Chemical Composition and Spectral Characteristics of Different Colored Spinel Varieties from Myanmar

Wang, Mengwei; Wang, Mingying; Qi, Yihui; Xue, Yuan; Shi, Guanghai

doi:10.3390/min14111124

Open AccessArticle

Chemical Composition and Spectral Characteristics of Different Colored Spinel Varieties from Myanmar

by

Mengwei Wang

¹,

Mingying Wang

^1,*,

Yihui Qi

¹,

Yuan Xue

² and

Guanghai Shi

^1,3

¹

School of Gemmology, China University of Geosciences, Beijing 100083, China

²

Crystal Structure Laboratory, Science Research Institute, China University of Geosciences, Beijing 100083, China

³

The Museum of China University of Geosciences, Beijing 100083, China

^*

Author to whom correspondence should be addressed.

Minerals 2024, 14(11), 1124; https://doi.org/10.3390/min14111124

Submission received: 14 October 2024 / Revised: 30 October 2024 / Accepted: 3 November 2024 / Published: 6 November 2024

(This article belongs to the Special Issue Gem Deposits: Mineralogical and Gemological Aspects, 2nd Edition)

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Abstract

:

With the growth of the Myanmar spinel market in recent years, spinels of colors other than red, including gray spinels, have gained increasing popularity. In this study, we performed conventional gemological, spectroscopic, and chemical analyses on the less commonly studied gray, red, pink, and purple spinels from Mogok in Myanmar to investigate their chemical composition and color mechanisms. The Raman and FTIR spectral analyses indicated that the samples contained oxides of Mg-Al end-members and that the spectral peak positions of different colors were essentially the same. According to the major, minor, and trace elements of samples determined via EPMA and LA-ICP-MS, the purple and gray samples had the most prominent Fe contents, the red spinels had the highest Cr contents, and the pink samples had high V+Cr contents, with a certain amount of Fe. The UV–visible spectra indicated that the absorption spectrum of the gray samples was predominantly influenced by the Fe_tot content, particularly Fe²⁺. The color rendering of the purple spinels was also intimately associated with Fe. The absorption spectrum of the gray spinels was weaker but more concentrated at 458 nm than that of the purple varieties. Cr³⁺ and V³⁺ in the red spinels produced broad bands near 400 nm and 540 nm, respectively, while light pink spinels exhibited Cr³⁺ and V³⁺ absorption spectra but featured an additional absorption band at 460 nm due to Fe. This study complements other research on the coloration mechanisms of multi-color spinels from Mogok, especially gray spinels.

Keywords:

spinel; chemical composition; spectral characteristics; Myanmar

1. Introduction

The ideal “normal” spinel structure can be regarded as a dense cubic packing of anions, having the general structural formula AB₂O₄ and belonging to the space group Fd3m[O_h⁷] (Z = 8) [1,2], as shown in Figure 1a. The divalent cations of group A occupy the tetrahedral empty spaces in an ordered manner, while the trivalent cations of group B incorporate into the octahedral empty spaces [3,4]. The ratio of the number of tetrahedral interstices to the number of octahedral interstices is 2:1, as is the ratio of the number of tetrahedral vacancies to the number of the octahedral ones. A variant of the spinel structure is the anti-spinel structure, whose molecular formula is B(AB)O₄, as exemplified by magnetite Fe²⁺Fe³⁺₂O₄ [5]. Aluminate represents the most common spinel structure; most gem-quality spinels on the market are composed of Mg-Al oxides. Figure 1b shows a ball-and-stick model of a MgAl₂O₄ cell, with the dashed lines indicating the prismatic edges of the cell, a = b = c = 8.08 Å. Complete isomorphism or incomplete isomorphism substitution of the various trace elements can take place at their structural sites. Specifically, complete isomorphism substitution can occur between Mg²⁺-Fe²⁺, Mg²⁺-Zn²⁺, and Al³⁺-Cr³⁺.

Due to its compact crystalline structure, spinel has a high hardness, which can reach 8 on the Mohs scale, making it wear-resistant. Indeed, good durability is one of the conditions for spinel to be used a decorative gemstone. In addition, gem-quality spinels are commercially attractive due to their rich colors, high transparency, and bright vitreous to sub-adamantine luster. In the gemstone market, red “Jedi” spinels are prized for their bright tone, intense saturation, and neon fluorescence. In ancient times, high-quality red spinels were mistaken for rubies, such as the Black Prince Ruby, which was collected by the British royal family in the 14th century and was not confirmed to be a spinel until the late 18th century. In 1915, Brgg [3] and NISHIKAWA [4] conducted research on the crystal structure of spinel. With the progress of science and technology, the research on spinel has also progressed. Today, about 600 records using spinel as a keyword can be found in the American Mineralogist database. Scholars have studied spinel in a wide range of fields, including, but not limited to, gemology, spectral characteristics, chemical properties, chromogenic mechanism [7,8,9,10,11], origin identification [12,13,14,15,16,17,18,19,20,21], heat treatment, and synthetic identification [22,23,24,25].

Globally, most spinels are sourced from Southeast Asia, Central Asia, and East Africa. Vietnam is renowned for its pink-purple spinels, with gem deposits mainly located in Luc Yen, An Phoi Province, which also produces rare natural cobalt-blue spinels. Large grains of spinels, among which pink colors are common, are found at the Kuh-i-Lal mine in Tajikistan. Tanzania is famous for the red-pink-colored spinels from the Ipanko area near Mahenge. Regions such as Afghanistan, Sri Lanka, Yunnan, China, Madagascar, and Kenya also produce gem-quality spinels [26].

Among the spinels produced in Myanmar, the pink, red, and purple varieties occupy a key position in the market due to their high saturation of vibrant colors. This country also produces spinels with a variety of hues that may present gray or orange tones, and less saturated colors have recently become popular in the gemstone market. Most research has focused on the chromogenic genesis and elemental analysis of red, pink, orange, purple, and cobalt-blue spinels of various origins, with fewer studies exploring less saturated gray spinels.

For this multi-method study, we selected gray, purple, red, and pink spinels from Myanmar and investigated their basic gemological, spectroscopic, and compositional characteristics to further analyze the influence of spinel chromophores on color formation, in particular to complement previous research on the spectroscopic characteristics and chemical composition of gray spinels. The factors responsible for the coloration of the spinel samples were analyzed using ultraviolet–visible spectrophotometry (UV-Vis-NIR). The structure and mineral composition of spinel were detected via laser Raman spectroscopy and Fourier Transform Infrared Spectroscopy (FTIR). The analysis of major and trace elements were identified using an Electron Probe Micro-Analyzer (EPMA) and Laser Ablation Inductively Coupled Plasma Mass Spectrometry (LA-ICP-MS).

2. Geological Settings

During the Cenozoic, a collision between the Indian and Eurasian Blocks generated the Myanmar Block [27,28]. The formation of active tectonics was dominated by the Sagaing Fault and other strike–slip faults and was associated with the Southeast Asian continental collision and subduction setting. The Sagaing Fault, formed through the diagonal convergence of the India–Sunda Plate, runs north–south and serves as an important and active tectonic boundary between the West Burma Block and the Shan–Thai Block in Myanmar [29] (Figure 2a). Myanmar can be divided from east to west into the Indo-Burma (Myanmar) Ranges, the Wuntho–Popa Arc, the Mogok–Mandalay–Mergui (MMM) Belt, and the Shan Plateau, which are the country’s four main geological provinces [30]. The active magmatism and complex geological activities here provide ideal conditions for mineralization. With an overall S-shaped belt distribution, the MMM Belt can reach the Himalayan syntaxis from the Andaman Sea to the north, including the east of the Sagaing Fault and west of the Paung Laung–Mawchi Zone, as well as the Mogok Metamorphic Belt (MMB) and the Slate Belt (Mergui Group) [30] (Figure 2b). The MMB was influenced by the Alpine–Himalayan Orogeny via the Tethyan collision zone [26,31], where significant regional crustal thickening formed major mountain belts, driving regional metamorphism, with high-grade regional metamorphic rocks and younger intrusive rocks as the main units [32,33].

In this study, spinel samples were obtained from the area of Mogok. The Mogok Stone Tract is located in the central part of the MMB and is well known for its famous gemstone belts. The area experienced high-temperature plastic deformation with numerous faults and complex lithological variations (Figure 2c). The Kabe gneisses extensively cover the Mogok region from the northeast to the center, while most of the remaining area consists of two intrusive granite units. For example, the Kabaing granite is located in the northwest and composed of biotite and biotite–muscovite granites [34]. Another unit, the Leuco-granite, consists of syenite, nepheline syenite, and tourmaline granite. The marble–calcium silicate belt is interspersed with and distributed to the south of this unit [32]. The main composition of calc–silicate is calc–silicate rock with local outcrops of diopside marble. The Wabyudaung Marble is dominated by white coarse crystalline calcite marble, which is commonly exposed throughout the Mogok gemstone district [32,33]. Mogok spinel is produced from marble rock [35], which is frequently found with ruby and other minerals including phlogopite, scapolite, and apatite. In addition to primary deposits, secondary spinel deposits are also found in placer or residual deposits, locally known as “byon” [36].

3. Materials and Methods

3.1. Sample Presentation

As shown in Figure 3, eighteen gem-quality spinel samples were selected from Myanmar and divided into a gray series (M-G1 to M-G4, total weight approximately 5.283 ct), purple series (labeled M-V1 to M-V4, total weight approximately 4.415 ct), red series (labeled M-R1 to M-R5, total weight approximately 6.260 ct), and pink series (labeled M-P1 to M-P5, total weight approximately 6.925 ct). All 18 spinel samples were of good clarity and faceted.

3.2. Spectral Analysis

Standard gemological analyses were conducted at the Gemological Experimental Teaching Center at the School of Gemmology, China University of Geosciences, Beijing. This analysis involved the use of a Fable refractometer with a sodium light source (589 nm) and sulfur-saturated CH₂I₂ contact solution. The refractive index was measured using the myopic method and recorded as the RI value. The jewels’ relative density was determined using the pure water weighing method and documented as specific gravity (SG) values. Fluorescence was detected by exposing the spinels to UV radiation with wavelengths of 365 nm (longwave) and 254 nm (shortwave).

Laser Raman spectroscopy tests were performed at the School of Materials, China University of Geosciences, Beijing. A HORIBA LabRAM HR Evolution device was utilized to accurately concentrate the samples using a 50× objective lens. The spectra were gathered at 3 to 5 places on the polishing plane, with the excitation source adjusted to 520 nm at 50 mW. The equipment had a resolution of 4 cm⁻¹, and the test range spanned 100–1200 cm⁻¹. The data plots were drawn with OriginPro 2024 software (OriginLab, Northampton, MA, USA).

Infrared spectroscopic tests were conducted at the Jewelry Appraisal Centre of China University of Geosciences, Beijing. The instrument used for the experiments was a Nicolet iS5 FTIR spectrometer manufactured by ThermoFisher Scientific, USA, which was tested via the reflectance method at room temperature with a resolution of 4 cm⁻¹, a scanning number of 32, and a wavelength range of 400–1200 cm⁻¹. The measured infrared spectra were analyzed using Ominic software, ver. 9.2 (Thermo Fisher Scientific, Waltham, MA, USA) and OriginPro 2024.

The UV–visible spectral test used a model UV-3600 ultraviolet–visible–near-infrared spectrophotometer, manufactured in Shimadzu, Japan. The Gemmology Experimental Teaching Center of the School of Gemmology at China University of Geosciences, Beijing, conducted an examination on the samples using the transmission method. This testing involved measuring the absorbance of wavelengths ranging from 200 nm to 900 nm. The default automatic sampling interval was used along with parameter settings for a 2 nm slit width, a light source conversion wavelength of 300 nm, and a grating conversion wavelength of 800 nm.

Peak fit v4.12 software (Palo Alto, CA, USA) was used to fit the measured absorbance data to the peaks. This software adjusted the baseline processing parameter to tol = 3%, smoothing the curves by Sm < 7.5%, and assumed that the shape of all fitted bands was Gaussian during the convolution process [37], controlling the fitting coefficients at r² > 0.99. The data plots were drawn with OriginPro 2024.

