Gemological and Spectral Characteristics of Gem-Quality Blue Gahnite from Nigeria

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Introduction
Te spinel supergroup has a complex composition with the general chemical formula AB 2 X 4 , where A and B are usually divalent and trivalent cations, respectively, and X is an anion.According to the diferent X anions, it is divided into the oxyspinel group(X � O 2− ), thiospinel group(X � S 2− ), and selenospinel group(X � Se 2− ) [1].Te spinel-type structure can be described as a slightly twisted cubic close-packed array of X anions, with the A and B cations occupying 1/8 of the tetrahedral coordination sites (T) and 1/2 of the octahedral coordination sites (M), respectively, so that the structure of the oxyspinel group can be described as When the B cation is aluminum, it is the aluminum spinel subgroup, in which magnesium and iron ions can be mixed in any proportion.Te  Gahnite, ideally with the chemical formula ZnAl 2 O 4 , has cubic symmetry and is a less common member of the spinel subgroup.Gahnite is mainly seen in material science studies, but no pure meta gahnite has been observed in nature [2][3][4][5][6], and only a few Zn-rich spinel minerals close to the ideal composition have been reported, and Fe 2+ substitution, as well as small amounts of Mg 2+ , Mn 2+ , Fe 3+ , and Cr 3+ , is also commonly found in samples, with Fe-and Zn-rich spinels commonly appearing as opaque black spinels.Although the chemical composition and petrogenesis of natural spinel [7] and the physical properties of synthetic gahnite [8][9][10] have been studied extensively by previous authors, systematic analysis of natural gahnite is rare, and its mineralogical and spectroscopic characteristics are rarely reported.Terefore, in this paper, single crystal gahnite from Nigeria was analyzed by basic gemological experiments, electron probe experiments, infrared refectance spectroscopy, laser Raman spectroscopy, photoluminescence spectroscopy, and UV-VIS spectroscopy to reveal its gemological attribution and spectroscopic characterization.

Description of Material.
Te experimental material of this project is 15 gahnite from Nigeria (see Figure 1), and the sample number is S01∼S15.Te samples were all produced in the Jemaa region of Nigeria (locally known as Gidan Wiya), where this gem-quality blue octahedral crystal was produced in a complex pegmatite 3.2 km northwest of the site, which is 18 meters wide, trending north-south and surrounded by dark hornblende banded gneiss.Other pegmatites in the area also produce dark green opaque gahnite [11].

General Gemological Analysis.
A gemological microscope, refractometer, single plate balance (hydrostatic weighing method), UV fuorescent lamp, and Charles flter (CCF) were used to observe the general gemological characteristics of the samples.

Laser Raman Spectroscopy (RS).
LabRAM HR Evolution high-resolution Raman spectrometer from Horiba, France, was used to perform Raman spectroscopy on the samples.Test conditions: excitation light source 532 nm, power 50 mW, confocal pinhole 100, objective 50x, grating inscription density 600 gr•mm −1 , measurement range 100∼1600 cm −1 , acquisition time 12 s, and cumulative times 3; silicon calibration was used before the test.

Photoluminescence (PL).
Te photoluminescence spectra of the samples were tested using a PL-Image instrument from Biaoqi Optoelectronics Technology Development Corporation (Guangzhou, China).Test conditions: laser light source wavelength 405 nm, integration time 200 ms, average times 20, smoothing width 2, and waveband 420∼980 nm.

Ultraviolet-Visible Absorption Spectroscopy (UV-VIS).
Ocean Optics GEM-3000 UV-VIS spectrometer was used to test the UV-VIS absorption spectrum of the samples.Test method: refection method.Test conditions: integration time 120 ms, averaging times 20, smoothing width 5, and waveband 220∼980 nm.Data processing: refectance was converted to absorbance using OPUS software.
Te above experiments were done in the gemstone and process materials laboratory at Tongji University.

Electron Microprobe Analysis (EPMA).
Te electron probe microscopy analyzer type JEOL JXA-8230 of Nippon Electronics Corporation was used to quantitatively analyze the mineral major elements of the sample.Test conditions: accelerating voltage 15 kV, electron beam current 10 nA, and electron beam class diameter 5 μm.Natural and synthetic minerals or oxides are used as standards, and data processing is performed using ZAF correction methods.Te experiment was completed at the State Key Laboratory of Marine Geology, Tongji University.

