Optical and Luminescence Spectroscopy of Varicolored Gem Spinel from Mogok, Myanmar and Lục Yên, Vietnam

Abstract

We studied 12 crystal fragments of natural spinel from Mogok, Myanmar and Lục Yên, Vietnam. All samples were crystal fragments of various shapes and sizes and several of them had gemological quality. Studied samples are enriched in Cr, V, Fe²⁺, Fe³⁺, Zn, which are responsible for its resulting color. They could be divided into groups of V-Cr spinels with Cr 0.001–0.006 apfu, V 0.001–0.004 apfu, and Fe spinels containing increased Fe²⁺ (0.001–0.017 apfu) and Fe³⁺ (0.004–0.012 apfu). Some samples show luminescence bands at 677, 685, 697, 710, and 718 nm assigned to Cr³⁺. The optical absorption spectra of spinels were divided into two groups of V-Cr and Fe spinels based on the dominant element acting on optical spectra. The optical spectrum of V-Cr spinels can be divided into two zones (1) 420–550 nm (V³⁺ and Cr³⁺ absorption); (2) 640–1000 nm (Fe²⁺-Fe³⁺ charge transfer). The optical absorption spectra of Fe spinels can also be divided into two zones (1) 410–650 nm (Fe²⁺-Fe³⁺ charge transfer) and (2) 770–1000 nm (Fe²⁺). This variation in chromophores results in the differences in color: V-Cr spinels are pink to red, Fe spinels are in shades of blue as well as yellow and pink.

1. Introduction

Due to their wide range of intense colors, high mechanical resistance, and high thermal and chemical stability, minerals belonging to the spinel group are actively sought as gemstones, while synthetic spinel powders are widely used as ceramic pigment. The variability in color displayed by spinels is due to their ability to accommodate a wide range of transition metal cations of different valence states at their structural sites (Fe, Cr, V, Co, etc.) [1]. Spinel color is caused by impurities; without impurities it is colorless. Its color can be brown, black, red, orange, yellow, green, blue, indigo, or purple. The luster is glassy to matt, with a white scratch, the spinel is isotropic [2].

The spinel-supergroup minerals have the general formula AB₂X₄ [3], where A represents divalent and B trivalent cations at the tetrahedrally and octahedrally coordinated sites in a structure with a cubic nearly close-packed arrangement. In spinel, the A site is occupied by Mg, B contains Al, and X site is occupied by O. Most natural and synthetic spinels are disordered, Al occupies tetrahedral site, and both Al and Mg occupies octahedral sites [4]. This disorder causes defects, electron traps, and vacancies in the crystal structure that complicate the interpretation of the spectra. In nature, the spinel can slowly cool enough to be arranged [5]. The cations occupy one-fourth of the tetrahedral interstices and half of the octahedral interstices in the cubic close packed array of oxide ions (space group Fd3m) [6]. The composition as well as cation distribution of spinel minerals and materials have a strong influence on their physical properties [7], as far as light absorption in concerned [1].

The goal of this paper is a multimethodological study of twelve spinel samples from two localities—Luc Yen, Vietnam and Mogok, Myanmar. These have a different coloring and chemical composition. Chromophores in spinel were identified by electron micro-probe analysis (EMPA) and optical absorption spectroscopy (OAS) to determine the main chromophores or their combination causing the resulting mineral coloration. Similarly, the main luminophores were studied by Raman spectroscopy.

2. Geological Settings

The rich tectonic history of Southeast Asia developed during several deformation episodes related to the closure of the Paleo-Tethys Ocean and, later, to the Himalayan orogeny. The geology of northern Vietnam is dominated by metamorphic rocks (mainly medium-grade mica schist, marble, and granulitic gneisses) inherited from these two major orogenic events: the Indosinian orogeny, and the Himalayan collision [8].

All of Luc Yen’s gem deposits are in the Lo Gam zone of Northern Vietnam. The structure of this unit results from the deformation of the Himalayan orogenesis superimposed on the pre-existing Indosinian structure [9]. The Lo Gam formation consists of a sedimentary series metamorphosed into marble, gneiss, calc-silicates, mica schist, and amphibolite. These metamorphic rocks are sometimes intruded by granitic and pegmatitic dykes [9,10,11]. The marbles are mainly calcitic and interlayered with Al-, V-, and Cr-rich amphibolite. Blue spinel in these primary deposits is not associated with ruby or red spinel [12]. There are numerous deposits in this area including ruby and spinel deposits, also fine tourmaline, sapphire, amazonite, pargasite, and humite [13].