3.3. Chemical Analysis

The primary elemental contents of the spinel samples were studied using quantitative electron probe micro-analysis with a Shimadzu EPMA-1720 electron probe at the Experimental Center of Science and Research Institute, China University of Geosciences, Beijing. The tests began under the conditions of an accelerating voltage of 15 kV, a beam current intensity of 10 nA, and a beam size of 5 μm. The results were calculated via the ZAF3 correction method to obtain the percentage of measured elemental oxides. The following standards were used for quantification: albite (Na), spinel (Mg, Al), diopside (Si, Ca), apatite (P), sanidine (K), rutile (Ti), vanadinite (V), chromite (Cr), rhodonite (Mn), garnet (Fe), cobaltite (Co), pentlandite (Ni), and willemite (Zn). Detection limits (d.l.) were 0.01 wt.% (Na₂O, SiO₂, CaO, MgO, Al₂O₃, P₂O₅, KO, TiO₂, V₂O₃, Cr₂O₃, MnO, FeO, CoO, NiO).

Trace element concentration was investigated by using a GeoLas HD 193 nm ArF excimer laser stripping system (Coherent, Saxonburg Boulevard, PA, USA) and Agilent 7900 quadrupole inductively coupled plasma mass spectrometer (Agilent Technologies, Santa Clara, CA, USA) at the Key Laboratory of Paleomagnetism and Paleomagnetism and Tectonic Reconstruction of the Ministry of Natural Resources [38,39]. Then, NIST610 and NIST612, as a set of standard samples, were tested under experimental conditions, e.g., using a 44 μm beam spot diameter, 2.8 J/cm² energy density, and 5 Hz stripping frequency [39]. Twenty polishing spots were selected for samples (M-G2, M-V4, M-R1, and M-P4) and were measured after every ten spots to assess the presence of trace elements. The detection limits (d.l.) apply in Appendix A Table A6.

4. Results

4.1. Basic Gemological Characteristics

Table 1 shows the results of the standard gemological analysis of all samples. The tested samples were cut to be faceted and show a sub-adamantine luster with good transparency. The measured refractive indices (RIs) range from 1.712 to 1.720 with no birefringence, and all samples are homogeneous. The refractive indices of the pink spinels are generally smaller than those of the other spinels investigated in this work. The measured specific gravity (SG) is between 3.54 and 3.62. The samples were sequentially placed in a UV fluorescent light box, and the intensity of the red fluorescence of the gray series was found to be weak to moderate. The red and purple series generally exhibited strong red fluorescence, while M-V3 and M-R4 showed weak red fluorescence under longwave light. Among the pink series samples, sample M-P5 showed bright red fluorescence, while the others showed bright pink fluorescence. The samples exhibited inert to moderate red fluorescence when exposed to shortwave light. The red spinel samples were examined by a portable spectrometer, revealing the presence of Cr spectra. The absorption bands exhibited significant variations in both breadth and intensity. The number of absorption lines visible in the red region varied depending on the observation conditions, while no absorption lines were observed in the blue region.

4.2. Spectroscopy

4.2.1. Raman Spectra

All spinel samples were analyzed via Raman spectroscopy. All samples have a chemical composition mainly composed of MgAl₂O₄. The Raman spectra of the red spinels, except for those of sample M-R4, were affected by fluorescence after 459 cm⁻¹, resulting in elevated peaks, which were corrected with the OriginPro 2024 software. The baseline was calibrated to eliminate fluorescence packet effects [19].

Three to five points were tested for each tetrachromatic spinel sample, and the range of 200–900 cm⁻¹ was selected for the collected spectra. We then plotted a comparison of Raman spectra with corrected baselines [19], as shown in Figure 4. The results indicate that all samples were unaffected by the colors, since the spectral lines show a constant trend. The overall spectral peaks were basically in the same position, but the peak intensities were slightly different. The ranges of Raman spectral peak positions among the measured samples are summarized in Table 2. The last row of the table provides a comparative analysis of the experimental data in the literature [40]. Natural spinel tends to be ordered, with energy band assignment patterns corresponding to four spectral peaks T_2g, E_g, T_2g’, and A_1g [18,41].

Figure 4a–d present the observed allocation of the four activity modes of the measured spinel. The absorption peaks at 309–312 cm⁻¹ were generated by the jump of Mg in the tetrahedral position. In the frequency range of 404–408 cm⁻¹, peaks were caused by the symmetric bending vibrations of Mg-O [19]. An enlarged view (Figure 5b) is also provided to facilitate identification of the shoulder peak near 385 cm⁻¹ [42]. This peak is attributed to the bending modes of Al³⁺ in tetrahedra [42], indicating that the main component of the samples is MgAl₂O₄ and exhibits cation disorder. The absorption peaks at 764–767 cm⁻¹ are a result of the symmetric stretching vibration of Mg-O [41,43]. The activity peak observed at T_2g’ is a result of the bending movements of the [AlO₆] octahedra [40,44,45] and/or the internal vibrations of the [MgO₄] tetrahedral unit [46]. However, there is presently no agreement on the specific mode responsible for this peak [19].

4.2.2. FTIR Spectra

The measured infrared spectra were analyzed using Ominic and OriginPro 2024 software (Figure 6). A wavenumber range of 400–1000 cm⁻¹ was selected to investigate the distribution of absorption peaks in the infrared reflectance spectra of Myanmar spinels with different colors, as tabulated in Table 3. The measured spinel was subjected to infrared spectral analysis, which revealed the presence of significant absorption bands corresponding to various crystal vibrational modes [47,48]. Since the spinel structure belongs to a perfectly symmetric isometric crystal system, where one tetrahedron shares oxygen atoms with three octahedra, neither tetrahedral nor octahedral vibrations of oxygen can occur independently [49,50,51]. The four primary spectral peaks can be divided into two categories. The first category consists of vibrations related to metal cations. For instance, the spectral peak at 542 cm⁻¹ is caused by the stretching vibration of Mg-O, while the peaks near 728 cm⁻¹ can be attributed to the stretching vibration of Al-O. The second category consists of lattice vibrations. For example, the high-frequency bands near 586 cm⁻¹ and 841 cm⁻¹ can be attributed to octahedral lattice vibrations caused by the movement of oxygen ions [47,52]. The presence of a shoulder at 473 cm⁻¹ suggests the existence of structural defects. However, further investigation is needed to determine their exact form [48].

4.2.3. UV-VIS-NIR Spectra

The UV–visible absorption wavelengths of spinel are mainly concentrated between 300 and 800 nm. Here, the data for UV–visible absorbance were normalized using OriginPro 2024 software, setting the x-axis to indicate the wavelength in nm and the y-axis the absorbance. Figure 7 shows the UV-VIS absorbance spectra of each color sample. The results show that the absorption spectra of spinels in each color group are very similar, while they differ significantly between groups.

Among the gray spinel samples, M-G3 and M-G4 are dominated by gray with a clear purple hue. The gray of M-G1 and M-G2 is relatively pure, but the absorbance spectral trends of the four samples are basically the same, showing absorption at 372 nm, 388 nm, 457 nm, and 552 nm, respectively. In purple spinel samples, the absorption peaks centered at 390 nm and 543 nm are more pronounced, with one weak absorption near 667 nm. Here, samples M-V1 and M-V4 are pinkish in color, and M-V2 has the highest purple saturation, with a large peak position difference. With the exception of M-R4, the red spinel samples exhibit prominent and wide absorption bands at 400 nm and 540 nm. These bands specifically absorb blue-violet and yellow-green light within the visible spectrum. The transmission of red light causes the spinel to appear red. M-R4 shows an orange-red tone, with the most prominent absorption peak at 392 nm. The measured pink spinel samples (M-P1 to M-P5) show absorption bands near 390 nm, 415 nm, and 540 nm, with an absorption peak at 656 nm. Moreover, the darker the color, the stronger the absorption and the more pronounced the peaks.

4.3. Chemical Composition

We selected uniformly colored spots without mineral inclusions from the samples (M-G2, M-V4, M-R1, and M-P4) with the most prominent color tone in the four-color group for the EPMA and LA-ICP-MS tests. At the same time, the sequence number was marked with 1–20 to analyze the major and trace elements. The measured data presented in Table A1, Table A2, Table A3, Table A4 and Table A5 (in Appendix A) show significant contents of MgO (27.69–29.15 wt.%) and Al₂O₃ (67.97–71.43 wt.%) in each sample, indicating that the selected samples contained a prevalent fraction of Mg-Al spinel end-member (MgAl₂O₄). However, due to the influence of isomorphous substitution, the contents of MgO and Al₂O₃ in the measured samples were generally lower than those in the ideal chemical formula. Table A1, Table A2, Table A3 and Table A4 present the structural formula of the spinel based on the average value of the measured cation number and the crystal structure characteristics of spinel-like minerals [42,53]. Cr³⁺ favors octahedral coordination due to its high spin with higher crystal field stabilization [42]. The main cations in group A are predominantly divalent cations, while in group B, there are mainly trivalent cations. Substitutions with Fe²⁺, Zn²⁺, Mn²⁺, and Mn³⁺ could also occur in group B due to a combination of factors such as ionic radius, charge, crystal field stabilization, and octahedral site preference energy [42,53]. In the calculations, the [FeO_tot] of the samples was considered as the total Fe content, independent of the valence state [54]. The number of other cations in the crystal chemical formula was calculated with the oxygen atom method [37,54]. Cr, V, Fe, and Zn in the different colored samples show higher contents than other trace elements measured by LA-ICP-MS, such as Ti and Ga.

The binary phase diagrams of the trace elements Cr, V, Fe, and Zn in the different colored spinel samples are reported in Figure 8 [19,55]. Here, the Fe contents of the gray samples are high at above 5000 ppm. The purple spinel samples are rich in Fe (4972.03–5329.65 ppm) and Zn (3811.45–4153.71 ppm), while gray spinel samples are poor in Zn (647.99–757.45 ppm). The Cr content is the most prominent in the red spinel samples (8132.58 –13,381.40 ppm), and the V content (1231.50–1745.79 ppm) is larger than that in the other three spinel color groups. Additionally, the pink samples contain higher levels of V (482.22–856.11 ppm) and Zn (969.22–1092.56 ppm) but less Cr (20.37–79.78 ppm) and Fe (480.78–561.55 ppm) compared to the levels in the other color groups.

5. Discussion

5.1. Chromogenic Mechanism

The spinel samples selected for this study contain Cr, V, and Fe as the main chromophores [10]. The gray and purple spinels are more closely associated with higher Fe contents. Under the influence of ligand-to-metal charge transfer (LMCT) O²⁻→Fe²⁺/Fe³⁺, the absorption intensity of the low-energy wing in the UV absorption band varies with Fe contents [56]. The coloration mechanism of the red and pink series is closely related to the amount of Cr and V [56].

5.1.1. Gray and Purple Spinels

The chromogenic element in gray and purple spinel is mainly Fe, which is associated with two oxidation states, Fe²⁺ and Fe³⁺, which absorb different wavelengths of visible light and undergo electron leaps to form their colors [57,58,59]. These absorption bands may be related to the spin-forbidden nature of ^TFe²⁺, ^MFe²⁺, and ^MFe³⁺, as well as the Fe²⁺–Fe³⁺ intervalence charge-transfer leaps and exchange-coupling pairs of leaps [11,56], which are subject to a combination of effects that together produce purple-colored transmission windows in the blue and red regions. When Fe is the main chromogenic element in spinel, the leap of ^TFe²⁺ generally leads to a purplish-blue color, while the color tends to be bluer with an increase in ^MFe³⁺ [37]. The Gaussian band is relatively wide in Figure 9a,b. The absorbance curves are shown in body-color tones of samples and light gray lines represent the fitted Gaussian shaped curves. The colored lines indicate the fitted bands and labeled using a-h. Due to the smooth spectral curve and uncertain shape of the high-energy fringes, there are unclear absorption bands during the fitting. Table 4 and Table 5 summarize the fitted bands and attributions of both.

In this study, Co was not considered as a chromogenic factor for the samples as it was present in minimal quantities. The gray spinel sample had poor saturation, with only M-G2 approaching a neutral gray, while the other samples possessed predominantly gray hues with a slight violet-blue coloration. Compared with the gray M-G2 spinel, the M-V4 sample had lower Fe content. Additionally, the light purple spinel was considered to be a low-Fe variety [60], but the spectral bands of both showed a consistent trend. The spectral band attributions can be mainly categorized as self-selected forbidden leaps of ^TFe²⁺, forbidden leaps of isolated ^MFe³⁺ and ^MFe²⁺–^MFe³⁺ intervalence charge-transfer translation (IVCT), and/or ^TFe²⁺–^MFe³⁺ exchange couple pair (ECP) [61,62].

Table 4. UV-Vis spectral peak positions of M-G2 (nm).

