energy dispersive spectrometer (EDS), simultaneous imaging and X-ray microanalysis became possible. However, long working distance and high current were needed because the position and small solid angle of the EDS detector.
Polylithionite from the rare-metal deposits of the Blachford Lake alkaline complex, N.W.T., Canada
Extensive deposits of rare lithophile elements are located within albitized and greisenized peralkaline granitb and syenite at Thor Lake, in the Blachford Lake complex east of Yellowknife, Northwest Territories in Canada. Five structural zones, affected by metasomatic alteration and locally also brecciation, host zoned assemblages with Be, Nb-Ta, REE, Y, Zr and Ga mineralization. Polylithionite is a major phase in several units of the mineralized zones; its abundance amounts to tens of thousands of tons. The chemistry, physical properties and structural characteristics of two polylithionite samples from widely separated locations are almost identical, and are close to the typical properties of the end-member lM polylithionite. Ti, Nb, Mg, Rb/Cs and Fe3*/Fe2* are much higher whereas Rb and Cs are remarkably lower than their ranges in lepidolites from orogenic to post-orogenic peraluminous granitic pegmatites. However, the compositional field of lM polylithionites is apparently transitional into that of lM lepidolites which partly overlaps with 2Mt and 2M, polytype ranges.