Table 1. Observational and measured properties

Infrared reflectance spectroscopy:

The infrared reflectance spectra were collected from different spots on the crystal, all yielded the same spectrum pattern as shown in figure 2. Gahnite reference spectra have been rare, one was published (Brunel & Vierne, 1970)^[2], the collected spectrum perfectly matched it, unambiguously identifying the 0.24 ct as gahnite. It is important to note, the spectrum significantly differ of that of spinel commonly found in gemology (see spinel database).

UV-VIS-NIR spectroscopy:

Since gahnite forms in cubic system, it is an isotropic mineral, therefore there was no need to get polarized UV-Vis-NIR spectra. The collected spectrum (figure 3) was rather complex. At a first glance, it was easy to recognize the Co²⁺/Fe²⁺ pattern in the 520-800 nm range and also the Fe³⁺ band at 462 nm (spinel database). The spectrum's characteristic was the wide band between 400 and 520 nm which was unsual and which formed with the Co²⁺/Fe²⁺ bands above 550 nm a transmission window centered around 520 nm giving the green color to the gemstone. The absorbance, being quite high even in the transmission window, explained the dark-green color, few light being transmitted.

The spinel absorption spectra have been studied for years to try to explain the right bands' assignments. Cobalt-bearing spinels have always risen interest and the cobalt spectra^[3],[4],[5] are teached to new gemologist since their characteristic pattern are even clearly observable using an handheld spectroscope. Many works about iron, cobalt, chromium and even manganese interplay in the absorptions were published by Taran et al. 2009^[6], D'Ippolito et al. 2013^[7], 2015^[8].

The bands at 542, (548), 580 and 625 nm are ascribed to d-d spin-allowed transitions ⁴A₂(F) → ⁴T₁(P) of tetrahedrally coordinated Co²⁺, and that at 575 and 595 nm are ascribed to spin-forbidden transitions ⁵E → ³T₂ of tetrahedrally coordinated Fe²⁺.  The band at 670 nm is ascribed to mechanisms involving Fe^2+/3+ pairs. Unlike in some other spinel, the wide band at 925 nm ascribed to the spin-allowed transitions ⁵T_2g → ⁵E_g of Fe²⁺ was not present.

As mentioned earlier, the characteristic of the spectrum was the wide band (or bands combination) between 400 and 520 nm. Verneuil synthetic grayish-blue spinel sometime shows a wide but most of time weak band either centered around 430 nm some other a narrow one around 480 nm usually attributed to Co²⁺. Cr³⁺/V³⁺-bearing spinel have 'globally' two bands, one at 390-420 nm and another one around 540 nm, producing the red color. In the gahnite spectrum the band seemed to be centered around 460 nm as the narrow Fe³⁺ 462 nm band. Was the band's side towards violet and UV real or was it an measurement error? The Cr³⁺/V³⁺ bands can mix/superimpose with Fe^2/3+/Co²⁺ bands without being distinctively observable. Chromium is present in the area of the collection of this gahnite since chromium minerals are found there such as eskolaite, a chromium oxyde (Cr₂O₃)^[1]. So, the cause of the color is still unclear, Fe^2/3+/Co²⁺ or Fe^2/3+/Co²⁺/Cr³⁺/V³⁺?

Photoluminescence spectroscopy:

The photoluminescence was studied with the three excitations sources: 405, 444 and 532 nm, the corresponding spectra are shown in figure 4. The 405 nm excitation produced an 'organ pipe' structure of emissions with multiple narrow peaks such as the 677, 687 and 699 nm ones. The organ pipe structure was rather wide compared to spinels since distinct emissions could be observed near 640, 650 and 655 nm, they were attributed to Cr³⁺ as for spinels. The 444 nm excitation induced the same organ pipe luminescence, significantly weaker than that induced by 405 nm source, but the most prominant peaks were the 650 and 655 nm ones. The other characteristic was the 512 nm emission in the green usually attributed to Mn²⁺ (⁴T₂(⁴G)→⁶A₁(⁶S), the absorption ⁶A₁(⁶S)→⁴T₂(⁴G) being between 440-460 nm). The 532 nm excitation induced a significant peak at 655 nm, that in the particular case was attributed to Co²⁺, weak peaks were also observed on the 655 nm peak's wing at 677, 687 and 699 nm assigned to Cr³⁺. Co²⁺ was said to be a luminescence quencher as Fe²⁺ and Ni²⁺, but according to Deren et al. 1991^[9] and Sardar et al. 2002^[10], Co²⁺ was responsible for such emission, anyway I have personally observed it only in spinels having Co²⁺ as shown by their absorption spectrum. It was not clear if vanadium, if present, could play a role in the emission, litterature is missing on the subject.

Conclusion:

Green gahnite is unusual it was worthwhile to study it, its color was not fully explained since it was not clear if Cr³⁺ and or V³⁺, and or even Mn²⁺ contributed to it but there is no doubt about Co²⁺ and Fe²⁺. Photoluminescence spectroscopy demonstrated the presence of Mn²⁺, Co²⁺ and Cr³⁺ (possibly V³⁺).