Sign	M-G2	Mozambique [61]	Attributions and Electronic Transition
a	374	371	Spin-forbidden ⁵E → ³E of ^TFe²⁺ [61]
b	393	385	Spin-forbidden ⁵E → ³T_2,³T₁ of ^TFe²⁺ [61]
c	456	458	⁶A_1g → ⁴A_1g, ⁴E_g of ^MFe^{3 +} [61]
d	537	/	Spin-forbidden ⁵E → ³T₂ of ^TFe²⁺ [60]
e	564	558	Spin-forbidden ⁵E → ³T₂ of ^TFe²⁺ [61,62]
f	655	660	^MFe²⁺-^MFe³⁺ IVCT [60,61]
g	680	/	⁶A_1g → ⁴T_2g of ^MFe³⁺ [62]

IVCT = intervalence charge-transfer translation.

Table 5. UV-Vis spectral peak positions of M-V4 (nm).

Sign	M-V4	670275 [56]	Attributions and Electronic Transition
a	375	372	Spin-forbidden ⁵E → ³E of ^TFe²⁺ [56]
b	386	387	Spin-forbidden ⁵E → ³T_2,³T₁ of ^TFe²⁺ [11,56]
c	423	409	Spin-forbidden ⁵E → ³T₁ possibly intensified by ECP transition ^TFe²⁺ [11,56]
d	463	458	⁶A_1g → ⁴A_1g, ⁴E_g of ^MFe³⁺ [11,56]
e	/	478	⁶A_1g → ⁴A_1g, ⁴E_g of ^MFe³⁺, possibly intensified by ECP transitions in ^TFe³⁺–^MFe³⁺ clusters and by spin-forbidden ⁵E → ³T₂, ³T₁, ³E in ^TFe²⁺ [56]
f	543	560	Spin-forbidden ⁵E → ³T₂ of ^TFe²⁺ [11,56]
g	579	/	Spin-forbidden ⁵E → ³T₁ of ^TFe²⁺ [11]
h	674	661	^MFe²⁺-^MFe³⁺ IVCT [56]

ECP = exchange couple pair; IVCT = intervalence charge-transfer translation.

5.1.2. Red and Pink Spinels

Cr³⁺ exists in the crystal as the 3d³ configuration, which is most stable [63]. In the Fd3m system of spinel, the octahedral sites containing Cr³⁺ undergo triangular distortion [58]. Considering the electrostatic interaction, the triangular symmetric crystal field, and the spin interaction [1], Cr³⁺ can be split from the ground state term ⁴F to ⁴A_2g, ⁴T_2g, and ⁴T_1g and further split under the action of the triangular symmetric crystal field. The spin interaction can cause high-intensity transitions and the selective absorption of visible light [64]. V³⁺ is the 3d² configuration. The ground state term ³F can be split into the three energy levels of ³T_1g, ³T_2g, and ³A_2g, enabling transitions to occur [65]. The similar lattice field coefficients of Cr and V produce similar characteristics in the spectrum and absorption band position. The orange-pink spinel’s color is mainly caused by the combined effect of transition metal cations Cr³⁺ and V³⁺ at the M point [8,9,64].

When Cr³⁺ is doped into the Al lattice of octahedral coordination, it induces a strong absorbance at 410 nm in the blue-violet region, while the transition from the ground state to the excited state induces a yellow-green absorption in the range of 550 nm. These two bands largely determine the color of the spinel, enabling it to show a reddish-pinkish tone [66]. When the content of V³⁺ is lower than that of Cr³⁺, the intensity of the two absorption bands is higher, which indicates stronger absorption of blue-violet and yellow-green light and thus a heavier red hue. If the content of V increases, the spinel will show an orange hue [67].

Figure 10a shows seven curves fitted with the M-R1 red spinel, with fitted bands labeled using a-g co-labeling. The absorbance curves obtained after de-baselining are indicated in body-color tones, the fitted Gaussian shaped curves are shown in gray, and the colored lines indicate the fitted bands.

The positions of the absorption peaks and their attributions are reported in Table 6. In the red spinel, the absorption bands resulting from ^MCr³⁺ and ^MV³⁺ spin-allowed leaps usually overlap [68], with each sub-band of the fit represented by a to f. Here, a and c are attributed to ^MCr³⁺ spinning at octahedral position d-d, resulting in electron leaps ⁴A_2g→⁴T_1g(F) [53,56] and spin-allowed electron leaps ⁴A_2g→⁴T_2g(F), resulting in absorptions at d and f [69]. The result at b is attributed to the ^MV³⁺ spin-allowed electron transition ³T_1g(F)→³T_1g(P) [53], while that at e is attributed to the ^MV³⁺ spin-allowed electron transition ³T₁(F)→³T₂(F) [26]. Combined with the LA-ICP-MS content analysis, we found that Cr content was much higher than that of V and Fe in the red spinel, while Cr dominated in its chromogenic effect. As Cr decreases and Fe increases, red saturation will decrease, with the spinel tending toward a magenta-pink color. The weak absorption at g is due to the charge transfer of Fe³⁺, resulting in a darkly colored sample with low brightness [56].

In the pink samples, the overall trend of the absorption bands and the fitted band positions were essentially the same as those in the red samples. However, weak absorption was produced between 450 and 500 nm. In total, nine curves were fitted, labeled as a.-i. (see Figure 10b and Table 7). The weak absorption produced between 450 and 500 nm is attributed to the spin-forbidden nature of ^TFe²⁺ and ^MFe³⁺ [11,70], whereas the main body is still affected by the assigned bands of Cr and V cations [56,70]. Here, the absorption position is basically the same as that of the red color, but the absorption intensity is significantly lower [56].

5.2. Trace Element Distribution of Different Colored Spinels

Gem-quality spinels often appear in a variety of colors based on the presence of various trace elements. For example, the blue color of natural spinels is due to Co and Fe [37,62]. In the present study, the Co contents determined by LA-ICP-MS were extremely low, all at the lower limit of color development (<100 ppm) [37], and could not contribute to the coloration of the gray, purple, red, and pink spinel samples.

In this study, Cr, V, and Fe are considered to be chromophores with oxidation states and also are the main chromogenic elements in the spinel samples [71]. Bivariate analysis was used to assess the potential correlation between the elements and measure the relationship between the variables using the Pearson correlation coefficient in the range of [−1, 1], where a positive value represents a positive correlation, and the opposite represents a negative correlation. The correlations were calculated using OriginPro 2024 software. In Table 8, r represents the correlation coefficient, *p is the parameter for determining the results of hypothesis testing, and *p ≤ 0.5 indicates that the results are significantly different. We observe a high negative correlation between Cr, V, and Al, suggesting that the substitution of Al³⁺ in octahedral vacancies occurs mainly in the form of trivalent cations. Negative correlations between the elements Fe and Mg show that Fe is mainly present in the form of Fe²⁺ [72].

Figure 11 shows the comparative percentage distribution of Cr, V, and Fe contents in the four color samples measured via LA-ICP-MS, expressed as a ternary phase diagram. Here, the gray spinel is clustered and distributed in the Fe corner of the ternary diagram; compared to the other colored samples, this spinel has higher Fe and lower Cr and V. The purple spinel is second only to the gray spinel in its Fe contents. The red spinel contains a significant amount of Cr and a certain amount of V. The pink spinel shows moderately low Cr, V, and Fe contents. With an increase in Fe, and a decrease in Cr and V content, the color dulls from red tones to purple tones and even gray tones.

Figure 12 shows the percentages of Cr+V, Fe, and Zn contents in the four samples. Figure 12a clearly indicates that Fe has the highest percentage among the common chromogenic elements in M-G2, indicating that the cause of gray coloration is closely related to Fe. The purple M-V4 also contains a high percentage of Fe as its main chromogenic element. We can conclude that the red coloring factors are influenced by Cr and V (Figure 12c). Cr, V, and Fe all affect the color development of the pink sample M-P4. In addition, the contents of Zn were found to be higher in the pink and purple spinel, suggesting that non-chromophore Zn can locally affect the crystal field and change the absorption of light [36].

6. Conclusions

This study analyzed four color-varied spinel groups from Mogok, Myanmar, using various techniques to focus on coloration mechanisms, based on trace element analyses and UV-Vis absorption spectra. The Raman and FTIR spectroscopy of spinel samples of different colors are basically consistent, with the main structural formula being MgAl₂O₄. Chemical analyses show that gem-quality spinels are almost exclusively composed of MgAl₂O₄, and the principal chromophores are Cr, V, and Fe; each chromogenic ion absorbs light energy at certain specific wavelengths and electron transition occurs, resulting in the formation of different colors. In this study, the red spinel is influenced by a combination of chromogenic elements Cr-V, ^MCr³⁺, and ^MV³⁺ occupying the octahedral sites of Al. Absorption in the visible spectrum is generally superimposed and shows strong absorption in the blue-violet and yellow-green visible regions, with higher Cr contents tending to be red in color. In addition, pink spinel is lighter in color due to the fact that Cr and V present lower concentrations and weak absorption of visible light, while Fe also plays a role in absorbing visible light, resulting in weak absorption at 450–500 nm. When the trace element Fe is more abundant than Cr in the body, the color changes to purple, dominated by the ^TFe²⁺ jump occupying the Mg tetrahedral sites, and becomes gray with an increase in ^MFe²⁺ and ^MFe³⁺. The main chromogenic element of gray and purple spinel is Fe, which produces absorption in the blue-green light region and orange, yellow, and red light region. However, compared with the absorption spectrum of purple spinel, the absorption of gray spinel near 458 nm is more concentrated, and the absorption near 560 nm and 660 nm is weaker. This study complements other research on the coloration mechanisms of multi-color spinels from Mogok, especially gray spinels.

Author Contributions

Conceptualization, M.W. (Mengwei Wang) and M.W. (Mingying Wang); Methodology, M.W. (Mengwei Wang) and M.W. (Mingying Wang); Resources, M.W. (Mingying Wang); Data curation, M.W. (Mengwei Wang); Writing—original draft preparation, M.W. (Mengwei Wang); Software, M.W. (Mengwei Wang) and Y.Q.; Visualization, M.W. (Mengwei Wang) and Y.Q.; Writing—review and editing, M.W. (Mengwei Wang), Y.X. and G.S.; Supervision, M.W. (Mingying Wang); Funding acquisition, M.W. (Mingying Wang) and G.S. All authors have read and agreed to the published version of the manuscript.

Funding

This research is funded by the National Natural Science Foundation of China (Grant No. 42273044).

Data Availability Statement

The article encompasses the data that are offered in this study.

Acknowledgments

We would like to thank the Gemological Institute and the Laboratory of the School of Materials of China University of Geoscience, Beijing, as well as the Jewelry Appraisal Centre, the Experimental Center of Science and Research Institute of China University of Geoscience, Beijing, and the Key Laboratory of Paleomagnetism and Paleotectonic Reconstruction of the Ministry of Natural Resources for providing experimental guidance.

Conflicts of Interest

The authors declare no conflicts of interest.

Appendix A

Table A1. Chemical composition of a gray spinel sample (M-G2) measured via EPMA (wt.%).

