Browsing by Author "Rozendaal, Abraham"

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    Characterization of coloured gemstones by X-ray micro computed tomography
    (MDPI, 2021-02-08) Heyn, Rene; Rozendaal, Abraham; Du Plessis, Anton; Mouton, Carene
    The monetary value of gemstones is based on five variables: rarity, cut, weight, color and clarity. The latter refers to internal impurities and defects. Fashion may also dictate demand and price. To enhance some of these features and value, gemstones are treated. Disclosure or nondisclosure thereof has been controversial and affected consumer confidence. Most of these treatments are difficult to detect with the naked eye and accurately quantify with traditional optical and analytical methods. X-ray micro computed tomography (micro-CT or μCT) is proposed as a relatively low cost, physically non-destructive and complementary method to detect and quantify clarity enhancement and also to provide a unique 3D fingerprint of each gemstone. A collection of 14 cut colored gemstones was selected. Micro-CT scans allowed fracture detection, their distribution and calculation of filler volume as well as 3D mapping of inclusions, surface and internal imperfections and artificially induced modifications. As a result the method allows the construction of a digital twin. X-ray exposure could however induce unwanted color changes. This effect was minimized or eliminated by optimizing dosage and exposure time.
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    Genesis of the stratiform BH-t Swartberg Cu-Pb-Zn-Ag deposit, Northern Cape, South Africa
    (Geology Applied to Mineral Deposits, 2013) Rudnick, Tarryn-Kim; Rozendaal, Abraham
    Swartberg is a metamorphosed Broken Hill- type Cu-Pb-Zn-Ag deposit in the north-west of South Africa. The mineralogy and stratigraphy closely resemble that of the nearby Broken Hill, Deeps and Gamsberg deposits. Swartberg is the least-studied of these and this and ongoing studies aim to complete the picture of the district by providing a clearer understanding of the deposit. It comprises two discrete, stacked stratiform orebodies, both of which are deformed by a recumbent, isoclinal F2 fold and refolded by an open F3 fold. The orebodies were deposited in a northeast-southwest trending basin, hinged to the northwest by a hydrothermal feeder fault. Both the upper orebody (UOB) and lower orebody (LOB) formed as syngenetic exhalatives, with the LOB representing an early, precursor exhalation stage to the more extensive stage that formed the UOB. The UOB displays a clear gradation from vent-source proximal chemogenic sediments in the F2 fold hinge, deposited under hotter, more reducing conditions, to distal material formed under cooler, oxidizing conditions. After deposition, the basin underwent several episodes of deformation and metamorphism to medium grades (pressure < 4.5kbar), during which the sulphides underwent limited mobilization into the F2 hinge zone.