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Chemometric Modeling of Trace Element Data for Origin Determination of Demantoid Garnets

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Chemometric Modeling of Trace Element Data for Origin Determination of Demantoid Garnets minerals Article Chemometric Modeling of Trace Element Data for Origin Determination of Demantoid Garnets Stefan G. Bindereif 1, Felix Rüll 1 , Stephan Schwarzinger 1 and Clemens Schwarzinger 2,* 1 North Bavarian NMR-Centre, University of Bayreuth, 95447 Bayreuth, Germany; [email protected] (S.G.B.); [email protected] (F.R.); [email protected] (S.S.) 2 Institute for Chemical Technology of Organic Materials, Johannes Kepler Univeristy Linz, 4040 Linz, Austria * Correspondence: [email protected]; Tel.: +43-732-2468-9011 Received: 21 October 2020; Accepted: 22 November 2020; Published: 24 November 2020 Abstract: The determination of country of origin poses a common problem in the appraisal of gemstones and is in many cases still based on the observation of inclusions and growth features of a gem, whereas chemical analysis is only done by major labs. We have used Laser Ablation Inductively Coupled Plasma Mass Spectrometry to analyze the trace element profiles of demantoid garnets from six different countries and identified a set of six elements, which are magnesium, aluminum, titanium, vanadium, chromium, and manganese, that are necessary to assign the mining regions with a good certainty. By using the logarithms of the trace element concentrations and subjecting them to chemometric modeling, we were able to separate the demantoids originating from Russia, Pakistan, Namibia, Iran, and Madagascar very well, leaving only Italy with some uncertainty. Results are presented for an “all origins” model as well as pair-wise comparison of two locations at a time, which lead to even better results. Keywords: multivariate statistics; chemometrics; origin determination; garnets; demantoid; LA-ICP-MS 1. Introduction The correct determination of origin of gemstones has gained more and more importance recently. The desire to know where a gem comes from is not only scientifically driven but to a very large extent by the trade. For a scientist it might be hard to understand that for example two blue sapphires, which have the same size, weight, color, clarity and cut are appraised differently only because they hail from different sources. The trade, however, introduces factors such as rarity and history into the appraisal process and thus the blue sapphire originating from e.g., Madagascar (currently producing) will be less valuable than the one that has been found in the famed Kashmir region (found in 1881 after a landslide and now exhausted) [1,2]. Additionally, ethical issues associated with gemstone production are becoming increasingly important. Hence, it will be necessary in certain cases to prove that gems do not come from conflict areas as those might be banned from trade in some countries, such as jade and rubies originating from Burma which were banned in the United States from 2008 to 2016. In classical gemology, mainly optical characteristics have been used to determine origins, and above all, inclusions. Tiny crystals which grew in the host gem material are dependent on the geological setting and thus provide a good tool to eliminate certain sources. For instance, in sapphires it is possible to assign certain origins by identifying inclusions [3]. However, the fact that inclusions are not always present, especially in high-priced gemstones, renders this approach problematic. In the last decades, to optimize the inclusion-based process, major labs have started to exploit more advanced technologies, among them the determination of chemical composition. By determining major and minor elements Minerals 2020, 10, 1046; doi:10.3390/min10121046 www.mdpi.com/journal/minerals Minerals 2020, 10, 1046 2 of 15 conclusions about e.g., sapphire genesis can be drawn, or the origin of copper bearing (Paraiba-type) tourmalines be determined [4]. The analytical arsenal used today includes techniques such as X-ray Minerals 2020, 10, x FOR PEER REVIEW 2 of 16 fluorescence (XRF), laser induced breakdown spectroscopy (LIBS), or laser ablation inductively coupled plasma massadvanced spectrometry technologies, (LA-ICP-MS). among them Eachthe determinat of theseion technologies of chemical composition. has its merits By butdetermining also weaknesses: whereas XRFmajor gives and minor robust elements values conclusions for major about elements, e.g., sa thepphire other genesis two can techniques be drawn, focusor the onorigin trace of elements. copper bearing (Paraiba-type) tourmalines be determined [4]. The analytical arsenal used today Having the lowest detection limits, typically in the ppb range, LA-ICP-MS further exhibits an excellent includes techniques such as X-ray fluorescence (XRF), laser induced breakdown