Mogok gem-rich area is located within the central part of the Metamorphic Belt in the Central Myanmar. This assemblage is composed of Paleozoic and Mesozoic high-grade metasediments and intrusive rocks [14,15,16] and forms part of the Mogok-Mandalay-Mergui belt. The Mogok Stone Tract is mainly composed of gneiss, marble, calc-silicate rocks and quartzite, intruded by various felsic to mafic igneous rocks [17]. Ruby, sapphire, and spinel are found in primary deposits (calc-silicate rocks and marbles, with spinel only forming in the latter) and secondary deposits such as alluvial and eluvial placers, as well as karstic sinkholes and caverns [18]. These are associated with beryl, actinolite, tourmaline, pargasite, phenakite, topaz, uvite, zircon, and many others [19].

3. Materials and Methods

We studied 12 crystal fragments of natural samples spinel from Mogok, Myanmar SP–M1 to 9 and Lục Yên, Vietnam SPV–1 to 3. Measurements were performed on unoriented samples.

The chemical composition of spinels and associated minerals was established using a Cameca SX100 electron microprobe in wavelength-dispersion mode at the Masaryk University (Brno, Czech Republic), Department of Geological Sciences, under the following conditions: accelerating voltage 15 kV, beam current 20 nA, and beam diameter 3 μm. The samples were analyzed with the following standards: hematite (Fe Ka), Mn₂SiO₄ (Mn Ka), TiO (Ti Ka), wollastonite (Ca Ka), andalusite (Al Ka), Mg₂SiO₄ (Mg Ka), titanite (Si Ka), chromite (Cr Ka), gahnite (Zn Ka), vanadinite (V Ka), Ni₂SiO₄ (Ni Ka), Co (Co Ka). The measured element detection limits ranged from 150 to 700 ppm. The spinel crystal-chemical formula was calculated based on 3 cations per formula unit, the Fe²⁺/Fe³⁺ ratio was calculated from the charge-balanced formula.

Raman spectroscopy was performed by a LabRAM–HR Evolution (Horiba Jobin–Yvon) spectrometer system with a Peltier cooled CCD detector and Olympus BX–41 microscope (Department of Geological Sciences, Masaryk University, Brno, Czech Republic). Raman spectra were excited by 473 nm frequency–doubled diode laser with power of 2.5 mW, dispersed by diffraction grating with density 600 gr/mm and a 520.6 cm⁻¹ silicon wafer enabled spectral calibration. Spectra ranged from 100 to 10,000 cm⁻¹ with acquisition time of 15 s per frame and 2 accumulations.

Optical (Vis-NIR) absorption spectra of samples in the region (400–750 nm) were measured with the GL Gem SpectrometerTM at room temperature in the Gemological Institute of Constantine the Philosopher University, Nitra, Slovakia. The UV absorption spectra of the polished samples within the regime 197–400 nm and resolution 1 nm, were measured using PERKIN-ELMER Lambda 35 UV/VIS double beam spectrophotometer in absorption mode, reference to air and 1 nm slit width at Faculty of Natural Sciences, Comenius University (Bratislava, Slovakia).

Both Raman and UV/Vis/NIR spectra were processed in Seasolve PeakFit 4.1.12 software. Raman and absorption bands were fitted by Voigt function with automatic background correction and Savitzky–Golay smoothing.

4. Results

We studied 12 crystal fragments of natural spinel from Mogok SP–M–1 to 9 and Lục Yên, Vietnam SPV–1 to 3 (

Table 1. Electron microprobe analyses of spinels, first provided in wt. % of elements and then recalculated on the basis of 3 anions, the Fe²⁺/Fe³⁺ ratio calculated from the charge-balanced formula.

