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.
Mesozoic rifting and basin inversion along the northern African Tethyan margin: an overview
Abstract The northern African Tethyan margin registered three major rifting episodes from the latest Palaeozoic-earliest Mesozoic to the earliest Cenozoic. Break-up of Gondwana was initiated in the late Carboniferous. Along the northern African-Arabian plate margin rifting propagated westward from the northeastern Arabian margin to Morocco during the Permian and Triassic, and was accompanied by Mid-Late Triassic-earliest Liassic extensive alkaline flow basalts. Rifting continued during the Liassic, e.g. in the Moghrebian Atlas troughs. A second stage of rifting occurred in the Late Jurassic and continued into, or was rejuvenated during, the Early Cretaceous. Along the east Mediterranean margin, some large E-W trending rifts formed, often with associated volcanism, e.g. southern Sirt and Abu Gharadig. Most researchers believe the oceanization of the eastern Mediterranean basin occurred at this time. Flexural subsidence affected the Moghrebian Atlas, including the Riffian-Tellian domain, where a thick flysch series was deposited. Two short-lived, but widespread and tectonically important, compressional or wrench-dominated events occurred during the Santonian and the latest Maastrichtian. From Morocco to Oman, most sedimentary basins were folded and inverted at these times. The Santonian event resulted from onset of the collision between the African-Arabian and Eurasian plates. New stress-fields favoured NE-SW extension along the African plate margin, generating or rejuvenating some rifts, e.g. in the Euphrates trough and in the Libya-Tunisia region (northwestern Sirt, Pelagian Sea). Rifting and magmatism continued in these regions into the Paleocene. During the Mesozoic, therefore, the northern margin of the African-Arabian plate registered both rifting resulting in the oceanization of the Tethys and rifting resulting from the initiation of the closure of the Tethys. The intraplate domain exhibited echoes of the tectonic events affecting the margin.