Publication details
Black Spinel-A Gem Material from Bo Phloi, Thailand
| Authors | |
|---|---|
| Year of publication | 2020 |
| Type | Article in Periodical |
| Magazine / Source | Journal of Gemmology |
| MU Faculty or unit | |
| Citation | |
| Web | https://www.univie.ac.at/Mineralogie/docs/BlackSpinel_JoG2020_37_1.pdf |
| Doi | http://dx.doi.org/10.15506/JoG.2020.37.1.66 |
| Keywords | Bo Phloi gem; Crystal chemistry; Thailand |
| Description | The Bo Phloi gem field in Kanchanaburi Province, western Thailand, is renowned mostly for its blue sapphire. Corundum production has now virtually ceased, so local gem cutters are focusing on high-quality black spinel, which is abundant from past stockpiles, to produce various jewellery items and carvings. A detailed mineralogical characterisation of this material showed that it is spinel sensu stricto (MgAl2O4) with elevated Fe contents (20.7 +/- 0.9 wt.% FeO, or Mg:Fe atomic ratio about 2:1). The enriched Fe causes the dark colouration of the material. Based on Mossbauer spectroscopic results, Fe occurs in both divalent and trivalent states, the latter occupying both (four-and six-coordinated) cation sites in the crystal structure. The Fe3+ contributes to a 'partially inverse' occupation of the cation sites. As a result, the Raman spectrum of Bo Phloi material does not resemble that of 'normal' spinel, but rather is similar to the spectra of other spinel-group minerals that tend to show some inversion in their cation-site occupation, such as hercynite, magnetite and magnesioferrite. |