Applications of portable x-ray fluorescence techniques to emerald chemistry: a potential tracer of provenance

Alvarez, Hugo Vicente Luzuriaga (2012) Applications of portable x-ray fluorescence techniques to emerald chemistry: a potential tracer of provenance. (MSc(R) thesis), Kingston University, .


Emeralds mined from Columbia, South America, are known worldwide for their colour. The emerald mines are located in two mineralization belts situated on the flanks of the eastern cordillera of the Columbian Andes. Emerald samples from the Eastern and Western Mineralization belts of Columbia, were analyzed using a Thermo Scientific Niton XL3t 900 portable X-Ray Fluorescence (XRF) analyzer. The non-destructive XRF analyzer is potentially a suitable instrument to create a trace-element chemical profile to assign geographical provenance to gemstones, due to its portability, internal calibration, rapid analysis time, and relatively low cost. After assessing the contents of Fe, Cr, V, Ca and K in seven geostandard reference material (JB-1, JG-1a, JGb-1, JP-1, NIM-L, NIM-P, SCo-1) pressed-powder pellets, it was determined that the portable XRF was precise and accurate when the reported contents in the pellets are above 250 ppm. A total of 80 and 30 emerald prisms from the Western and Eastern Mineralization Belts, respectively, were analyzed with the instrument. The portable XRF was capable of quantifying contents of the three main chromophores: Fe, Cr, and V. After constructing bivariate plots, it was found that most emerals from the Eatern Belt have Cr/V ratios > 1.5, and those samples from the Western Belt are distributed mostly at Cr/V < 1.5. In Fe-Cr-V ternary plots, the Eastern samples are distributed towards the V apex, whereas those from the Western Belt are distributed towards the Cr apex. These observations are supported by SEM-WDS data and reported literature values. Although the emerald samples were not fully dissolved following HF-HClO[sub]4-H[sub]3BO[sub]3 digestion, Cr/V values obtained by inductively coupled plasma - mass spectrometry (ICP-MS) were compared to the Cr/V values obtained by XRF, yielding a linear regression curve with an R[sup]2 value of 0.778, and supporting the results obtained by portable XRF. It is concluded that emeralds from the Eastern Mineralization Belt have higher chromium to vanadium contents than the Western emeralds. A pilot project involved the comparison of portable XRF, SEM-WDS, and literatuyre values for Columbian (Eastern and Western Mineralization Belts), Pakistani, and Tanzanian emeralds. The portable XRF results indicate that emeralds from Pakistan and Tanzania have chromium and iron as their main chromophores, with very low levels of vanadium, resulting in high Cr/V values. In comparison, Columbian emeralds have lower Cr/V values due to their much higher vanadium content. The SEM-WDS results showed than Tanzanian emeralds have a high Mg content. The experimental values were in agreement with reported literature values, leading to the conclusion that the portable XRF analyzer is a low-cost, easy to use, non-destructive instrument that can be used to identify the provenance of emeralds.

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