	SP–M1	SP–M2	SP–M3	SP–M4	SP–M5	SP–M6	SP–M7	SP–M8	SP–M9	SPV–1	SPV–2	SPV–3
SiO2	0.05	0.07	0.08	0.04	0.06	0.02	0.05	0.04	0.05	0.03	0.06	0.04
TiO2	0.01	0.01	0.01	0.03	0.14	0.00	0.00	0.00	0.01	0.04	0.00	0.08
Al2O3	71.28	71.64	71.29	71.48	71.12	71.06	70.27	70.36	70.80	70.87	70.52	70.86
V2O3	0.12	0.12	0.01	0.08	0.24	0.11	0.04	0.05	0.01	0.19	0.06	0.07
Cr2O3	0.00	0.06	0.07	0.04	0.07	0.02	0.04	0.03	0.02	0.17	0.31	0.12
Fe2O3	0.14	0.03	0.20	0.11	0.22	0.15	0.65	0.36	0.38	0.22	0.46	0.35
FeO	0.12	0.05	0.05	0.00	0.11	0.00	0.85	0.65	0.11	0.08	0.00	0.31
MnO	0.00	0.01	0.00	0.00	0.00	0.00	0.01	0.00	0.00	0.02	0.01	0.02
MgO	28.12	28.23	28.19	28.25	28.30	28.39	27.34	27.49	28.01	28.08	28.18	27.92
ZnO	0.11	0.16	0.06	0.33	0.00	0.07	0.28	0.13	0.03	0.11	0.10	0.26
NiO	0.01	0.01	0.01	0.03	0.03	0.00	0.00	0.00	0.00	0.02	0.00	0.00
CoO	0.02	0.05	0.00	0.00	0.00	0.00	0.00	0.00	0.02	0.02	0.00	0.00
CaO	0.01	0.00	0.01	0.02	0.01	0.01	0.00	0.00	0.00	0.00	0.01	0.00
Total	99.98	100.44	99.99	100.40	100.31	99.82	99.54	99.12	99.42	99.83	99.71	100.02
Si4+	0.001	0.002	0.002	0.001	0.001	0.001	0.001	0.001	0.001	0.001	0.001	0.001
Al3+	1.995	1.996	1.994	1.992	1.985	1.989	1.987	1.992	1.993	1.988	1.981	1.987
V3+	0.002	0.002	0.000	0.002	0.004	0.002	0.001	0.001	0.000	0.004	0.001	0.001
Cr3+	0.000	0.001	0.001	0.001	0.001	0.000	0.001	0.001	0.000	0.003	0.006	0.002
Fe3+	0.003	0.001	0.004	0.002	0.006	0.003	0.012	0.006	0.007	0.005	0.008	0.009
Ti4+	0.000	0.000	0.000	0.000	0.000	0.000	0.000	0.000	0.000	0.000	0.000	0.000
Mg2+	0.000	0.000	0.000	0.003	0.003	0.006	0.000	0.000	0.000	0.000	0.003	0.000
B	2.000	2.000	2.000	2.000	2.000	2.000	2.000	2.000	2.000	2.000	2.000	2.000
Ti4+	0.000	0.000	0.000	0.000	0.003	0.000	0.000	0.000	0.000	0.001	0.000	0.002
Fe2+	0.002	0.001	0.001	0.000	0.000	0.000	0.017	0.013	0.002	0.000	0.000	0.003
Mn2+	0.000	0.000	0.000	0.000	0.000	0.000	0.000	0.000	0.000	0.000	0.000	0.000
Mg2+	0.995	0.995	0.998	0.993	0.996	0.998	0.978	0.984	0.997	0.996	0.998	0.990
Zn2+	0.002	0.003	0.001	0.006	0.000	0.001	0.005	0.002	0.000	0.002	0.002	0.005
Ni2+	0.000	0.000	0.000	0.000	0.001	0.000	0.000	0.000	0.000	0.000	0.000	0.000
Co2+	0.000	0.001	0.000	0.000	0.000	0.000	0.000	0.000	0.000	0.000	0.000	0.000
Ca2+	0.000	0.000	0.000	0.000	0.000	0.000	0.000	0.000	0.000	0.000	0.000	0.000
A	1.000	1.000	1.000	1.000	1.000	1.000	1.000	1.000	1.000	1.000	1.000	1.000

). All samples were crystal fragments of various shapes and sizes and several of them had gemological quality.