	P₂O₅	SiO₂	TiO₂	Al₂O₃	V₂O₃	Cr₂O₃	MgO	CaO	MnO	FeO_tot	CoO	NiO	ZnO	Na₂O	K₂O	Total
M-G2-1	<d.l.	<d.l.	<d.l.	70.65	<d.l.	0.02	28.35	<d.l.	0.03	0.67	0.03	<d.l.	<d.l.	0.01	0.02	99.77
M-G2-2	0.02	0.03	<d.l.	69.78	<d.l.	0.04	28.07	<d.l.	0.01	0.63	0.02	<d.l.	0.19	0.01	<d.l.	98.80
M-G2-3	<d.l.	<d.l.	0.06	70.35	<d.l.	0.01	28.08	<d.l.	<d.l.	0.71	<d.l.	<d.l.	0.07	<d.l.	<d.l.	99.27
M-G2-4	0.02	0.07	0.05	70.97	0.06	0.01	28.54	<d.l.	0.06	0.62	<d.l.	<d.l.	0.03	<d.l.	<d.l.	100.42
M-G2-5	0.05	<d.l.	<d.l.	70.66	0.02	0.04	28.35	0.01	<d.l.	0.65	0.02	<d.l.	<d.l.	<d.l.	<d.l.	99.78
M-G2-6	0.03	0.01	0.03	70.85	0.01	0.02	28.64	<d.l.	0.05	0.65	<d.l.	0.01	0.24	0.01	0.01	100.55
M-G2-7	<d.l.	0.02	0.06	70.51	<d.l.	<d.l.	28.25	<d.l.	0.06	0.63	<d.l.	<d.l.	0.02	<d.l.	<d.l.	99.56
M-G2-8	0.06	<d.l.	<d.l.	70.54	0.04	0.05	28.45	<d.l.	0.05	0.54	0.04	<d.l.	0.08	<d.l.	<d.l.	99.84
M-G2-9	<d.l.	0.03	0.03	70.68	0.02	0.03	28.47	0.01	0.07	0.64	<d.l.	0.02	0.12	<d.l.	0.01	100.13
M-G2-10	<d.l.	0.04	0.03	70.10	<d.l.	0.07	28.54	<d.l.	0.02	0.70	0.02	<d.l.	0.20	0.01	<d.l.	99.71
M-G2-11	0.04	<d.l.	0.01	70.14	0.01	0.01	28.31	<d.l.	0.03	0.61	<d.l.	0.05	0.13	<d.l.	<d.l.	99.33
M-G2-12	0.01	0.03	0.04	70.14	<d.l.	0.04	28.32	<d.l.	0.03	0.62	<d.l.	0.01	0.12	<d.l.	<d.l.	99.35
M-G2-13	0.01	0.01	<d.l.	70.80	<d.l.	0.02	28.47	<d.l.	<d.l.	0.62	<d.l.	0.07	0.06	<d.l.	<d.l.	100.05
M-G2-14	0.03	0.01	<d.l.	70.23	<d.l.	0.05	28.44	0.02	0.05	0.65	<d.l.	0.03	0.11	<d.l.	0.01	99.62
M-G2-15	0.04	0.05	<d.l.	70.59	<d.l.	0.02	28.34	<d.l.	0.03	0.62	<d.l.	<d.l.	0.15	0.01	0.01	99.85
M-G2-16	0.01	<d.l.	0.03	70.37	0.05	0.06	28.39	<d.l.	0.07	0.63	<d.l.	<d.l.	0.08	<d.l.	<d.l.	99.69
M-G2-17	0.02	<d.l.	<d.l.	70.80	0.01	0.07	28.37	<d.l.	0.08	0.68	0.04	<d.l.	0.10	<d.l.	<d.l.	100.16
M-G2-18	<d.l.	0.01	0.02	70.25	<d.l.	<d.l.	28.19	<d.l.	0.02	0.62	<d.l.	<d.l.	0.15	<d.l.	<d.l.	99.25
M-G2-19	<d.l.	0.01	0.02	69.54	<d.l.	0.10	28.04	0.01	0.02	0.72	<d.l.	0.03	0.08	0.02	0.01	98.60
M-G2-20	0.03	0.02	0.04	70.08	0.03	<d.l.	28.41	<d.l.	0.02	0.62	<d.l.	0.05	0.20	<d.l.	<d.l.	99.50
Cations on the basis of 4 oxygens
	P	Si	Ti	Al	V	Cr	Mg	Ca	Mn	Fe_tot	Co	Ni	Zn	Na	K	Total
M-G2-1	0.000	0.000	0.000	1.985	0.000	0.000	1.007	0.000	0.001	0.013	0.001	0.000	0.000	0.000	0.000	3.008
M-G2-2	0.000	0.001	0.000	1.981	0.000	0.001	1.008	0.000	0.000	0.013	0.000	0.000	0.003	0.000	0.000	3.008
M-G2-3	0.000	0.000	0.001	1.986	0.000	0.000	1.003	0.000	0.000	0.014	0.000	0.000	0.001	0.000	0.000	3.006
M-G2-4	0.000	0.002	0.001	1.981	0.001	0.000	1.007	0.000	0.001	0.012	0.000	0.000	0.001	0.000	0.000	3.006
M-G2-5	0.001	0.000	0.000	1.984	0.000	0.001	1.007	0.000	0.000	0.013	0.000	0.000	0.000	0.000	0.000	3.006
M-G2-6	0.001	0.000	0.001	1.978	0.000	0.000	1.011	0.000	0.001	0.013	0.000	0.000	0.004	0.000	0.000	3.009
M-G2-7	0.000	0.001	0.001	1.985	0.000	0.000	1.006	0.000	0.001	0.013	0.000	0.000	0.000	0.000	0.000	3.006
M-G2-8	0.001	0.000	0.000	1.980	0.001	0.001	1.010	0.000	0.001	0.011	0.001	0.000	0.001	0.000	0.000	3.007
M-G2-9	0.000	0.001	0.001	1.980	0.000	0.001	1.009	0.000	0.001	0.013	0.000	0.000	0.002	0.000	0.000	3.008
M-G2-10	0.000	0.001	0.001	1.974	0.000	0.001	1.016	0.000	0.000	0.014	0.000	0.000	0.004	0.000	0.000	3.011
M-G2-11	0.001	0.000	0.000	1.980	0.000	0.000	1.011	0.000	0.001	0.012	0.000	0.001	0.002	0.000	0.000	3.008
M-G2-12	0.000	0.001	0.001	1.980	0.000	0.001	1.011	0.000	0.001	0.012	0.000	0.000	0.002	0.000	0.000	3.008
M-G2-13	0.000	0.000	0.000	1.983	0.000	0.000	1.009	0.000	0.000	0.012	0.000	0.001	0.001	0.000	0.000	3.008
M-G2-14	0.001	0.000	0.000	1.978	0.000	0.001	1.013	0.000	0.001	0.013	0.000	0.000	0.002	0.000	0.000	3.010
M-G2-15	0.001	0.001	0.000	1.982	0.000	0.000	1.006	0.000	0.001	0.012	0.000	0.000	0.003	0.000	0.000	3.007
M-G2-16	0.000	0.000	0.001	1.980	0.001	0.001	1.010	0.000	0.001	0.012	0.000	0.000	0.001	0.000	0.000	3.008
M-G2-17	0.000	0.000	0.000	1.983	0.000	0.001	1.005	0.000	0.002	0.013	0.001	0.000	0.002	0.000	0.000	3.007
M-G2-18	0.000	0.000	0.000	1.984	0.000	0.000	1.007	0.000	0.000	0.012	0.000	0.000	0.003	0.000	0.000	3.007
M-G2-19	0.000	0.000	0.000	1.979	0.000	0.002	1.009	0.000	0.000	0.014	0.000	0.001	0.001	0.001	0.000	3.009
M-G2-20	0.001	0.001	0.001	1.976	0.000	0.000	1.013	0.000	0.000	0.012	0.000	0.001	0.004	0.000	0.000	3.009
Crystal Chemical Formula (Average)						(Mg₁.₀₀₉Fe₀.₀₁₃Zn₀.₀₀₂) _∑1.₀₂₄(Al₁.₉₈₁Cr₀.₀₀₁Mn₀.₀₀₁) _∑1.₉₈₃O₄

Table A2. Chemical composition of a purple spinel sample (M-V4) measured via EPMA (wt.%).

	P₂O₅	SiO₂	TiO₂	Al₂O₃	V₂O₃	Cr₂O₃	MgO	CaO	MnO	FeO_tot	CoO	NiO	ZnO	Na₂O	K₂O	Total
M-V4-1	<d.l.	<d.l.	0.03	69.68	0.06	0.05	27.69	<d.l.	<d.l.	0.60	<d.l.	0.04	0.57	0.01	0.01	98.71
M-V4-2	<d.l.	0.02	<d.l.	70.15	0.09	0.10	27.98	0.01	<d.l.	0.67	<d.l.	0.03	0.56	<d.l.	<d.l.	99.60
M-V4-3	0.01	0.06	0.02	70.11	0.05	0.08	28.13	<d.l.	0.02	0.65	<d.l.	<d.l.	0.58	0.03	<d.l.	99.72
M-V4-4	<d.l.	0.02	0.04	69.72	0.12	0.16	27.74	<d.l.	0.02	0.66	<d.l.	<d.l.	0.54	0.01	<d.l.	99.03
M-V4-5	<d.l.	0.03	<d.l.	70.11	0.05	0.16	27.94	0.01	<d.l.	0.67	0.01	0.04	0.61	0.01	<d.l.	99.65
M-V4-6	0.04	<d.l.	<d.l.	70.53	0.11	0.14	28.06	<d.l.	0.04	0.64	<d.l.	0.02	0.61	0.02	<d.l.	100.20
M-V4-7	<d.l.	<d.l.	0.02	70.11	<d.l.	0.13	28.02	<d.l.	0.03	0.64	0.06	0.04	0.54	0.02	<d.l.	99.60
M-V4-8	<d.l.	0.03	<d.l.	70.45	0.08	0.02	28.25	<d.l.	0.02	0.60	0.04	0.01	0.63	0.02	<d.l.	100.14
M-V4-9	0.04	0.02	0.04	70.73	0.05	0.17	28.00	<d.l.	0.03	0.65	0.03	0.05	0.52	0.02	<d.l.	100.35
M-V4-10	0.02	<d.l.	<d.l.	69.73	0.09	0.06	28.01	<d.l.	<d.l.	0.61	<d.l.	0.02	0.71	0.03	<d.l.	99.27
M-V4-11	<d.l.	0.02	0.02	70.31	0.07	0.14	27.98	0.01	0.04	0.61	0.04	<d.l.	0.60	0.05	<d.l.	99.88
M-V4-12	<d.l.	<d.l.	0.06	70.40	0.09	0.08	28.03	<d.l.	0.04	0.67	<d.l.	<d.l.	0.69	0.04	<d.l.	100.08
M-V4-13	0.04	0.01	0.04	70.52	0.04	0.08	28.32	0.01	0.01	0.57	<d.l.	<d.l.	0.53	0.01	<d.l.	100.18
M-V4-14	0.01	<d.l.	<d.l.	69.82	0.04	0.09	27.93	<d.l.	0.08	0.60	0.04	<d.l.	0.63	0.02	<d.l.	99.28
M-V4-15	<d.l.	0.03	0.03	69.69	0.07	0.13	27.87	<d.l.	0.02	0.64	0.02	<d.l.	0.71	0.03	<d.l.	99.23
M-V4-16	0.05	0.04	<d.l.	70.14	0.07	0.10	27.72	<d.l.	0.01	0.66	<d.l.	0.01	0.48	0.03	0.01	99.32
M-V4-17	<d.l.	0.02	0.08	70.25	0.08	0.10	27.93	<d.l.	0.05	0.71	<d.l.	0.02	0.53	0.02	0.01	99.79
M-V4-18	<d.l.	0.02	0.08	70.46	0.06	0.11	28.23	<d.l.	0.08	0.74	<d.l.	0.02	0.61	0.02	0.01	100.41
M-V4-19	<d.l.	<d.l.	<d.l.	70.43	0.08	0.09	28.35	<d.l.	0.05	0.62	<d.l.	0.06	0.58	0.02	0.01	100.29
M-V4-20	0.02	<d.l.	0.02	70.71	0.05	0.14	28.13	<d.l.	0.02	0.66	<d.l.	0.03	0.48	0.02	<d.l.	100.29
Cations on the basis of 4 oxygens
	P	Si	Ti	Al	V	Cr	Mg	Ca	Mn	Fe_tot	Co	Ni	Zn	Na	K	Total
M-V4-1	0.000	0.000	0.000	1.984	0.001	0.001	0.997	0.000	0.000	0.012	0.000	0.001	0.010	0.000	0.000	3.007
M-V4-2	0.000	0.000	0.000	1.980	0.002	0.002	0.999	0.000	0.000	0.013	0.000	0.001	0.010	0.000	0.000	3.008
M-V4-3	0.000	0.001	0.000	1.977	0.001	0.001	1.003	0.000	0.000	0.013	0.000	0.000	0.010	0.001	0.000	3.009
M-V4-4	0.000	0.000	0.001	1.980	0.002	0.003	0.996	0.000	0.000	0.013	0.000	0.000	0.010	0.001	0.000	3.007
M-V4-5	0.000	0.001	0.000	1.979	0.001	0.003	0.998	0.000	0.000	0.013	0.000	0.001	0.011	0.000	0.000	3.008
M-V4-6	0.001	0.000	0.000	1.980	0.002	0.003	0.996	0.000	0.001	0.013	0.000	0.000	0.011	0.001	0.000	3.007
M-V4-7	0.000	0.000	0.000	1.980	0.000	0.003	1.001	0.000	0.001	0.013	0.001	0.001	0.010	0.001	0.000	3.009
M-V4-8	0.000	0.001	0.000	1.978	0.002	0.000	1.003	0.000	0.000	0.012	0.001	0.000	0.011	0.001	0.000	3.010
M-V4-9	0.001	0.001	0.001	1.982	0.001	0.003	0.992	0.000	0.001	0.013	0.001	0.001	0.009	0.001	0.000	3.005
M-V4-10	0.000	0.000	0.000	1.976	0.002	0.001	1.004	0.000	0.000	0.012	0.000	0.000	0.013	0.001	0.000	3.010
M-V4-11	0.000	0.000	0.000	1.980	0.001	0.003	0.997	0.000	0.001	0.012	0.001	0.000	0.011	0.002	0.000	3.008
M-V4-12	0.000	0.000	0.001	1.979	0.002	0.001	0.997	0.000	0.001	0.013	0.000	0.000	0.012	0.002	0.000	3.008
M-V4-13	0.001	0.000	0.001	1.978	0.001	0.001	1.005	0.000	0.000	0.011	0.000	0.000	0.009	0.001	0.000	3.008
M-V4-14	0.000	0.000	0.000	1.979	0.001	0.002	1.001	0.000	0.002	0.012	0.001	0.000	0.011	0.001	0.000	3.009
M-V4-15	0.000	0.001	0.001	1.977	0.001	0.003	1.000	0.000	0.000	0.013	0.000	0.000	0.013	0.001	0.000	3.009
M-V4-16	0.001	0.001	0.000	1.984	0.001	0.002	0.992	0.000	0.000	0.013	0.000	0.000	0.009	0.001	0.000	3.004
M-V4-17	0.000	0.000	0.001	1.980	0.002	0.002	0.996	0.000	0.001	0.014	0.000	0.000	0.009	0.001	0.000	3.007
M-V4-18	0.000	0.000	0.001	1.975	0.001	0.002	1.001	0.000	0.002	0.015	0.000	0.000	0.011	0.001	0.000	3.010
M-V4-19	0.000	0.000	0.000	1.976	0.001	0.002	1.006	0.000	0.001	0.012	0.000	0.001	0.010	0.001	0.000	3.011
M-V4-20	0.000	0.000	0.000	1.982	0.001	0.003	0.997	0.000	0.000	0.013	0.000	0.001	0.008	0.001	0.000	3.007
Crystal Chemical Formula (Average)						(Mg₀.₉₉₉Fe₀.₀₁₃Zn₀.₀₁₀Mn₀.₀₀₁Na₀.₀₀₁) _∑1.₀₂₄(Al₁.₉₇₉Cr₀.₀₀₂V₀.₀₀₁) _∑1.₉₈₂O₄

