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Investigating the colour of spinel: 1. Red gem-quality spinels (“balas”) from Ratnapura (Sri Lanka)

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Abstract

The rationale of red colour in spinel was studied by an energy dispersive X-ray microfluorescence apparatus (μ-EDXRF), equipped with the Kumakhov polycapillary focusing optics, on a set of 13 rough gemstones from the Ratnapura beds (Sri Lanka). Unpolished and uncut cm-sized stones were scanned non-destructively over 50 μm wide areas of natural flat surfaces sequentially oriented according to four different settings, which are all related to the octahedron form. All gemstones show slightly variable contents of Cr, Ga, Fe, Ti, V, and Zn, but they are substantially homogeneous. Mg could not be determined, whereas Al, although poorly reflecting, could be measured accurately enough, owing to its very high amount. Argon was detected too, but it was disregarded as being due to air contamination. Minor elements show a remarkably regular relationship with the Al contents, but for deviations that are systematic in some gemstones and are random outliers in others. These behaviours suggest the presence of occasional small substitutions in the tetrahedral A site that are additional to the octahedral B site substitutions characterizing the gem set. The entire μ-EDXRF data set was processed by multivariate statistical methods (cluster analysis, principal components and discriminant analysis) to establish the main chemical reason of the red colour and of the hues visibly modifying that colour in some gems. Statistical methods converge into identifying four groups of gemstones, one of them including most samples and the other three consisting of one gemstone each. Statistics also helps in explaining their mutual differences: the red colour characterizing all these spinel gemstones essentially derives from the random isovalent substitution of Al by Cr + Fe + V, whilst V in excess contributes to hue through Frenkel defects based on the heterovalent substitutions of 2Al by Zn + V.

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Acknowledgments

The μ-EDXRF apparatus is a prototype conceived, set and maintained by a cooperation between INFN and UNISANTIS. We thank Unità Funzionale di Fisica Sanitaria e Medicina del Lavoro, INFN-LNF, for letting us use it. This study is an extended and improved version of the dissertation presented by AR to Università Roma Tre to fulfil the requirements to her degree in Geological Sciences. The careful revision of a unknown referee and, particularly, the very valuable comments and suggestions of the handling editor greatly improved this manuscript, while stressing our mind to conceive future possible developments.

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Authors and Affiliations

  1. INFN, Laboratori Nazionali di Frascati, Casella Postale 40, 00044, Frascati, RM, Italy

    Astrik Gorghinian, Annibale Mottana, Adolfo Esposito & Augusto Marcelli

  2. Dipartimento di Scienze Geologiche, Università degli Studi Roma Tre, Largo S. Leonardo Murialdo 1, 00146, Roma, Italy

    Annibale Mottana, Anastasia Rossi & Florentina Maria Oltean

Authors
  1. Astrik Gorghinian
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  2. Annibale Mottana
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  3. Anastasia Rossi
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  4. Florentina Maria Oltean
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  5. Adolfo Esposito
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  6. Augusto Marcelli
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Correspondence to Astrik Gorghinian.

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Gorghinian, A., Mottana, A., Rossi, A. et al. Investigating the colour of spinel: 1. Red gem-quality spinels (“balas”) from Ratnapura (Sri Lanka). Rend. Fis. Acc. Lincei 24, 127–140 (2013). https://doi.org/10.1007/s12210-013-0223-7

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