4.1. Chemical Composition

The most abundant elements in the spinel samples are Al and Mg corresponding to the mineral formula of spinel end member (

Table 2. Raman shifts (cm⁻¹) of Raman-active bands in the studied samples. Assignment of the spinel band modes was made according to [20].

Mineral	Symmetry	SP–M1	SP–M2	SP–M3	SP–M4	SP–M5	SP–M6
						141	143
Spinel	T2g	311	312	312	311	308	309
						354
Spinel	Eg	406	406	406	406	404	404
						437
Spinel	T2g	664	665	665	664	662	664
Spinel	A1g	766	767	766	766	764	763
						1005
		SP–M7	SP–M8	SP–M9	SPV–1	SPV–2	SPV–3
Calcite/Dolomite				141
Calcite/Dolomite				277
Spinel	T2g	309	310	309	311	311	310
Spinel	Eg	404	404	405	406	406	406
Spinel	T2g	664	665	664	664	665	664
Spinel	A1g	765	765	765	766	766	766
Calcite/Dolomite				1086

). Spinel is enriched in Cr, V, Fe²⁺, Fe³⁺, and Zn, which are responsible for its resulting color. V and Cr-bearing spinels (SP–M1, 2, 4, 5, and 6, and SPV–1, 2, and 3) contain increased Cr (0.001–0.006 apfu), V (0.001–0.004 apfu), and low content of Fe²⁺ and Fe³⁺. Fe-bearing spinels (SP–M3, 7, 8, and 9) contain increased Fe²⁺ (0.001–0.017 apfu), Fe³⁺ (0.004–0.012 apfu).

4.2. Raman Spectra

The Raman bands of spinel (Figure 1) were assigned according to [20] (

Table 3. Color of the spinel samples and dominant chromophore.

Sample	Provenance	Color	Determined Chromophore
SP–M1	Mogok, Myanmar	Light grayish pink	Vanadium–chromium
SP–M2	Mogok, Myanmar	pale orange	Vanadium–chromium
SP–M3	Mogok, Myanmar	very pale pink	Iron (Fe3+)
SP–M4	Mogok, Myanmar	purplish pink	Vanadium–chromium
SP–M5	Mogok, Myanmar	very dark red	Vanadium–chromium
SP–M6	Mogok, Myanmar	very soft orange	Vanadium–chromium
SP–M7	Mogok, Myanmar	very dark cyan	Iron (Fe2+)
SP–M8	Mogok, Myanmar	dark blue	Iron (Fe2+)
SP–M9	Mogok, Myanmar	light grayish yellow	Iron (Fe3+)
SPV–1	Lục Yên, Yên Bái, Vietnam	reddish pink	Vanadium–chromium
SPV–2	Lục Yên, Yên Bái, Vietnam	strong pink	Chromium–vanadium
SPV–3	Lục Yên, Yên Bái, Vietnam	grayish pink	Chromium–vanadium

). The 308–312 and 662–665 cm⁻¹ band are attributed to T_2g symmetry, 404–406 cm⁻¹ band to E_g symmetry, 763–767 cm⁻¹ bands to A_1g symmetry. The measured values have deviations of up to 5 cm⁻¹ compared to the reference literature. Measured Raman spectra also differ from published by the absence of some vibrations, depending on the sample orientation relative to the laser beam.

4.3. Luminescence Spectra

Luminescence spectra of the spinel samples SP–M1 to 9 and SPV–1 to 3 was measured on the Raman spectrometer with 432 nm laser. The most intensive region in the luminescence spectra (Figure 2) of spinels is between 650 and 730 nm, with the most intensive bands at 686–687 nm. The luminescence region can be divided into five parts: (1) region 650–680 nm with the most intensive luminescence in the area 675–677 nm; (2) region 680–690 nm with the most intensive luminescence in the area 686–687 nm; (3) region 690–705 nm with maximum at 698 nm; (4) region 705–715 nm with maximum at 707 nm (5) region 715–730 nm with maximum at 717 nm. Luminescence bands at 677, 685, 697, 710, and 718 nm were assigned to Cr³⁺ according to [21]. The luminescence spectra of spinels have a major difference in intensity, due to the influence of luminophores.

