Core-to-rim chemical zoning of a Paraíbatype tourmaline from Abulo, Ethiopia: Implications for origin determination

@misc{peretti2026coretorim,
  title = {{Core-to-rim chemical zoning of a Paraíbatype tourmaline from Abulo, Ethiopia: Implications for origin determination}},
  author = {A. Peretti and S. J. Oberholzer and J. Allaz and D. Günther},
  howpublished = {ChemRxiv},
  year = {2026},
  month = oct,
  doi = {10.26434/chemrxiv.15010291/v1},
  url = {https://chemrxiv.org/doi/full/10.26434/chemrxiv.15010291/v1},
  note = {CC BY},
}

TY  - UNPB
TI  - Core-to-rim chemical zoning of a Paraíbatype tourmaline from Abulo, Ethiopia: Implications for origin determination
AU  - A. Peretti
AU  - S. J. Oberholzer
AU  - J. Allaz
AU  - D. Günther
PY  - 2026
DA  - 2026/10/12
PB  - ChemRxiv
DO  - 10.26434/chemrxiv.15010291/v1
UR  - https://chemrxiv.org/doi/full/10.26434/chemrxiv.15010291/v1
AB  - Copper-bearing tourmaline of Paraíba type is now mined at Abulo in southern Ethiopia. We report the core-to-rim characterisation of one colour-zoned crystal attributed to this locality, obtained in Bangkok in June 2026. A sawn slab about 12 mm across and 5 mm thick was e x t e n s i v e l y analysed by electron microprobe (113 points), LA-ICP-MS analysis (45 spots on two traverses and 58 isotopes, totaling 2,565 concentration values with silicon as internal standard and GSE and NIST 610 as external standards), LA-ICP-TOFMS imaging (13,500 measuring spots of 24 µm and 55 isotopes, totalling 729,000 concentration values with silicon as internal standard and GSE and NIST 610 as external standard, about 505,000 of them above limits of detection), Raman spectroscopy (24 spectra) and a screening test by ED-XRF. The crystal is an Al–Li tourmaline of the elbaite series, including elbaite, fluor-elbaite, and elbaite with a liddicoatite component. Fluorine at the W site is 0.385 apfu in the core and exceeds the 0.5 apfu boundary to fluor-elbaite in the zones next to the core (0.51–0.53 apfu); 25 of 111 points lie within 1of that boundary. Ca/(Ca + Na) increases from core to rim but never exceeds σ 0.43. Lithium, measured by LA-ICP-MS, increases from about 7,840 µg/g in the core to about 10,000 µg/g in the outer zones. With this lithium taken zone by zone, boron and H O taken from 2 the formula and the unanalysed trace elements added as oxides, the analytical totals of the microprobe are 99.2 ± 0.5 wt.% over 111points.The trace elements are quantified on this measured matrix, with silicon measured by microprobe as internal standard using GSE and NIST610 as external standards. To our knowledge for the first time, silicon concentrations were measured across a growth profile of this material, in each of the five growth zones of the crystal. Themeasurementsprovide36.3 wt.% SiO2 in the core, 35.2–35.4 wt.% in the dark-blue zone, 35.5–35.8 wt% in the "neon"-blue zone, 35.9 wt% in the pink zone and 36.0 wt% in the yellowbrown zone. The concentrations of the trace elements are significantly zoned. Copper zone averages decrease from 16,200–25,700 µg/g in the core and the dark-blue zones around it to 1,420–1,820 µg/g in the yellow-brown outer zone. The iron concentration d e c r e a s e sfrom 4,130–4,150 µg/g to below detection. The bismuth concentration increases from 26–27 to 1,370 µg/g and the lead concentration from 146–160 to 1,650–1,810 µg/g. I nc o n t r a s t ,Gallium (629–741 µg/g as zone averages) and vanadium (58–158 µg/g at all 45 spots) vary far less. The element ratios used in origin determination therefore change by up to an order of magnitude inside this one crystal. For example, Cu/Ga changes from 2.3 to 41, Pb/Bi from 1.2 to 5.9 Peretti, Oberholzer, Allaz and Günther: Colour-zoned Paraíba-type tourmaline from Ethiopia. Draft of 9 October 2026 2 / 78 and Fe/Mn starts below detection to 1.1 w h i c ha r ecalculated as zone averages. The imaging confirms the element ratios of the spot analyses and separates the elements that are present in the tourmaline's composition from those carried by inclusions, which are detected above the matrix composition of the tourmaline. Of the rare earth elements, only the light rare earths, La to Sm, are present in the tourmaline's composition. Yttrium is also present in the tourmaline, at about 0.6–0.9 µg/g outside the core and lower in the core. The other elements found in the tourmaline include Li, Be, Sn, Ga, Nb, Ta and B, all of which are typically found in LCT pegmatites. Compared with our 2009 reference profiles for Brazil, Mozambique and Nigeria, the crystal from Ethopia has higher vanadium and gallium concentrations and evolves toward bismuth rather than manganese. A high vanadium concentration is independently reported for Ethiopian material by other laboratories, and the vanadium test proposed there holds across the growth zones of this crystal, with the core at the limitofdetection. The gallium difference from Mozambique is small, whereas the published Ethiopian gallium concentration extends to 280 µg/g. Because the comparison is based on a very detailed analysis of five growth zones, the reported observations are very significant and represent more information than the analyses of larger sample sets. Therefore, this information is crucial as origin criteria. Individual analyses of samples could represent mixtures of various growth zones and would not easily be identified withoutknowledgeaboutthegrowth stage. Any discriminant therefore has to be implemented into the growth-stage concentration profiles of this study as reference. A limited Raman data set, placed on the element maps, shows that the framework bands shift with the copper concentration and that the OH bands follow the fluorine and lithium concentrations; these relations are shown for one crystal.
ER  -