Table A3. Chemical composition of a red spinel sample (M-R1) measured via EPMA (wt.%).

	P₂O₅	SiO₂	TiO₂	Al₂O₃	V₂O₃	Cr₂O₃	MgO	CaO	MnO	FeO_tot	CoO	NiO	ZnO	Na₂O	K₂O	Total
M-R1-1	0.06	<d.l.	0.14	68.14	0.29	1.36	28.53	0.01	<d.l.	0.17	<d.l.	0.03	0.11	<d.l.	<d.l.	98.83
M-R1-2	<d.l.	0.03	0.11	67.97	0.19	1.99	28.76	<d.l.	0.03	0.17	0.06	0.02	0.15	0.02	<d.l.	99.50
M-R1-3	<d.l.	0.02	<d.l.	69.01	0.28	1.48	28.55	<d.l.	0.04	0.17	<d.l.	0.01	0.14	0.01	0.01	99.71
M-R1-4	<d.l.	0.01	0.06	68.89	0.31	1.32	28.40	0.03	<d.l.	0.18	<d.l.	0.02	0.26	<d.l.	0.01	99.47
M-R1-5	0.03	0.02	0.02	69.17	0.35	1.44	28.82	0.01	0.02	0.15	0.03	<d.l.	0.13	<d.l.	0.01	100.20
M-R1-6	<d.l.	<d.l.	0.13	68.73	0.31	1.45	28.48	0.01	0.03	0.20	<d.l.	<d.l.	0.15	0.01	<d.l.	99.50
M-R1-7	0.02	0.01	0.06	69.22	0.38	1.28	28.68	<d.l.	<d.l.	0.18	<d.l.	0.04	0.17	<d.l.	0.01	100.03
M-R1-8	0.04	0.02	0.12	69.13	0.30	1.50	28.73	<d.l.	0.04	0.16	0.01	<d.l.	0.20	<d.l.	<d.l.	100.24
M-R1-9	0.02	0.03	0.13	69.04	0.33	1.54	28.72	<d.l.	<d.l.	0.12	<d.l.	<d.l.	0.03	0.01	<d.l.	99.98
M-R1-10	0.06	0.03	0.05	69.36	0.30	1.45	28.42	0.01	<d.l.	0.16	<d.l.	<d.l.	0.10	<d.l.	<d.l.	99.94
M-R1-11	0.02	<d.l.	0.17	69.46	0.28	1.35	28.54	<d.l.	<d.l.	0.19	0.02	0.02	<d.l.	<d.l.	<d.l.	100.05
M-R1-12	0.04	0.02	0.08	68.94	0.30	1.41	28.55	<d.l.	0.03	0.19	0.03	0.02	0.10	0.01	<d.l.	99.71
M-R1-13	0.01	<d.l.	0.05	68.96	0.32	1.62	28.53	0.01	<d.l.	0.24	<d.l.	0.04	0.07	<d.l.	<d.l.	99.84
M-R1-14	0.01	<d.l.	0.13	69.20	0.30	1.44	28.77	<d.l.	0.02	0.14	0.03	<d.l.	0.34	0.01	<d.l.	100.38
M-R1-15	0.02	0.02	0.10	69.09	0.33	1.61	28.81	<d.l.	0.02	0.23	0.06	0.02	0.25	<d.l.	0.01	100.56
M-R1-16	<d.l.	0.01	0.12	68.70	0.30	1.73	28.72	0.01	<d.l.	0.16	<d.l.	<d.l.	0.08	<d.l.	<d.l.	99.83
M-R1-17	0.05	0.03	0.03	68.59	0.29	1.58	28.62	<d.l.	0.03	0.14	0.01	0.12	0.06	<d.l.	<d.l.	99.56
M-R1-18	<d.l.	0.01	0.07	69.15	0.35	1.44	28.79	0.01	<d.l.	0.17	0.03	<d.l.	0.12	0.01	<d.l.	100.16
M-R1-19	<d.l.	<d.l.	0.07	68.91	0.32	1.47	28.57	<d.l.	<d.l.	0.23	<d.l.	0.02	0.05	<d.l.	0.01	99.64
M-R1-20	0.01	0.02	0.03	69.29	0.26	1.45	28.47	<d.l.	<d.l.	0.16	<d.l.	<d.l.	0.33	0.01	0.01	100.04
Cations on the basis of 4 oxygens
	P	Si	Ti	Al	V	Cr	Mg	Ca	Mn	Fe_tot	Co	Ni	Zn	Na	K	Total
M-R1-1	0.001	0.000	0.003	1.940	0.006	0.026	1.028	0.000	0.000	0.003	0.000	0.001	0.002	0.000	0.000	3.010
M-R1-2	0.000	0.001	0.002	1.928	0.004	0.038	1.032	0.000	0.001	0.003	0.001	0.000	0.003	0.001	0.000	3.013
M-R1-3	0.000	0.000	0.000	1.948	0.005	0.028	1.019	0.000	0.001	0.003	0.000	0.000	0.003	0.001	0.000	3.009
M-R1-4	0.000	0.000	0.001	1.950	0.006	0.025	1.017	0.001	0.000	0.004	0.000	0.000	0.005	0.000	0.000	3.008
M-R1-5	0.001	0.000	0.000	1.943	0.007	0.027	1.024	0.000	0.000	0.003	0.001	0.000	0.002	0.000	0.000	3.009
M-R1-6	0.000	0.000	0.002	1.945	0.006	0.027	1.019	0.000	0.001	0.004	0.000	0.000	0.003	0.000	0.000	3.008
M-R1-7	0.000	0.000	0.001	1.948	0.007	0.024	1.021	0.000	0.000	0.004	0.000	0.001	0.003	0.000	0.000	3.009
M-R1-8	0.001	0.000	0.002	1.942	0.006	0.028	1.021	0.000	0.001	0.003	0.000	0.000	0.004	0.000	0.000	3.008
M-R1-9	0.000	0.001	0.002	1.943	0.006	0.029	1.022	0.000	0.000	0.002	0.000	0.000	0.001	0.001	0.000	3.008
M-R1-10	0.001	0.001	0.001	1.952	0.006	0.027	1.011	0.000	0.000	0.003	0.000	0.000	0.002	0.000	0.000	3.004
M-R1-11	0.000	0.000	0.003	1.952	0.005	0.026	1.014	0.000	0.000	0.004	0.000	0.000	0.000	0.000	0.000	3.005
M-R1-12	0.001	0.000	0.001	1.946	0.006	0.027	1.019	0.000	0.001	0.004	0.000	0.000	0.002	0.000	0.000	3.008
M-R1-13	0.000	0.000	0.001	1.945	0.006	0.031	1.018	0.000	0.000	0.005	0.000	0.001	0.001	0.000	0.000	3.008
M-R1-14	0.000	0.000	0.002	1.943	0.006	0.027	1.022	0.000	0.000	0.003	0.001	0.000	0.006	0.000	0.000	3.010
M-R1-15	0.000	0.000	0.002	1.938	0.006	0.030	1.022	0.000	0.000	0.005	0.001	0.000	0.004	0.000	0.000	3.010
M-R1-16	0.000	0.000	0.002	1.938	0.006	0.033	1.025	0.000	0.000	0.003	0.000	0.000	0.001	0.000	0.000	3.009
M-R1-17	0.001	0.001	0.001	1.940	0.006	0.030	1.024	0.000	0.001	0.003	0.000	0.002	0.001	0.000	0.000	3.009
M-R1-18	0.000	0.000	0.001	1.944	0.007	0.027	1.023	0.000	0.000	0.003	0.000	0.000	0.002	0.001	0.000	3.010
M-R1-19	0.000	0.000	0.001	1.947	0.006	0.028	1.021	0.000	0.000	0.005	0.000	0.000	0.001	0.000	0.000	3.008
M-R1-20	0.000	0.001	0.001	1.951	0.005	0.027	1.014	0.000	0.000	0.003	0.000	0.000	0.006	0.000	0.000	3.007
Crystal Chemical Formula (Average)					(Mg₁.₀₂₁Fe₀.₀₀₄Zn₀.₀₀₃) _∑1.₀₂₇(Al₁.₉₄₄Cr₀.₀₂₈V₀.₀₀₆Ti₀.₀₀₂) _∑1.₉₈₀O₄

Table A4. Chemical composition of a pink spinel sample (M-P4) measured via EPMA (wt.%).