4.4. Optical Absorption Spectroscopy

The optical absorption spectra of spinels were measured separately in UV and Vis-NIR regions. All spectra display presence of the intensive broad band (absorption edge) below 300 nm which can be attributed to ligand-metal charge transfer of both O²⁻ → Fe²⁺ and O²⁻ → Fe³⁺. Moreover, SP-M1 and SPV-2 samples have also relatively broad bands, but significantly less-intensive bands at 360–380 nm caused by spin-allowed electronic d–d transitions ⁴A_2g → ⁴T_1g(F) in Cr³⁺ at the M sites.

Based on the spectral features in the Vis-NIR region, samples can be divided into two groups V-Cr (with V-Cr spinels divided according to the chromophore dominance into V-Cr or Cr-V spinels) and Fe spinels (Fe²⁺ dominated and Fe³⁺ dominated), based on the dominant element acting on optical spectra. Vanadium–chromium spinels have two major absorption bands and a third less intense absorption band (Figure 3;

Table 4. Assignment of the absorption bands for V and Cr spinel samples to chromophores.

SP–M1	SP–M2	SP–M4	SP–M5	SP–M6	SPV–1	SPV–2	SPV–3	Assignment
398		399	396	397	399	397		V3
425		424	484	439	448	424	437	V3+, Cr3+
	468		509	465				Cr3+
554	544	542	546	543	543	541	543	V3+, Cr3+
		648
782	770		773	785	784		779	Fe2+
		807				803		Fe2+

). Therefore, the optical spectrum can be divided into two zones (1) 420–550 nm; (2) 640–1000 nm. In the first zone, there are two significant absorption bands which are caused by the V³⁺ and Cr³⁺. The second absorption features occur in the infrared region due to electron intervalence charge transfers (IVCT) Fe²⁺↔Fe³⁺ and Fe²⁺ [22]. The most intensive absorption band is in the violet-blue to blue region and the green to yellow-green region. There is less intensive absorption band in the near infrared and ultraviolet regions.

The optical spectra of Fe-spinels (Figure 4,

Table 5. Assignment of the absorption bands for Fe spinel samples to chromophores.

SP–M3	SP–M7	SP–M8	SP–M9	Assignment
438	415	422	426	Fe2+
464	461	461	461	Fe3+
541	557	553	557	Fe2+
641	647	639	641	IVCT Fe2+↔Fe3+
778	775	770	795	Fe2+
	914	908	932	Fe2+

) are different, they are more complicated, and they have more absorption regions. In this case, the absorption can also be divided into two zones (1) 410–650 nm and (2) 770–1000 nm. In the first zone, there are two absorption regions at 422–464 nm and 541–647 nm, the most intense absorption bands at 541, 553 (SP–M3, SP–M8) and 461 nm (SP–M7, SP–M9). This zone belongs to IVCT Fe²⁺↔Fe³⁺ and Fe²⁺. The third region, the infrared region, with two absorption bands at 770–795 and 908–932 nm, is due to Fe²⁺ [22]. The most intense absorptions are in the violet-blue to blue, yellow green to red, and weak absorption in the ultraviolet and near infrared regions.

5. Discussion

5.1. Raman Spectroscopy

The molecular vibrations in spinel supergroup belong to the following symmetry species: A_1g(R) + E_g(R) + F_1g + 3F_2g(R) + 2A_2u + 2E_u + 5F_1u(IR) + 2F_2u, where R (Raman active) and IR (infrared active) denote spectral activity. Spinel structure has a symmetry center; therefore, the active modes exclude each other. Five of the nine optical modes, 1A_g + 1E_g + 3T_2g, are Raman active, and four, 1A_g + 1E_g + 3T_2g, are infrared active [5,20].

However, only four Raman bands have been reported for a natural spinel. The missing fifth band at 492 cm⁻¹ was obtained only in the spectrum of synthetic spinel. When the sample is heated, a band appears in the spectrum at 727 cm⁻¹ [5].

Spinel has Raman vibrational modes in the spectral region from 100 to 800 cm⁻¹. We observed strong Raman bands in the regions of 400–500 cm⁻¹, and 700–800 cm⁻¹; these were attributed to E_g and A_1g, respectively.

Some studied samples—SP–M5, 6, and 9—exhibited different bands than commonly found in published Al-spinel literature [5,23,24,25]. It is possible that during the measurement, some mineral inclusion, which was located below the surface of the sample and, therefore, not detected by EMPA, was measured.