spectroscopy (LIBS), linearity ofor uplaser to ablation eight orders inductively of magnitude. coupled plasma By contrast,mass spectrometry higher concentrations(LA-ICP-MS). Each very of these often present a problemtechnologies for this technique. has its merits However,but also weaknesses: having whereas access XRF to both,gives robust major values components for major elements, as well as trace elements, isthe desirable other two techniques for obtaining focus on the trace overall elements. chemical Having compositionthe lowest detection in a limits, greater typically context. in the ppb range, LA-ICP-MS further exhibits an excellent linearity of up to eight orders of magnitude. By In our study we have focused on the determination of geographic origin of demantoids, which is contrast, higher concentrations very often present a problem for this technique. However, having the well acceptedaccess to both, trade major name components of the green as well variety as trac ofe elements, andradite is desirable (Ca3Fe 2for(SiO obtaining4)3) garnets. the overall Demantoids have first beenchemical found composition in the Russianin a greater Urals context. in the river Bobrovka near the village Elizavetinskoye, close to Niznij TagilIn (or our often study just we have called focused Tagil) on in the the determination middle of of the geographic 19th century origin [of5, 6demantoids,]. Further which deposits in the is the well accepted trade name of the green variety of andradite (Ca3Fe2(SiO4)3) garnets. Demantoids Urals were later found in Poldnevskoye (Poldnevaya), Korkodin, and Ufaley—all within a range of have first been found in the Russian Urals in the river Bobrovka near the village Elizavetinskoye, 100 km ofclose Ekaterinburg. to Niznij Tagil Only (or often in thejust 1990scalled Tagil) a second in the middle major of deposit the 19th wascentur foundy [5,6]. Further in the Erongodeposits region of Namibia [7in], the which Urals togetherwere later withfound Russia in Poldnevskoye represents (Poldnevaya), most of the Korkodin, cut gems and availableUfaley—all inwithin today’s a market (Figure1)[range8]. Smaller of 100 km deposits of Ekaterinburg. of gem Only quality in the 1990 materials a second can major also deposit be found was infound Val in Malenco the Erongo in Italy [9], region of Namibia [7], which together with Russia represents most of the cut gems available in today’s Antetezambato in Madagascar [10,11], Khuzdar, Baluchistan in Pakistan [12,13], as well as in Belqeys market (Figure 1) [8]. Smaller deposits of gem quality material can also be found in Val Malenco in and Kerman,Italy [9], both Antetezambato located in in Iran Madagascar [14,15]. [10,11], Certainly, Khuzda therer, Baluchistan are many in Pakistan more [12,13], deposits as well of as demantoid garnets onin the Belqeys globe and which Kerman, are both not mentionedlocated in Iran here [14,15]. as toCertainly, this date there they are domany not more play deposits a major of role in the gem market.demantoid garnets on the globe which are not mentioned here as to this date they do not play a major role in the gem market. Figure 1. FigureDemantoid 1. Demantoid gemstones gemstones (0.75–3.23 (0.75–3.23 ct)ct) from Ru Russiassia (top (top row)row) and Namibia and Namibia (bottom row) (bottom row) represent the majority of today’s market. represent the majority of today’s market. Geologically, these deposits can be separated into two distinct groups, namely in (1) serpentinite Geologically,hosted demantoids, these deposits such as canthe beones separated from the Urals, into twoVal Malenco distinct and groups, Baluchistan namely and in(2) (1)those serpentinite hosted demantoids,originating from such skarns, as thesuch ones as the from sites in the Namibia Urals, and Val Madagascar. Malenco First and successful Baluchistan attempts and in (2) those originatingdetermining from skarns, the origin such of asdemantoids the sites based in Namibia on chemical and fingerprinting Madagascar. have Firstshown successful promising attempts results, although only simple characteristics such as the Mn/Ti ratio and Cr and/or V content have in determiningbeen used the [8,16]. origin In this of demantoidsstudy we have carried based out on systematic chemical quantitative fingerprinting trace-element have analysis shown of promising results, although only simple characteristics such as the Mn/Ti ratio and Cr and/or V content have been used [8,16]. In this study we have carried out systematic quantitative trace-element analysis of demantoid garnets from six different origins using LA-ICP-MS and subsequent chemometric modeling for targeted classification without using inclusion-based assumptions. Minerals 2020, 10, 1046 3 of 15 2. Materials and Methods Representative rough demantoid samples have been collected from Tubussis (Namibia), Ufaley (Russia), Karkodin
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