Results and Discussion
3.1.Gemological Characteristics.Samples magnifed observation of more visible surface triangular etching and triangular growth seats (see Figure 2), refractive index RI 1.792∼1.794,a strong vitreous luster, and generally high transparency.Long and short-wave UV fuorescence is inert, with no change under the Charles flter under incandescent light.Te specifc gravity is 4.45∼4.66(see Table 1).
Comparing the RI value of 1.718 and SG value of 3.06 for common gem-quality magnesia-alumina spinel, it can be found that due to the high zinc content in the mineral, the refractive index and relative density values increase correspondingly with the increase of zinc-to-magnesia-like substitution (zinc atomic mass 65.38 and magnesium atomic mass 24.31).

Chemical Composition.
Te chemical composition of 15 gahnite was tested using an electron microprobe, and the results are presented in Table 2.   Te data show that the main chemical composition of the Nigerian gahnite samples is Al 2 O 3 and ZnO with an average composition of 55.64% and 40.32%, containing a small amount of FeO with an average composition of 2.33%, in addition to trace amounts of Na, Mg, Co, Mn, Cr, Cu, Si, K, and Ca elements.Te chemical formula was calculated to be Zn (0.90) Fe (0.06) Al (1.99) O 4 , mainly composed of 93.57% ZnAl 2 O 4 , and the other end elements of the aluminum spinel subgroup contained less content, respectively, 6.13% FeAl 2 O 4 , 0.19% MgAl 2 O 4 , and 0.11% MnAl 2 O 4 .Terefore, the gahnite in this experiment belongs to purer gahnite end elements.

Infrared Spectroscopy.
Te infrared refectance spectra of gahnite S01 to S15 were tested, and the results are shown in Figure 3.
Te Nigerian gahnite samples all have midinfrared absorption bands near 510 cm −1 , 559 cm −1 , and 664 cm −1 , which are consistent with the infrared spectral characteristics of oxide minerals, showing a small number of wide bands below 800 cm −1 .According to the theoretical analysis, the spinel group minerals have four infrared active vibrational peaks, all expressed as T 1u , which are called ] 1 , ] 2 , ] 3 , ] 4 , according to the frequency.Te infrared spectra of the minerals in the group are characterized by two strong and wide absorption bands from 400 to 600 cm −1 and two sharp and weak bands below 400 cm −1 .
Te absorption frequencies of the gahnite samples in this paper were compared with those of the previously studied gahnite (see Table 3).Terefore, the absorptions at 510 cm −1 and 559 cm −1 shown in this paper belong to the Zn-O stretching vibration and its bending vibration, respectively, which are characteristic absorption peaks of Zn-O.Te absorption at 664 cm −1 is attributed to the bending vibration of Al-O.Although the absorption peaks in the fngerprint region of 400∼1000 cm −1 are present in all the spinel group, gahnite is diferent from natural Mg-Al spinel with absorption peaks at 544 cm −1 and 584 cm −1 caused by Al-O stretching vibration and 727 cm −1 and 845 cm −1 caused by Mg-O stretching vibration [13], and these characteristic peaks may be red-shifted with Co 2+ , Fe 2+ , and Zn 2+ -like substitution of Mg 2+ in the [MgO 4 ]tetrahedra [14].

Raman Spectroscopy.
Te Raman spectra of gahnite S01 to S15 were tested, and the results are shown in Figure 4.
Te theory concludes that the simple positive vibrational modes of the general oxide spinel group are A 1g + E g + 3T 2g + 4T 1u + T 1g + 2A 2u + 2E u + 2T 2u , where A 1g + E g + 3T 2g are the fve Raman active vibrational mode and 4T 1u is infrared active.Te characteristic absorption peaks of the Raman spectra of Nigerian gahnite show three of the fve Raman active modes of the spinel group.Te samples all   mainly acts in the low-frequency part below 250 cm −1 , while the part over 250 cm −1 is mainly afected by O and Al, and the contribution of O is greater than that of Al [15], so the above characteristic absorption peaks are caused by the Al-O bending vibration of the (AlO 6 ) octahedron, which belongs to the characteristic Raman spectral peaks of gahnite.Te Raman spectral peak data of gahnite in this work were compared with those of ZnAl 2 O 4 crystals (theory), natural blue spinel (Fe, Zn chromogenic), natural blue spinel (Co chromogenic), natural pink spinel (Cr chromogenic), and synthetic spinel (see Table 4).As can be seen from the table, although the Raman spectra cannot distinguish between the diferent colors of spinel, they can efectively spinel and gahnite can be efectively identifed among the natural blue spinel.Raman spectroscopy is efective as a means of characterizing the ordered-disordered phase transition of spinel [17]: no 727 cm −1 Raman peak for the symmetric stretching vibration attributed to (AlO 4 ) tetrahedra in gahnite, no structural disorder observed, and no 226 cm −1 Raman peak associated with the Mg-Al disorder process as observed in spinel and the N 3 peak 720 cm −1 that would strengthen with increasing heating temperature, so the gahnite of this experiment is highly ordered.

