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.
Magmatic vs. hydrothermal origins for zircon associated with tantalum mineralization in the Tanco pegmatite, Manitoba, Canada
Abstract Complex textures in zircon associated with Ta oxides have been used to assess the processes at the origin of zircon crystallization and associated Ta mineralization in pegmatites, in particular the role of magmatic vs. hydrothermal processes. The Tanco pegmatite is used as an example because its zircon is devoid of post-magmatic alteration and its petrogenesis is well constrained. Zircon in primary units is metamict with high U-Pb-Th contents. By contrast, in secondary assemblages affected by latemagmatic micaceous alteration, zircon is devoid of radiogenic elements, but it may contain abundant Ta2O5 (up to 4.7 wt%). The incorporation of Ta into zircon may occur through coupled substitution mechanisms involving other minor elements such as P, Al, Mn, or Li. The presence of Ta accounts for the distorted structures in the Ta-rich zircon, as revealed in high-resolution TEM images. Four zircon types occur sequentially in single zircon crystals, which permits a new model for zircon growth and evolution in rare element pegmatites to be advanced: (1) zircon (Z1) is Ta-rich and crystallized possibly before discrete Ta phases; (2) Z1 was overgrown by regularly zoned zircon Z2 and Z3, which show lower Ta and extreme Zr/Hf fractionation (HfO2 up to 38.9 wt%), suggesting crystallization from a highly fractionated melt, possibly at the onset of Ta mineralization; and (3) close to the solidus, the aqueous fluid at the origin of micaceous alteration corroded the distorted structure of Z1, and a low- Ta zircon (Z4) replaced Z1 by dissolution-reprecipitation, whereas Z2 and Z3 resisted this alteration. Tantalum was no longer stable in the zircon structure and crystallized as Ta-oxide inclusions in the reequilibrated zircon (Z4).