Since Mogok spinels are formed in marbles, or carbonates, there is a probable occurrence of Ca-mineral inclusions in spinels. They were typically present as colorless, irregularly shaped inclusions. In the sample SP–M9, we recorded calcite or dolomite inclusions. Raman bands at 141, 277, and 1086 cm⁻¹ are consistent with the published data [26]. The band at 1086 cm⁻¹ is assigned to the ν₁(CO₃)²⁻ symmetric stretching mode. Bands at 141 and 277 cm⁻¹ are at lower wavelengths and are the result of external group vibrations (CO₃)²⁻, which include rotational and translational oscillation of these groups [27].

Identification of the 143 cm⁻¹ Raman band origin in SP–M6 is more complicated. It may be (CO₃)²⁻ vibration in carbonate, while other bands did not manifest in the spectrum due to crystal orientation. However, it could also be a different inclusion, possibly anatase. Weak Raman bands were reflected in the area at 510 and 635 cm⁻¹, which are consistent with the published data of spinel [26].

5.2. Luminescence Spectroscopy

Luminescence features were observed in the Raman spectra of spinel in Figure 2. Luminescence bands at 677, 685, 697, 710, and 718 nm were assigned to Cr³⁺ [21]. Highly increased intensity of spinel luminescent bands is associated with a laser effect that occurs at 685 nm [28]. The luminescence spectrum of spinel usually has three parts: (a) the purely electronic R-lines, assigned collectively to the most intense centers [29]; (b) a series of N-lines (Stokes or anti-Stokes line) associated with eight Cr³⁺–Cr³⁺ pair interactions and with Cr³⁺ at sites differing from the primary site, the concentration-dependent N-lines are caused by chromium pairs, the structure-dependent N-lines arise from six coordinated Cr³⁺ ions whose short range orders are distorted [30]; (c) series of phonon side bands on the low energy side of the R-lines that relate to the vibrational modes of the spinel crystal [29].

Thus, for a mineral to be luminescent the following three conditions must be satisfied at once: (1) a suitable type of crystal lattice favorable to form emission centers; (2) sufficient content of luminescence centers and (3) a small number of quenchers. The most important quenchers in minerals are Fe²⁺, Co²⁺, and Ni²⁺, which have intense charge-transfer bands [21], while Fe³⁺ forms luminescent centers with concentrations up to approximately 1% [31]. Ferrous iron replaces Mg²⁺ at the tetrahedral site and Fe³⁺ substitutes Al³⁺ at the octahedral site [32]. Differences in shift are minimal, but the biggest difference is in the intensity of bands (Figure 2). Different luminescence intensities are due to the variable content of Fe (Table 2) which quenches the luminescence intensity. This quenching occurs because the energy is transferred from Cr³⁺ to Fe²⁺ and is transformed to heat [33].

Spinel SPV–1, 2, and 3 samples from Vietnam have the most intense luminescence resulting from the highest content of Cr³⁺ (0.0020.006 apfu, 0.12–0.31 wt.% Cr₂O₃). The SPV–2 sample with 0.006 apfu Cr³⁺, 0.008 apfu Fe³⁺, and Fe²⁺ below 0.001 apfu has the highest intensity within all the samples studied. Other spinels from Vietnam are very similar with very intense Cr³⁺ luminescence, which cannot be effectively quenched at the almost absent Fe²⁺.

However, there is not a straightforward dependence of Cr³⁺ content and the luminescence intensity in spinels from Mogok. The SP–M2 sample has the most intensive luminescence bands. Although the Cr³⁺ is not very abundant (0.001 apfu Cr³⁺, 0.06 wt.% Cr₂O₃), the Fe²⁺ content (0.001 apfu Fe²⁺, 0.05 hm.% FeO) is not high enough for the effective quenching of the luminescence. Although the intensity of luminescence is related to the Cr³⁺ content, Fe²⁺ must also be considered. This is the case of the SP–M8 sample with a moderate luminescence intensity. The content of Fe²⁺ (0.013 apfu, 0.65 wt.% FeO) is relatively higher compared to previously mentioned samples, however, still not sufficient to completely quench luminescence of Cr³⁺ (0.001 apfu, 0.03 wt.% Cr₂O₃) and Fe³⁺ (0.006 apfu, 0.36 wt.% Fe₂O₃) luminescent centers. Similarly, in SP–M7, Fe²⁺ (0.017 apfu, 0.85 hm.% FeO) has even higher contents than SP–M8, its intensity is significantly lower. The SP–M9 sample with low amount of Cr³⁺ (Cr₂O₃ 0.02 wt.%) has the least intense luminescence; the content of Fe²⁺ (0.002 apfu, FeO 0.11 wt.%), although not the highest among studied samples, contributed to the effective reduction of luminescence. In this case, Fe³⁺ is not in the position of the luminescent center but rather contributes to its reduction.