Photoluminescence Spectroscopy.
Te photoluminescence spectra of gahnite S01 to S15 were tested, and the results are shown in Figure 5.
Five photoluminescence peaks at 675 nm, 686 nm, 698 nm, 708 nm, and 717 nm were present in all the samples in the 650∼750 nm band, although there were intensity differences in the fuorescence emission lines of diferent samples.Grinberg et al. have revealed that lasers of wavelengths less than 600 nm can excite Cr 3+ ions in the crystal feld of spinel-phase strong octahedral crystals [18], which results in fve absorptions visible in the 650∼750 nm band [19]: (I) 650∼680 nm, (II) 680∼690 nm, (III) 690∼702 nm, (IV) 702∼712 nm, and (V) 712∼725 nm, which is consistent with the test results in this paper.Electron microprobe analyses showed that the gahnite samples all contained unequal amounts of Cr 2 O 3 (Table 2), and thus, the gahnite showed a photoluminescence spectrum that was highly similar to that of the other chromium-doped spinel species [19][20][21].
Widmer et al. [20] referred to the 2 E ⟶ 4 A 2 fuorescence emission lines produced by spin-forbidden transitions of Cr 3+ ions in the spinel structure as R-and N-lines, of which the R-line of the gahnite in the present study is located at 686.3 nm, whereas that of the natural spinel is usually located at 685.6 nm, and the diference in the position of the R-line should be attributed to the diferences in the cell parameters of the diferent spinel species.
Malícková et al. grouped the fuorescence spectra of Cr 3+ in natural spinel into three categories [19]: (a) purely electronic R-lines, known as the most intense luminescence centers, (b) a series of N-lines associated with eight pairs of Cr 3+ -Cr 3+ interactions related to the disruption of shortrange ordering of crystals [22], and (c) a series of phonon side bands on the low-energy side of the R-lines (R-PSB), related to the vibrational modes of spinel crystals.
Due to the strong octahedral position preference energy of Cr 3+ , it is almost certain that in the spinel structure Cr 3+ ions displace Al only at the octahedral position (M), behaving as an ideal [CrO 6 ] octahedron, i.e., Cr {ideal}, at which point the PL spectra contain only the R-line as well as the R-PSB [21].
In reality, however, Cr 3+ ions are also present in tetrahedral positions (T) causing lattice distortion.Te degree of inversion increases, the lower the proportion of Cr {ideal}, the higher the degree of lattice disorder, and the stronger the corresponding N-lines and the weaker the R-lines.Te photoluminescence spectra of natural spinel at room temperature in Table 5 show this characteristic of Cr 3+ ion inversion.Te gahnite sample in the II region only sees the obvious and sharp R-line at 686.3 nm, without splitting the N-line, and almost no broadened spectral lines and split peaks.Combined with the Raman spectra, it can be shown that the Zn and Al of the gahnite samples of the present study are almost completely ordered in the T-site and M-site of the tetrahedral and octahedral coordination.
3.6.Ultraviolet-Visible Absorption Spectroscopy.Te wide variety of color variations displayed by the spinel group is due to its structure's ability to accommodate many transition metal cations of diferent valencies distributed in tetrahedral and octahedral coordination in the spinel structure.Te color of spinel is infuenced by the element type, valence state, and occupancy, with the color-causing element in blue spinel often thought to be primarily iron or cobalt.Te gahnite in this experiment contains FeO and CoO at an average of 2.33% and 0.04%, respectively.
Te UV-VIS spectra of gahnite S01 to S15 were tested, and the results are shown in Figure 6.