	P₂O₅	SiO₂	TiO₂	Al₂O₃	V₂O₃	Cr₂O₃	MgO	CaO	MnO	FeO_tot	CoO	NiO	ZnO	Na₂O	K₂O	Total
M-P4-1	<d.l.	0.04	0.01	70.51	0.12	<d.l.	28.57	<d.l.	<d.l.	0.11	0.08	<d.l.	0.06	0.01	<d.l.	99.51
M-P4-2	<d.l.	0.01	0.01	70.74	0.07	0.03	28.77	<d.l.	0.04	<d.l.	<d.l.	0.04	0.25	0.02	<d.l.	99.98
M-P4-3	<d.l.	0.03	0.04	70.70	0.09	0.03	28.57	0.02	<d.l.	0.09	0.02	0.02	0.23	<d.l.	<d.l.	99.84
M-P4-4	0.04	0.05	0.02	71.00	0.10	0.03	28.65	0.01	0.01	0.09	0.01	<d.l.	0.22	<d.l.	<d.l.	100.21
M-P4-5	<d.l.	<d.l.	<d.l.	71.04	0.10	0.03	28.81	<d.l.	0.05	0.15	0.03	0.05	0.23	<d.l.	<d.l.	100.48
M-P4-6	<d.l.	0.01	<d.l.	71.01	0.05	0.02	28.75	<d.l.	<d.l.	0.10	0.02	0.06	0.12	<d.l.	<d.l.	100.14
M-P4-7	<d.l.	0.02	<d.l.	70.73	0.13	0.02	28.58	<d.l.	0.03	0.02	0.05	0.04	0.19	0.01	<d.l.	99.83
M-P4-8	<d.l.	<d.l.	<d.l.	70.50	0.14	<d.l.	28.44	0.02	<d.l.	0.02	0.02	0.02	0.23	0.01	<d.l.	99.38
M-P4-9	0.05	0.02	0.03	70.83	0.15	0.06	29.02	0.01	<d.l.	0.02	<d.l.	0.02	0.31	0.02	0.01	100.55
M-P4-10	0.01	<d.l.	<d.l.	70.93	0.09	0.02	28.59	0.01	0.06	0.06	<d.l.	0.02	0.11	0.02	<d.l.	99.89
M-P4-11	<d.l.	<d.l.	<d.l.	70.73	0.08	0.02	29.15	0.01	0.09	0.05	0.03	<d.l.	0.17	0.02	0.01	100.34
M-P4-12	<d.l.	0.04	0.06	71.12	0.09	<d.l.	28.64	<d.l.	0.04	0.01	<d.l.	<d.l.	0.11	<d.l.	<d.l.	100.11
M-P4-13	0.01	<d.l.	<d.l.	70.97	0.13	0.03	28.82	<d.l.	<d.l.	0.05	0.04	<d.l.	0.29	0.01	<d.l.	100.35
M-P4-14	<d.l.	0.04	0.03	71.43	0.06	<d.l.	28.83	0.02	0.03	0.09	0.03	0.04	0.23	0.01	<d.l.	100.83
M-P4-15	<d.l.	0.01	<d.l.	70.48	0.16	<d.l.	28.68	0.01	<d.l.	0.04	<d.l.	0.03	0.10	0.02	0.01	99.52
M-P4-16	0.03	0.01	<d.l.	70.78	0.11	<d.l.	29.12	<d.l.	<d.l.	0.07	0.05	0.07	0.28	0.01	<d.l.	100.52
M-P4-17	0.05	0.01	0.02	70.64	0.11	0.02	28.94	<d.l.	0.03	0.07	<d.l.	0.02	0.13	0.02	<d.l.	100.06
M-P4-18	<d.l.	<d.l.	0.10	70.74	0.15	0.01	28.69	0.01	<d.l.	0.08	0.01	0.02	0.12	0.03	<d.l.	99.97
M-P4-19	0.01	0.02	0.08	70.35	0.11	<d.l.	28.63	0.51	0.02	0.07	0.10	<d.l.	0.26	<d.l.	0.01	100.14
M-P4-20	0.05	0.04	<d.l.	70.61	0.06	0.01	28.80	0.01	<d.l.	<d.l.	<d.l.	<d.l.	0.14	0.01	<d.l.	99.72
Cations on the basis of 4 oxygens
	P	Si	Ti	Al	V	Cr	Mg	Ca	Mn	Fe_tot	Co	Ni	Zn	Na	K	Total
M-P4-1	0.000	0.001	0.000	1.982	0.002	0.000	1.016	0.000	0.000	0.002	0.002	0.000	0.001	0.000	0.000	3.007
M-P4-2	0.000	0.000	0.000	1.981	0.001	0.000	1.019	0.000	0.001	0.000	0.000	0.001	0.004	0.001	0.000	3.009
M-P4-3	0.000	0.001	0.001	1.982	0.002	0.001	1.013	0.000	0.000	0.002	0.000	0.000	0.004	0.000	0.000	3.006
M-P4-4	0.001	0.001	0.000	1.982	0.002	0.001	1.012	0.000	0.000	0.002	0.000	0.000	0.004	0.000	0.000	3.005
M-P4-5	0.000	0.000	0.000	1.981	0.002	0.001	1.016	0.000	0.001	0.003	0.001	0.001	0.004	0.000	0.000	3.008
M-P4-6	0.000	0.000	0.000	1.984	0.001	0.000	1.016	0.000	0.000	0.002	0.000	0.001	0.002	0.000	0.000	3.007
M-P4-7	0.000	0.000	0.000	1.983	0.003	0.000	1.013	0.000	0.001	0.000	0.001	0.001	0.003	0.001	0.000	3.007
M-P4-8	0.000	0.000	0.000	1.985	0.003	0.000	1.013	0.000	0.000	0.000	0.000	0.000	0.004	0.000	0.000	3.006
M-P4-9	0.001	0.000	0.000	1.973	0.003	0.001	1.023	0.000	0.000	0.000	0.000	0.000	0.005	0.001	0.000	3.009
M-P4-10	0.000	0.000	0.000	1.986	0.002	0.000	1.012	0.000	0.001	0.001	0.000	0.000	0.002	0.001	0.000	3.006
M-P4-11	0.000	0.000	0.000	1.974	0.001	0.000	1.029	0.000	0.002	0.001	0.001	0.000	0.003	0.001	0.000	3.012
M-P4-12	0.000	0.001	0.001	1.986	0.002	0.000	1.011	0.000	0.001	0.000	0.000	0.000	0.002	0.000	0.000	3.004
M-P4-13	0.000	0.000	0.000	1.981	0.002	0.001	1.017	0.000	0.000	0.001	0.001	0.000	0.005	0.000	0.000	3.008
M-P4-14	0.000	0.001	0.001	1.983	0.001	0.000	1.012	0.000	0.000	0.002	0.001	0.001	0.004	0.001	0.000	3.007
M-P4-15	0.000	0.000	0.000	1.981	0.003	0.000	1.020	0.000	0.000	0.001	0.000	0.001	0.002	0.001	0.000	3.008
M-P4-16	0.001	0.000	0.000	1.973	0.002	0.000	1.027	0.000	0.000	0.001	0.001	0.001	0.005	0.000	0.000	3.012
M-P4-17	0.001	0.000	0.000	1.976	0.002	0.000	1.024	0.000	0.001	0.001	0.000	0.000	0.002	0.001	0.000	3.009
M-P4-18	0.000	0.000	0.002	1.980	0.003	0.000	1.016	0.000	0.000	0.002	0.000	0.000	0.002	0.001	0.000	3.007
M-P4-19	0.000	0.000	0.001	1.971	0.002	0.000	1.015	0.013	0.000	0.001	0.002	0.000	0.004	0.000	0.000	3.011
M-P4-20	0.001	0.001	0.000	1.980	0.001	0.000	1.021	0.000	0.000	0.000	0.000	0.000	0.002	0.000	0.000	3.007
Crystal Chemical Formula (Average)					(Mg₁.₀₁₇Zn₀.₀₀₃Fe₀.₀₀₁Ca₀.₀₀₁) _∑1.₀₂₂(Al₁.₉₈₀V₀.₀₀₂) _∑1.₉₈₂O₄

Table A5. Chemical composition of a spinel measured via LA-ICP-MS (ppm).

Components/ppm		Li	Be	Ti	V	Cr	Fe	Co	Mn	Ni	Zn	Ga
Gray	M-G2-1	5.97	4.87	19.70	47.72	238.77	5331.33	0.59	163.80	11.47	746.55	213.09
	M-G2-2	<d.l.	5.67	18.28	46.46	184.27	5271.68	0.83	159.40	10.96	689.92	203.86
	M-G2-3	2.85	5.30	23.64	45.82	210.44	5430.95	0.65	160.64	8.93	669.50	209.48
	M-G2-4	2.97	4.82	17.97	47.93	188.15	5189.37	0.54	156.37	11.75	683.67	195.15
	M-G2-5	2.02	4.56	19.20	47.86	186.77	5287.11	0.65	159.30	12.25	671.87	186.81
	M-G2-6	3.55	3.96	18.46	44.47	204.62	5282.00	0.49	161.28	11.70	711.68	196.94
	M-G2-7	4.41	5.92	21.26	46.95	224.58	5255.63	0.48	160.92	11.37	734.47	208.37
	M-G2-8	4.28	4.12	18.52	43.84	199.64	5024.43	0.51	155.29	12.19	687.51	191.15
	M-G2-9	5.23	4.37	20.01	46.05	202.21	5273.68	0.69	161.57	9.87	713.19	195.62
	M-G2-10	2.09	4.52	17.73	46.29	208.68	5339.46	0.56	160.88	9.23	708.01	187.74
	M-G2-11	2.88	4.81	21.35	45.60	206.69	5134.89	0.76	155.91	10.04	648.44	191.74
	M-G2-12	3.27	3.75	18.86	47.06	189.25	5183.16	0.65	159.43	11.14	667.21	185.71
	M-G2-13	4.36	3.76	20.11	45.52	213.40	5157.30	0.57	157.66	12.93	687.90	196.60
	M-G2-14	5.57	4.56	17.20	47.66	229.63	5297.94	0.63	165.34	11.41	730.89	203.55
	M-G2-15	3.50	3.82	18.52	47.24	188.85	5154.78	0.66	159.45	10.93	666.58	186.20
	M-G2-16	3.00	3.48	18.90	46.37	222.03	5255.47	0.49	160.85	9.98	744.29	198.32
	M-G2-17	4.60	2.61	19.80	47.25	250.73	5332.99	0.51	165.98	10.73	757.45	220.12
	M-G2-18	3.93	4.12	19.01	45.95	218.03	5174.64	0.52	159.17	9.28	727.08	197.73
	M-G2-19	5.25	4.25	18.44	46.00	212.69	5453.71	0.70	164.74	9.93	709.27	211.33
	M-G2-20	2.41	6.47	22.37	46.22	214.25	5208.29	0.58	157.94	10.09	647.99	199.36
Purple	M-V4-1	4.99	20.25	91.53	378.76	665.43	5033.22	2.58	78.67	3.48	3811.45	97.32
	M-V4-2	5.35	21.08	86.47	378.10	670.21	5273.59	2.69	81.69	2.56	4100.31	97.83
	M-V4-3	6.29	21.40	87.04	382.24	683.11	5229.33	2.73	79.53	3.14	4123.71	97.31
	M-V4-4	7.94	24.20	86.05	376.50	666.84	5241.85	2.67	80.74	2.88	4080.61	96.41
	M-V4-5	7.36	23.20	76.90	375.91	652.67	5289.63	2.67	79.49	<d.l.	4153.71	97.52
	M-V4-6	4.59	22.49	88.28	378.08	673.35	5196.45	2.78	79.17	<d.l.	4040.58	98.62
	M-V4-7	5.84	22.23	83.58	382.94	671.36	5168.45	2.46	80.57	2.91	4110.20	96.52
	M-V4-8	3.31	20.30	90.34	378.00	679.89	5003.27	2.59	78.42	<d.l.	3904.85	96.58
	M-V4-9	5.03	23.07	86.28	378.29	689.56	5232.52	2.74	78.00	<d.l.	4134.76	95.22
	M-V4-10	5.98	25.12	83.59	373.09	665.58	5144.82	2.89	77.40	3.39	4068.31	94.50
	M-V4-11	5.79	20.70	84.06	373.88	672.41	5149.82	2.41	81.46	<d.l.	3935.79	93.32
	M-V4-12	5.69	20.49	82.49	372.46	683.10	5243.92	2.99	80.62	3.68	4074.04	99.26
	M-V4-13	3.83	21.66	86.85	376.93	664.48	4972.03	2.84	74.87	<d.l.	3897.50	94.10
	M-V4-14	5.15	23.57	85.60	376.19	658.94	5043.31	2.62	75.90	3.59	3965.34	94.10
	M-V4-15	4.89	20.83	87.36	373.19	659.77	5204.60	2.53	78.87	<d.l.	4083.47	95.56
	M-V4-16	5.06	21.51	82.76	375.31	680.37	5242.03	2.79	79.74	<d.l.	4084.65	101.00
	M-V4-17	6.88	21.81	87.44	376.09	656.24	5296.03	2.84	80.22	3.62	4117.65	99.34
	M-V4-18	6.97	21.69	87.83	377.25	657.91	5329.65	2.85	80.50	3.29	4117.22	99.60
	M-V4-19	6.57	22.31	81.02	376.55	673.04	5158.28	2.69	78.45	4.24	4000.08	97.34
	M-V4-20	7.24	21.09	82.31	380.36	669.98	5239.40	2.69	78.55	4.56	4044.74	96.40
Red	M-R1-1	6.05	1.35	492.56	1703.86	9067.98	1464.94	1.32	5.10	99.61	993.64	86.45
	M-R1-2	7.06	<d.l.	519.18	1709.91	9065.57	1465.87	1.13	6.32	104.04	969.93	82.89
	M-R1-3	7.59	1.42	560.76	1231.50	13,381.40	1463.60	1.08	6.19	74.30	1037.86	84.30
	M-R1-4	3.56	<d.l.	506.76	1500.57	9656.06	1470.48	1.27	6.90	81.84	1022.81	89.40
	M-R1-5	6.32	1.34	514.73	1698.63	9193.21	1442.39	1.37	6.10	99.17	971.78	83.37
	M-R1-6	5.35	1.29	495.31	1701.42	9113.66	1483.76	1.22	7.22	89.18	1005.95	88.48
	M-R1-7	5.90	0.91	516.62	1700.16	9598.35	1471.19	1.25	6.20	107.88	1010.20	87.08
	M-R1-8	5.26	<d.l.	496.68	1597.97	9549.67	1455.27	1.35	5.72	83.83	996.89	87.02
	M-R1-9	19.09	1.35	447.50	1585.98	8670.95	758.92	4.45	0.00	139.38	895.85	81.43
	M-R1-10	3.96	1.24	519.06	1685.90	9245.54	1458.20	1.06	6.48	106.47	976.29	82.74
	M-R1-11	8.54	<d.l.	489.81	1703.08	10,247.99	1475.43	1.25	7.02	103.40	1021.46	86.39
	M-R1-12	6.25	2.50	497.21	1672.13	9175.45	1479.35	1.31	6.85	94.48	1023.11	88.18
	M-R1-13	5.33	1.45	494.21	1666.43	9208.83	1509.49	1.03	5.19	96.91	992.31	88.30
	M-R1-14	8.23	1.44	478.58	1745.79	9329.76	1035.55	0.87	6.86	148.72	914.13	80.66
	M-R1-15	4.84	1.74	510.07	1698.75	8131.58	1484.59	1.15	6.08	100.81	976.98	86.96
	M-R1-16	4.87	<d.l.	514.94	1686.76	8173.35	1457.16	1.17	6.51	103.08	979.13	85.36
	M-R1-17	5.07	1.39	505.39	1594.87	9952.34	1438.22	1.04	6.93	92.88	992.86	86.33
	M-R1-18	7.80	1.15	524.63	1602.93	11,391.87	1470.28	1.22	7.04	96.85	1008.29	83.38
	M-R1-19	6.11	1.38	510.38	1667.97	8599.97	1490.37	1.05	6.83	104.38	1003.67	88.52
	M-R1-20	6.26	1.77	499.45	1601.14	10,571.17	1453.87	1.20	6.81	93.84	1019.18	85.05
Pink	M-P4-1	4.30	2.25	53.75	782.49	37.47	535.29	0.21	7.57	<d.l.	1009.19	117.43
	M-P4-2	3.91	2.86	13.42	508.77	33.43	480.78	0.18	7.78	<d.l.	994.89	112.54
	M-P4-3	3.82	1.66	37.65	721.78	36.01	513.55	0.16	8.39	<d.l.	1014.34	119.22
	M-P4-4	4.09	2.36	13.57	520.19	32.77	517.96	0.22	7.96	2.89	1075.01	124.47
	M-P4-5	7.44	1.74	83.30	856.11	50.48	561.55	0.23	8.76	<d.l.	1092.56	122.29
	M-P4-6	4.06	4.68	6.12	486.87	36.54	523.23	0.19	8.77	2.56	1077.83	122.38
	M-P4-7	<d.l.	3.49	14.91	537.48	33.66	491.54	0.14	8.13	3.50	1000.92	115.23
	M-P4-8	4.01	2.66	24.65	595.09	36.23	490.82	0.21	7.29	<d.l.	969.22	113.96
	M-P4-9	3.53	3.58	11.61	522.32	37.64	491.99	0.18	7.64	<d.l.	1030.82	118.72
	M-P4-10	3.09	1.85	29.73	649.88	33.23	500.59	0.15	8.15	<d.l.	1002.25	117.18
	M-P4-11	4.45	2.79	32.91	694.88	79.78	497.85	<d.l.	7.98	2.91	1000.84	117.86
	M-P4-12	2.61	3.49	18.72	542.56	24.99	488.42	<d.l.	7.54	<d.l.	1005.27	113.25
	M-P4-13	2.79	4.02	6.34	482.22	34.23	500.47	0.22	7.42	<d.l.	1021.53	114.94
	M-P4-14	5.56	3.10	40.33	726.56	37.13	511.47	0.25	7.88	<d.l.	1002.78	120.40
	M-P4-15	4.71	2.72	23.13	560.96	33.54	497.65	0.16	7.40	<d.l.	1011.53	115.34
	M-P4-16	3.28	3.44	6.45	482.31	35.92	504.58	0.16	8.08	<d.l.	1057.41	117.36
	M-P4-17	4.04	4.05	10.06	510.70	30.03	505.09	0.23	8.85	2.95	1031.40	115.56
	M-P4-18	5.21	1.70	38.07	699.70	36.11	507.92	0.22	7.90	2.40	1004.80	116.94
	M-P4-19	<d.l.	2.97	9.22	489.64	35.06	501.59	0.14	8.21	2.94	1042.77	114.12
	M-P4-20	3.81	2.74	17.46	532.32	20.37	487.05	<d.l.	7.05	<d.l.	1019.26	115.11