5.3. Optical Absorption Spectroscopy

Colors of spinels can be explained by the crystal field formalism which involves predominantly ionic crystals containing ions with unpaired electrons, elements with partially filled d shells: V, Cr, Mn, Fe, Co, Ni, and Cu. From these transition elements, iron is the dominant color contributor in minerals [34].

Octahedrally coordinated d⁵ ions (Fe³⁺, Mn²⁺) exhibit no ground-state crystal-field splitting and are not expected to show any colors. Their compounds are weakly colored at the best due to other factors. The free-ion ground state of octahedrally coordinated d⁶ ions (Fe²⁺) splits into two, and these ions will show one absorption band, corresponding to a transition between these two energy levels. If all or part of this band is in the visible region, compounds containing these cations will display color. The d² (V³⁺) and d³ (Cr³⁺) ions have two free-ion terms of the same multiplicity, ³F and ³P for d², and ⁴F and ⁴P for d³. In the crystal field, the ³P or ⁴P states with the higher energy give rise to ³T_1g and ⁴T_1g states. Spin-allowed transitions to these additional energy levels give rise to three absorption bands in the case of V³⁺. In Cr³⁺ ions, the transition to the ⁴T state is at a high energy and is not recorded in the visible spectrum [35].

According to their dominant chromophore, the studied spinels can be divided to vanadium–chromium and iron spinels.

5.3.1. V and Cr Spinels

Vanadium–chromium or chromium–vanadium-bearing spinel samples (Figure 3) show a color typically changing from orange to red or magenta [22].

The optical spectra of this group spinels are approximately similar, consisting of the main absorption band, which is divided into two regions, absorption in the infrared region and sharp but weak absorption in the ultraviolet region. In Table 4, absorption bands are assigned to the dominant chromophores, or interactions. For SP–M1, SP–M2, SP–M4, SP–M5, and SP–M6 (Figure 3a–e) spinels with dominant vanadium, the first absorption bands at 424–468 and 542–554 nm were assigned to spin allowed d–d ³T_1g(³F) → ³T_1g(³P) and ³T_1g(³F) → ³T₂(³F) transitions in V³⁺ at the M sites. Chromium-enriched spinel (SPV–1-3) absorption bands at 424–448 and 541–543 nm were assigned to spin-allowed d–d transition ⁴A_2g → ⁴T_1g(⁴F) and ⁴A_2g → ⁴T_2g(⁴F) in Cr³⁺ at the M sites (Figure 3f–h). All the spectra studied had weaker and broad absorption band in infrared region, caused by oxygen to metal O^2-–Fe²⁺ electronic charge transfer transition [22].

The color of spinel depends on the concentration of chromophores and their interactions in the structure. Vanadium dominant spinels are orange, red, pink and their various shades. Orange-colored SP–M2 and SP–M6 spinels with the increased iron content have broad absorption band without significant transmission in the blue region. The pink-shaded SP–M1 and SP–M4 samples are similar, only SP–M4 has stronger transmission at 480 nm (blue region) and 620 nm causing a strong purplish pink color. Chromium content in this sample causes more intensive transmission in the yellow-green region, while SP–M1 has Cr below the detection limit. Red spinel SP–M5 has absorption band shifted to 480 nm. Transmission occurs from 600 nm to NIR region. The SP–M5 sample has the highest contents of V, Cr, and also Fe. The high iron and vanadium contents result in a very saturated and dark color of the spinel.