Journal of Spectroscopy
Te samples all showed absorption bands at 400 nm, 444 nm, 489 nm, and 609 nm, among which 444 nm and 489 nm were the most obvious, which were caused by the d-delectron leap of T Fe 2+ ( 5 E ⟶ 5 T 2 ) [19,[24][25][26], and the absorption band at 400 nm should be due to splitting.Te blue-gray hue of the sample is mainly caused by the spinforbidden electronic transitions in T Fe 2+ and M Fe 2+ ↔ M Fe 3+ , and the smaller color saturation of the sample may be due to the lower elemental content of Fe 2+ and Fe 3+ [27,28].
N-order derivative spectra can efectively separate the mutual envelope or overlapping peaks and weak absorption shoulder peaks in the absorption spectrum and give accurate localization and discrimination.Te increase of the derivative order resolves the spectrum increase accordingly, but the signalto-noise ratio decreases.In spectral analysis, the second-order derivative spectrum is the result of the second diferentiation of the primary derivative spectrum.It is often used to improve the resolution and accuracy of the raw spectral signal and can produce a sharper response to the nature of the chemical bonds in the sample.Specifcally, the second derivative spectrum can help distinguish absorption peaks that are close to the peak top and have similar peak widths, thus improving the resolution of the peaks in the spectrum.To confrm that the weak absorption at 609 nm is related to Co 2+ , the second-order derivative spectrogram of sample S01 was processed as an example, and the results are shown (see Figure 7).Te separation of the weak absorption originally located at 609 nm can be seen by the second-order derivative spectrum showing three absorption peaks at 561 nm, 580 nm, and 609 nm, which are further identifed as the typical 500∼620 nm Co 2+ absorption band.It is because Co 2+ in the T-site in the gahnite tetrahedron permits spin-allowed transitions 4 A 2 (F) ⟶ 4 T 1 (P) [24,25].Te latest study demonstrates the efect of Co 2+ on the color of Nigerian gahnite, explaining the blue-to-green color change process during the heating process [28].Natural blue spinel (Co chromogenic) 300∼330 nm: the d electron transition of Fe 3+ ( 6 A 1g ⟶ 4 A 1g ) [24,25] 440∼430 nm: the outer electron transition of V 3+ 500∼650 nm: areas of concentration of strong and pronounced peaks 595 nm: spin-forbidden transitions of T Fe 2+ ( 5 E ⟶ 3 T 1 ) 550, 585, 625 nm: spin-allowed transitions of T Co 2+ ( 4 A 2 ⟶ 4 T 1 )

Journal of Spectroscopy
Te positions of the absorption bands of the UV-VIS absorption spectra of gahnite in this work were compared with those of natural blue spinel (Fe, Zn chromogenic) and natural blue spinel (Co chromogenic) compared in this paper (see Table 6).Comparing the natural spinel, the maximum absorption band of T Fe 2+ of gahnite drifts towards lower energy to 489 nm.

Conclusions
Nigerian gahnite samples are granular blue octahedral crystals with refractive indices (RI) of 1.792∼1.794,specifc gravity (SG) of 4.45∼4.66,and no fuorescence.RI and SG values are positively correlated with the degree of homogeneous substitution of zinc for magnesium.
Te Te infrared spectra of the Nigerian gahnite samples showed midinfrared absorption bands near 510 cm −1 , 559 cm −1 , and 664 cm −1 in the fngerprint region all corresponding to Zn-O stretching vibration, bending vibration, and bending vibration of Al-O, respectively.Te Raman spectra showed three of the fve Raman active modes of the spinel group, with characteristic Raman absorption peaks located at 418 cm −1 , 508 cm −1 , and 660 cm −1 , corresponding to E g , T 2g(2) , and T 2g(3) modes, respectively, and the higher degree of Zn and Al ordering of Gahnite in this paper was found by comparison.Te photoluminescence spectra show the common Cr 3+ -activated fuorescence splitting peaks of natural spinel, of which the 686 nm (R-line) fuorescence peak is obvious and sharp.Te UV-VIS absorption spectra showed absorption bands located at 400 nm, 444 nm, 489 nm, and 609 nm, among which 444 nm and 489 nm were the most obvious, caused by the d-d electron leap of T Fe 2+ ( 5 E ⟶ 5 T 2 ), and the blue-gray tones of the samples are mainly caused by the spin-forbidden electronic transitions in T Fe 2+ and M Fe 2+ ↔ M Fe 3+ ; the weak absorption peak at 609 nm was determined to be associated with Co 2+ by derivative spectra.

Figure 2 :
Figure 2: Surface triangular etching and triangular growth seats of gahnite.

Table 3 :
Infrared spectral absorption wavenumber (cm −1 ) comparison of gahnite.Raman absorption peaks at 418 cm −1 and 660 cm −1 , corresponding to the E g mode, high-frequency T 2g(3) mode, and another small peak at 508 cm −1 , which belongs to the medium-frequency T 2g(2) mode.According to previous studies, in the Raman spectrum of ZnAl 2 O 4 , Zn 2+

Table 6 :
UV-VIS spectral absorption bands' comparison of the blue spinel.