<d.l. = below detection limits.

Table A6. The detection limits (d.l.) of LA-ICP-MS (ppm).

d.l./ppm		Li	Be	Ti	V	Cr	Fe	Co	Mn	Ni	Zn	Ga
Gray	M-G2-1	2.36	0.94	1.24	0.16	2.42	10.84	0.14	1.15	2.49	1.25	0.23
	M-G2-2	3.34	0.97	1.11	0.17	2.79	11.83	0.16	1.47	3.03	1.00	0.21
	M-G2-3	2.39	0.96	0.78	0.19	2.42	10.57	0.13	1.55	2.86	1.06	0.20
	M-G2-4	2.65	0.67	1.51	0.14	3.11	13.22	0.14	1.38	2.88	1.28	0.26
	M-G2-5	2.41	0.65	1.64	0.18	2.73	10.67	0.10	1.30	2.65	1.17	0.21
	M-G2-6	2.84	0.63	2.16	0.15	2.06	12.31	0.17	1.33	2.77	0.76	0.27
	M-G2-7	2.46	1.16	0.69	0.17	2.36	9.24	0.18	1.51	2.60	1.32	0.25
	M-G2-8	2.78	0.90	1.05	0.13	2.62	8.89	0.18	1.32	2.77	0.68	0.22
	M-G2-9	2.82	1.09	0.74	0.16	2.37	10.92	0.08	1.33	2.96	0.77	0.28
	M-G2-10	2.90	1.40	0.75	0.13	2.29	11.90	0.15	1.29	2.61	1.32	0.24
	M-G2-11	2.54	0.00	1.29	0.14	2.48	12.22	0.14	1.36	2.80	1.29	0.20
	M-G2-12	3.10	1.31	1.52	0.22	2.36	11.93	0.15	1.50	2.37	0.71	0.29
	M-G2-13	2.59	0.95	1.53	0.16	2.44	10.94	0.11	1.32	2.12	1.32	0.25
	M-G2-14	2.62	0.68	1.10	0.14	2.86	10.30	0.16	1.38	2.69	0.42	0.24
	M-G2-15	3.01	1.25	1.76	0.21	2.34	11.48	0.14	1.44	2.31	0.77	0.28
	M-G2-16	2.84	0.66	1.08	0.16	2.53	10.32	0.14	1.38	3.01	1.27	0.19
	M-G2-17	3.21	1.90	1.89	0.16	2.99	12.22	0.15	1.50	2.81	0.83	0.30
	M-G2-18	2.87	0.96	0.78	0.20	2.83	10.83	0.14	1.72	2.73	0.82	0.23
	M-G2-19	2.36	1.48	1.07	0.09	2.85	10.75	0.11	1.41	2.89	1.18	0.28
	M-G2-20	2.73	0.00	0.76	0.15	2.75	9.29	0.20	1.44	2.93	1.02	0.24
Purple	M-V4-1	2.74	0.68	1.34	0.18	2.58	11.16	0.10	1.43	2.76	0.99	0.40
	M-V4-2	2.64	0.65	1.05	0.12	2.42	11.80	0.12	1.39	2.28	1.26	0.26
	M-V4-3	2.68	0.00	0.00	0.14	2.44	11.57	0.14	1.59	3.08	0.88	0.27
	M-V4-4	2.59	1.12	1.05	0.08	2.60	10.44	0.09	1.32	2.83	1.65	0.21
	M-V4-5	2.79	0.66	1.82	0.20	2.50	10.63	0.10	1.44	2.91	0.56	0.26
	M-V4-6	2.56	1.37	1.37	0.16	2.80	11.23	0.08	1.53	2.90	1.02	0.21
	M-V4-7	2.73	0.99	0.80	0.16	2.34	11.85	0.16	1.33	2.46	0.74	0.25
	M-V4-8	2.58	0.69	0.79	0.13	2.77	11.61	0.11	1.45	3.11	1.59	0.26
	M-V4-9	2.91	1.37	1.12	0.16	2.62	11.97	0.14	1.45	3.04	1.16	0.35
	M-V4-10	2.52	0.00	2.29	0.11	2.75	11.84	0.13	1.49	2.66	0.44	0.26
	M-V4-11	3.26	0.88	1.00	0.18	3.63	11.86	0.20	1.88	3.96	2.37	0.43
	M-V4-12	2.96	0.00	2.11	0.19	3.03	11.89	0.11	1.83	3.14	1.30	0.29
	M-V4-13	3.25	1.34	0.89	0.21	3.02	13.79	0.16	1.68	3.22	1.68	0.32
	M-V4-14	2.44	0.00	0.83	0.08	2.62	12.32	0.17	1.46	2.56	1.22	0.26
	M-V4-15	3.12	0.72	1.40	0.14	2.84	10.36	0.13	1.55	2.54	1.13	0.22
	M-V4-16	3.10	1.18	1.11	0.14	2.98	12.04	0.14	1.63	2.67	0.93	0.26
	M-V4-17	2.64	1.57	1.41	0.18	2.71	11.81	0.10	1.81	3.00	1.50	0.31
	M-V4-18	2.66	1.61	1.44	0.18	2.69	11.69	0.10	1.82	3.04	1.26	0.31
	M-V4-19	2.36	1.23	2.08	0.20	2.65	10.65	0.08	1.50	2.45	1.05	0.28
	M-V4-20	3.10	1.26	1.18	0.16	2.95	11.20	0.13	1.66	2.85	1.19	0.33
Red	M-R1-1	2.76	0.61	1.46	0.18	3.03	15.35	0.10	1.80	2.41	1.15	0.18
	M-R1-2	3.03	1.07	1.18	0.19	3.07	13.91	0.15	1.53	2.64	1.03	0.26
	M-R1-3	2.94	1.36	1.50	0.20	2.68	16.25	0.12	1.40	2.75	1.03	0.25
	M-R1-4	2.87	1.86	1.39	0.23	2.89	12.04	0.12	1.53	2.87	1.28	0.22
	M-R1-5	2.68	0.64	1.21	0.16	3.17	13.03	0.11	1.55	2.51	1.05	0.22
	M-R1-6	2.59	0.65	1.02	0.19	2.99	12.87	0.13	1.37	2.67	1.21	0.34
	M-R1-7	3.04	0.91	1.84	0.20	3.07	11.03	0.11	1.33	2.30	1.12	0.27
	M-R1-8	2.90	1.24	1.22	0.25	2.63	13.10	0.10	1.25	2.77	1.30	0.24
	M-R1-9	2.78	1.35	1.35	0.21	2.76	14.29	0.06	1.37	2.63	1.12	0.25
	M-R1-10	3.22	0.93	1.95	0.23	2.79	12.60	0.13	1.19	2.54	1.33	0.22
	M-R1-11	2.67	1.08	2.65	0.20	2.69	10.80	0.12	1.42	2.54	1.34	0.16
	M-R1-12	2.99	1.50	1.46	0.18	2.45	12.85	0.13	1.24	2.99	1.01	0.22
	M-R1-13	2.45	0.78	1.08	0.15	2.37	11.32	0.14	1.49	2.53	0.91	0.21
	M-R1-14	2.52	1.40	1.47	0.19	2.34	10.46	0.11	1.41	2.54	1.37	0.22
	M-R1-15	2.74	1.33	1.28	0.17	2.37	13.44	0.15	1.39	2.68	1.28	0.31
	M-R1-16	3.00	1.45	1.28	0.15	2.73	12.33	0.12	1.41	2.66	1.54	0.17
	M-R1-17	2.43	0.63	1.24	0.14	2.80	9.68	0.17	1.48	2.66	1.36	0.24
	M-R1-18	2.60	1.11	1.47	0.20	2.67	12.59	0.08	1.32	2.61	1.53	0.22
	M-R1-19	2.95	0.92	1.48	0.18	2.70	14.29	0.14	1.34	2.83	1.46	0.29
	M-R1-20	2.79	1.28	1.79	0.17	3.00	12.78	0.12	1.61	3.09	0.87	0.22
Pink	M-P4-1	2.98	0.69	1.37	0.18	2.53	11.65	0.10	1.67	2.99	1.35	0.28
	M-P4-2	2.35	1.13	1.08	0.19	3.04	12.23	0.11	1.20	3.01	1.12	0.19
	M-P4-3	3.35	1.21	1.39	0.19	3.66	12.43	0.15	1.80	2.96	0.94	0.30
	M-P4-4	2.85	0.68	1.33	0.17	2.86	12.43	0.13	1.46	2.85	1.44	0.26
	M-P4-5	2.77	0.00	1.57	0.19	2.99	11.41	0.10	1.43	3.04	1.24	0.33
	M-P4-6	2.88	0.93	1.65	0.12	2.69	11.10	0.13	1.46	2.53	0.95	0.28
	M-P4-7	2.51	0.65	1.47	0.22	2.40	11.35	0.15	1.24	2.69	0.40	0.30
	M-P4-8	2.36	0.00	0.75	0.14	2.57	11.83	0.13	1.29	2.80	1.39	0.23
	M-P4-9	2.85	0.00	0.77	0.16	2.84	13.02	0.15	1.56	2.67	1.47	0.36
	M-P4-10	2.91	1.28	0.75	0.15	2.20	11.82	0.13	1.34	2.86	0.87	0.19
	M-P4-11	2.87	0.93	1.49	0.16	2.60	11.61	0.17	1.53	2.65	1.26	0.21
	M-P4-12	2.72	0.00	1.28	0.18	2.31	11.89	0.15	1.49	2.51	1.16	0.29
	M-P4-13	2.80	0.92	1.80	0.24	2.56	10.56	0.11	1.48	2.51	0.78	0.22
	M-P4-14	2.49	0.65	1.46	0.16	2.47	11.17	0.10	1.61	2.72	1.26	0.22
	M-P4-15	2.70	1.14	3.99	0.18	2.27	11.09	0.13	1.38	2.83	0.69	0.25
	M-P4-16	3.03	0.86	1.19	0.14	2.57	10.27	0.12	1.49	2.48	0.99	0.34
	M-P4-17	2.45	1.10	1.25	0.12	2.95	12.54	0.14	1.35	2.46	1.06	0.30
	M-P4-18	2.74	1.30	1.17	0.17	2.48	10.19	0.14	1.35	2.37	0.71	0.22
	M-P4-19	2.46	1.15	0.76	0.16	2.80	11.61	0.11	1.42	2.39	1.24	0.18
	M-P4-20	2.94	1.36	1.69	0.15	2.34	11.42	0.13	1.26	2.72	0.94	0.23