Spinels from Vietnam are pink to red with different shades but have usually higher Cr than V content (Table 1). The SPV–1 sample has relatively stronger absorption in blue to green region than SPV–2; thus, SPV–1 is red and SPV–2 intensively pink. The SPV–3 sample has relatively strong absorption in the whole visible light spectrum due to the highest content of Fe and the lowest contents of Cr. Consequently, the color of this sample is grey.

In the NIR region at around 770–800 nm, the studied samples display broad absorption band likely associated with Fe²⁺ spin-forbidden ⁵E → ³T₁ electron transition [22].

5.3.2. Fe Spinels

Iron in spinels occurs in two valence states—as Fe³⁺ and Fe²⁺ [34]. Iron is the main chromophore among the iron group of studied spinels. The differences in spectra result from variations in the Fe³⁺/Fe²⁺ ratio, and also from the presence of minor Cr³⁺ and V³⁺. Studied iron spinels can be divided into two types: Fe³⁺-dominant SP–M3 and SP–M9 samples (Figure 4a,d) and Fe²⁺ dominant SP–M7 and SP–M8 samples (Table 5, Figure 4b,c). Spinels containing Fe²⁺ have shades of blue (SP–M7,8) and samples with predominant Fe³⁺ content have shades of pink to yellow (SP–M3,9).

Absorption bands in spinel spectra are likely caused by electronic transitions in isolated Fe²⁺ and Fe³⁺ cations, as well as by pairs of Fe ions [36,37]. The absorption bands at 464 and 461 nm can be attributed to ⁶A_1g → ⁴A_1g, ⁴E_g (⁴G) transition in ^VIFe³⁺ [36]. However, it is not excluded that, at least partly, absorption may be intensified by exchange-coupled interaction in Fe³⁺–Fe³⁺ pairs [37]. Therefore, we cannot discriminate between electronic transitions in isolated ions and electron exchange in coupled pairs of Fe³⁺ as the cause of these band in spectra of spinels. The intensity of these band does not strictly correlate with the amount of Fe³⁺.

This discrepancy may be because in iron-enriched spinels, a part of ferric iron enters tetrahedral sites of the spinel structure. In such case, oscillator strength of the electronic transition of tetrahedral ions may be several orders of magnitude higher than in octahedral [36] and the proper absorption bands may be significantly enhanced [37]. Energy of ⁶A_1g → ⁴A_1g, ⁴E_g transition may be very similar in ^VIFe³⁺ and ^IVFe³⁺, and therefore both transitions may be seen as a single band in the spectra [37]. In the spectra of low-iron spinels, these bands may also be caused by spin-forbidden transitions of tetrahedral Fe²⁺. Therefore, in most cases this relatively sharp distinct peak in spinel spectra is a superposition of two or even three bands of close energies, caused by spin-forbidden transitions [38]. Samples SP–M7 and SP–M8 with the highest Fe²⁺ contents (0.17 and 0.13 apfu) have the most intensive absorption in this area. In contrast, SP–M3 sample has the lowest iron content from Fe–spinels and the absorption band at 464 nm is negligible.

Absorption bands at 541–557 nm in the SP–M3, 7, 8 samples belong to the spin forbidden transition ⁵E(D) → ³T₂(H) of Fe²⁺ [37,39,40]. Bands at 639–647 nm are assigned to ^VIFe²⁺–^VIFe³⁺ IVCT and/or ^VIFe²⁺–^IVFe³⁺ exchange-coupled pair transitions [39,41]. This argument is well supported by the presence of the absorption band at 908–932 nm attributed to a spin-allowed transition ⁵T_2g → ⁵E_g in ^VIFe²⁺ [39,40,41].

6. Conclusions

Spectroscopic comparison of spinels showed a direct influence of transition metals (luminophores and chromophores) on spectra. The chemical composition and chromophores, luminophores in the structure largely affects the resulting spectra. The effect of luminophores on luminescence spectra was most pronounced in intensity. The dependence of the number of luminophores and the intensity of luminescence was confirmed, where Fe acted predominantly as a quencher. We have seen the effect of quenching but also the enhancement of luminescence due to Cr³⁺. Optical spectra were divided according to the dominant chromophore into V and Cr spinels, Fe spinels. These differences visible in the spectra and the chemical composition has confirmed this division.