Research Article
Gemological and Spectral Characteristics of Gem-Quality Blue Gahnite from Nigeria
Table 6
UV-VIS spectral absorption bands’ comparison of the blue spinel.
| Species | Absorption bands | Source |
| Gahnite | 444 nm, 489 nm: d-d electron leap of TFe2+ (5E ⟶ 5T2) | This work | 609 nm: spin-allowed transitions of TCo2+ (4A2 ⟶ 4 T1) |
| Natural blue spinel (Fe, Zn chromogenic) | 458 nm strong absorbent band: characteristic absorption band of Fe | [24, 25] | 555 nm: related to Fe2+ |
| Natural blue spinel (Co chromogenic) | 300∼330 nm: the d electron transition of Fe3+ (6A1g ⟶ 4A1g) | [24, 25] | 440∼430 nm: the outer electron transition of V3+ | 500∼650 nm: areas of concentration of strong and pronounced peaks | 595 nm: spin-forbidden transitions of TFe2+ (5E ⟶ 3T1) | 550, 585, 625 nm: spin-allowed transitions of TCo2+ (4A2 ⟶ 4T1) |
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