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Figure 1. (a) Schematic diagram of the spinel polyhedral model; (b) schematic diagram of the spinel ball-and-stick model. Modified and designed using Vesta software, ver. 3.5.8 [6]. The source parameters pertaining to the MgAl₂O₄ crystal structure were obtained from the Crystallography Open Database.

Figure 2. (a) Location of major gem mining areas in Myanmar; (b) simplified map showing the distribution of the MMM Belt; (c) map centered on Mogok including most of the gem fields (modified from the literature [30,32]).

Figure 3. The 18 faceted spinel samples (photos by Yihui Qi).

Figure 4. Raman spectral peaks of spinel samples: (a) M-G; (b) M-V; (c) M-R; (d) M-P.

Figure 5. (a) Raman spectra; (b) partial enlargement of Raman spectral peaks from 380 to 420 cm⁻¹.

Figure 6. Infrared spectrum of spinel samples. R% = reflectance: (a) M-G; (b) M-V; (c) M-R; (d) M-P.

Figure 7. Representative UV–visible absorbance. Abs. = Absorption. (a) M-G; (b) M-V; (c) M-R; (d) M-P.

Figure 8. Binary diagram of trace elements in Myanmar spinels: (a) Cr-V; (b) Zn-Fe.

Figure 9. Band analysis of UV-Vis spectra: (a) M-G2; (b) M-V4. (a-h indicate the peaks of the fitted bands).

Figure 10. UV-Vis spectra analysis of absorbance: (a) M-R1; (b) M-P4. (a–i indicate the peaks of the fitted bands).

Figure 11. Ternary phase diagram of V—Cr—Fe.

Figure 12. The percentages of Cr+V, Fe and Zn contents in the investigated spinels: (a) M-G2; (b) M-V4; (c) M-R1; (d) M-P4.

Table 1. List of the main gemological properties of the analyzed samples.

No.	Color	Cut	Carat	RI	SG	Fluorescence Reaction
No.	Color	Cut	Carat	RI	SG	365 nm	254 nm
M-G1	Light gray, slight violet-blue hue	Cushion Brilliant	1.125	1.718	3.60
M-G2	Nearly neutral gray	Cushion Brilliant	1.075	1.714	3.59
M-G3	Gray, slight violet-blue hue	Cushion Brilliant	1.430	1.718	3.59
M-G4	Dark gray, slight violet-blue hue	Cushion Brilliant	1.608	1.718	3.60
M-V1	Purple	Modified Triangular Brilliant	1.170	1.716	3.62
M-V2	Purple	Modified Triangular Brilliant	1.290	1.717	3.62
M-V3	Purple	Cushion Brilliant	0.965	1.715	3.60
M-V4	Light purple	Emerald Cut	0.990	1.717	3.62
M-R1	Red	Cushion Brilliant	1.235	1.719	3.58
M-R2	Red, medium orange tone	Emerald Cut	1.370	1.718	3.61
M-R3	Red	Cushion Brilliant	1.165	1.717	3.58
M-R4	Dark orange-red	Cushion Brilliant	1.395	1.716	3.62
M-R5	Red	Cushion Brilliant	1.085	1.720	3.58
M-P1	Light pink	Cushion Brilliant	1.365	1.712	3.60
M-P2	Pink	Cushion Brilliant	1.495	1.713	3.59
M-P3	Light pink	Cushion Brilliant	1.610	1.713	3.58
M-P4	Pink	Cushion Brilliant	1.144	1.714	3.55
M-P5	Pink, slight orange tone	Cushion Brilliant	1.310	1.715	3.54

Table 2. List of main Raman positions in the investigated samples (cm⁻¹).

Samples	T_2g	E_g	T_2g’	A_1g	Samples	T_2g	E_g	T_2g’	A_1g
M-G1	309	404	663	765	M-R1	309	404	664	764
M-G2	309	404	663	764	M-R2	310	404	663	764
M-G3	310	404	663	764	M-R3	310	404	664	764
M-G4	310	404	663	765	M-R4	309	404	663	764
M-V1	310	404	664	762	M-R5	310	404	664	764
M-V2	309	404	663	764	M-P1	311	408	665	766
M-V3	310	406	663	764	M-P2	311	406	664	765
M-V4	310	404	663	765	M-P3	312	407	665	766
Polarized Raman [40]	312	407	666	767	M-P4	311	406	664	767
Polarized Raman [40]	312	407	666	767	M-P5	312	406	665	766

Table 3. List of the most important FTIR absorption peaks in the investigated samples (cm⁻¹).

Samples	“Shoulder”	Broad Peak	“Shoulder”	Sharp Peak	“Weak Shoulder”
M-G	471	542	586	735	843
M-V	474	545	587	731	849
M-R	472	543	586	726	845
M-P	473	541	588	731	843

Table 6. UV-Vis spectral peak positions of M-R1 (nm).

Sign	M-R1	891292d [56]	Attributions and Electronic Transition
a	381	387	⁴A_2g→⁴T_1g(F) of ^MCr³⁺ [53,56]
b	398	393	³T_1g(F)→³T_1g (P) of ^MV³⁺ [53]
c	428	422	⁴A_2g→⁴T_1g(F) of ^MCr³⁺ [56]
d	513	529	⁴A_2g→⁴T_2g(F) of ^MCr³⁺ [56,69]
e	542	533	³T₁(F)→³T₂(F) of ^MV³⁺ [26]
f	571	561	⁴A_2g→⁴T_2g(F) of ^MCr³⁺ [56,69]
g	680	668	Fe²⁺-Fe³⁺ exchange interaction [56]

Table 7. UV-Vis spectral peak positions of M-P4 (nm).

Sign	M-P4	88169	Attributions and Electronic Transition
a	389	389	Spin-allowed ⁴A_2g→⁴T_1g(F) of ^MCr³⁺ [70]
b	409	402	Spin-allowed ³T_1g(F)→³T_1g(P) of ^MV³⁺ [56]
c	428	422	Spin-allowed ⁴A_2g→⁴T_1g(F) of ^MCr³⁺
d	460	/	A_1g → ⁴A_1g, ⁴E_g of ^MFe³⁺, possibly intensified by ECP transitions in ^TFe³⁺–^MFe³⁺ clusters and by spin-forbidden ⁵E → ³T₂, ³T₁, ³E in ^TFe²⁺ [11]
e	488	/	Spin-forbidden ⁵E→³T₁ of ^TFe²⁺ [11]
f	533	540	Spin-allowed ³T₁(F)→³T₂(F) of ^MV³⁺ [56]
g	572	579	Spin-allowed⁴A_2g→⁴T_2g(F) of ^MCr³⁺ [56]
h	658	686	Fe²⁺-Fe³⁺ exchange interaction [56,70]
i	774	794	Spin-forbidden ⁵T_2g → ⁵E_g of ^TFe²⁺ [11]

Table 8. Bivariate correlation analysis of the spinel elemental contents.

		Cr	V	Fe
Mg	r	0.28	0.42	−0.78
Mg	*p	≤0.5	≤0.5	≤0.5
Al	r	−0.85	−0.75	0.15
Al	*p	≤0.5	≤0.5	–

*p: the parameter for determining the results of hypothesis testing.

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Wang, M.; Wang, M.; Qi, Y.; Xue, Y.; Shi, G. Chemical Composition and Spectral Characteristics of Different Colored Spinel Varieties from Myanmar. Minerals 2024, 14, 1124. https://doi.org/10.3390/min14111124

AMA Style

Wang M, Wang M, Qi Y, Xue Y, Shi G. Chemical Composition and Spectral Characteristics of Different Colored Spinel Varieties from Myanmar. Minerals. 2024; 14(11):1124. https://doi.org/10.3390/min14111124

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Wang, Mengwei, Mingying Wang, Yihui Qi, Yuan Xue, and Guanghai Shi. 2024. "Chemical Composition and Spectral Characteristics of Different Colored Spinel Varieties from Myanmar" Minerals 14, no. 11: 1124. https://doi.org/10.3390/min14111124

APA Style

Wang, M., Wang, M., Qi, Y., Xue, Y., & Shi, G. (2024). Chemical Composition and Spectral Characteristics of Different Colored Spinel Varieties from Myanmar. Minerals, 14(11), 1124. https://doi.org/10.3390/min14111124

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Chemical Composition and Spectral Characteristics of Different Colored Spinel Varieties from Myanmar

Abstract

1. Introduction

2. Geological Settings

3. Materials and Methods

3.1. Sample Presentation

3.2. Spectral Analysis

3.3. Chemical Analysis

4. Results

4.1. Basic Gemological Characteristics

4.2. Spectroscopy

4.2.1. Raman Spectra

4.2.2. FTIR Spectra

4.2.3. UV-VIS-NIR Spectra

4.3. Chemical Composition

5. Discussion

5.1. Chromogenic Mechanism

5.1.1. Gray and Purple Spinels

5.1.2. Red and Pink Spinels

5.2. Trace Element Distribution of Different Colored Spinels

6. Conclusions

Author Contributions

Funding

Data Availability Statement

Acknowledgments

Conflicts of Interest

Appendix A

References

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Samples	“Shoulder”	Broad Peak	“Shoulder”	Sharp Peak	“Weak Shoulder”
M-G	471	542	586	735	843
M-V	474	545	587	731	849
M-R	472	543	586	726	845
M-P	473	541	588	731	843

Samples	“Shoulder”	Broad Peak	“Shoulder”	Sharp Peak	“Weak Shoulder”
M-G	471	542	586	735	843
M-V	474	545	587	731	849
M-R	472	543	586	726	845
M-P	473	541	588	731	843

Samples	“Shoulder”	Broad Peak	“Shoulder”	Sharp Peak	“Weak Shoulder”
M-G	471	542	586	735	843
M-V	474	545	587	731	849
M-R	472	543	586	726	845
M-P	473	541	588	731	843