Growth and Alteration of Uranium-Rich Microlite
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EXPERIMENTAL PROCEDURES The microlite specimens were first characterized optically with a polarizing petrographic microscope. Subsequently, polished thin sections (30 g.tm thick) were studied with an Environmental Scanning Microscope (Philips XL30 ESEM) using a field emission gun as electron source. A portion of one of the large zoned microlite crystals was broken into 0.25 mm sized fragments which were carefully sorted by color and morphology into two samples: a core sample containing only fragments from the U-free center of the crystal, and a rim sample with only fragments from the U-rich rim. Each sample was finely powdered, mounted on a zero background Si plate (using acetone), and analyzed on a Scintag X2 automated powder X-ray diffractometer equipped with a Peltier solid-state detector using Cu-Ka radiation at 45 kV and 40 mA. Data were collected from 2-70° 20 in step scan mode (0.020 steps at 10 s/step). Electron probe microanalysis was performed using a Cameca SX-50 microprobe which is equipped with four wavelength dispersive spectrometers and was operated at 20 kV and a beam current of 20 nA measured on a Faraday cage (beam size --2 gtm). Samples and standards were coated with 200 A of carbon. Synthetic oxides were used as standards for all elements except for Al and Fe, which were calibrated with mineral standards. For most elements, data collection time was 30 s on the peaks, and 15 s on background positions above and below the peaks. The raw data were corrected on-line by the PAP correction procedure [9]. For the transmission electron microscopy, microlite fragments were crushed between two clean glass slides. A small amount of alcohol was added and the particle-containing solution was pipetted onto lacy-carbon nickel grids. The samples were examined in a JEOL 2000 FXII TEM operated at 200 kV with a LaB 6 filament. Electron diffraction patterns were taken with a charge coupled device (CCD) camera which allows imaging at resolutions comparable to photographic film. Compositional analysis was performed using an IXRF Iridium II digital pulse processor with an attached Noran Instruments detector. RESULTS Microlite is a characteristic mineral in the early Paleozoic (approximately 500-450 Ma) lithium pegmatites of the Mozambique Belt in East Africa, where it occurs in association with albite, spodumene, lithian mica, potassium feldspar and quartz [10, 11]. These pegmatites are strongly altered as a result of tropical weathering. However, microlite is resistant to weathering and is preserved as euhedral crystals in a clay mineral matrix which consists mainly of kaolinite. We have examined Th-free microlite from an altered pegmatite which intruded into a greenschist country rock in the Mutala area, Alto Ligonha (Mozambique). Here, the euhedral microlite crystals have a vitreous luster and exhibit a pronounced growth zoning (Fig. 1), characterized by a light green U-free core which is surrounded by a brown U-rich rim containing up to 17 wt.% U0 2 (Fig. 2). The core exhibits conjugate sets of straight cracks which
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