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Article

Color Genesis and Compositional Characteristics of Color-Change Sapphire from Fuping, China

1
Beijing Institute of Economics and Management, Beijing 100102, China
2
State Key Laboratory of Geological Processes and Mineral Resources, China University of Geosciences, Beijing 100083, China
3
School of Gemology, China University of Geosciences, Beijing 100083, China
4
Southern Marine Science and Engineering Guangdong Laboratory (Guangzhou), Guangzhou 511458, China
5
Shandong Academician Workstation of Diamond Mineralization Mechanism and Exploration, Shandong No. 7 Exploration Institute of Geology and Mineral Resources, Linyi 276006, China
6
Key Laboratory of Deep-Earth Dynamics, Institute of Geology, Chinese Academy of Geological Sciences, Beijing 100037, China
7
Department of Earth Sciences, Karakoram International University, Gilgit 15100, Pakistan
*
Authors to whom correspondence should be addressed.
Crystals 2022, 12(4), 463; https://doi.org/10.3390/cryst12040463
Submission received: 28 February 2022 / Revised: 20 March 2022 / Accepted: 22 March 2022 / Published: 27 March 2022
(This article belongs to the Special Issue Gem Crystals)

Abstract

:
The color-change sapphire occurs in sillimanite-biotite gneiss in Fuping County Hebei province, China, is one of most famous sapphire deposits in China. However, the color genesis, mechanisms of color changing and compositional characteristics of the sapphire remain enigmatic. In this contribution, the coloration in the Fuping sapphire, color changing mechanisms and, compositional characteristics were studied by conventional gemological instruments in conjunction with ultraviolet-visible spectroscopy and Laser ablation inductively coupled plasma mass spectrometer. The results show that the Fuping sapphire is characterized by purple-blue-to-purple-red changed effect and column-shaped, waist drum-shaped with higher degree of euhedral crystal. Reddish brown rutile inclusions with 120° crossed cleavage are commonly observed. The dominant coloring element of the Fuping sapphire is Fe3+, and subordinate elements are Fe2+, Cr3+ and V3+. The color-change effect is caused by trace elements Cr3+ and V3+. The chemical compositions of Fuping color-change sapphires are analogous to those of metamorphic blue sapphires. When geochemically compared with sapphires from Isalo/Ilakaka deposit in Madagascar, Ratnapura deposit in Sri Lanka and Mogok in Myanmar, the Fuping color-change sapphires have distinctly higher rare element contents of Fe, Cr and Ga.

1. Introduction

Sapphires are of both commercial and scientific interest due to their beautiful and variable color, high hardness, less production, and their specific optical effect. Research on the physical properties of materials has been ongoing, and the difference in the structure type of materials leads to the discoloration of their response to light [1,2]. Previous research on the mechanism of color changing in sapphires has mainly focused on trace elements versus visible light absorption and identified the color-causing elements in the color-change sapphires based on the difference in the absorption of visible light by sapphires [3]. However, for color-change sapphires of different origins, the geochemical compositions and chromogenic elements are significantly different due to various geological formation environments [4,5,6,7]. Therefore, it is necessary to study the geochemical compositions and chromogenic mechanism of color-change sapphires in different regions.
Sapphires from the Fuping deposit in Hebei province, China have long been mined since Fuping corundum was used as “Jie Yu Sha (jade sand)” in jade processing during the Qing dynasty [8]. In previous studies, the Fuping sapphire was classified as A metamorphic type [9], or as a migmatite-type deposit [9,10,11], and corundum genesis is considered to be formed through the crystallization of Al-enriched melts in the process of migmatization [11,12], in a slightly high refractive index environment [10]. The Fuping sapphires commonly exhibit a star effect with high contents of Fe and Ga trace elements, and a low content of Ti [12]. However, the color-change mechanism of Fuping sapphires and compositional characteristics remain ambiguous. In this paper, absorption spectra and chemical compositions of the Fuping color-change sapphires were presented using microscopes, ultraviolet (UV) fluorescent spectroscopy, refractometers, scales, ultraviolet-visible (UV-Vis) spectroscopy, and laser ablation inductively coupled plasma mass spectrometer (LA-ICP-MS). Our data and interpretation provide a robust insight into the color genesis and compositional characteristics of the Fuping color-change sapphire.

2. Materials and Methods

2.1. Materials

Six color-change sapphire samples (FU-1, FU-2, FU-3, FU-4, FU-5, and FU-6) were taken from Fuping sapphire deposit. The wall rocks are the Archean metamorphic rocks of the Fuping Group, which mainly constitutes the hinterlands of the Taihang Mountain. The Fuping Group is subdivided into upper and lower subgroups. The upper subgroup is composed of Guanyintang and Songjiakou Formations, which mainly comprise of granulite and marble. The lower subgroup is mainly distributed in Fuping, Chenzhuang and west of Pingshan, and consists of Suojiazhuang, Chenzhuang, and Wangjiagou Formations. Their main rock suits are plagioclase amphibolite, biotite plagiogneiss, shallow granulite, and biotite granulite, of which sapphire deposits are only distributed in the Guanyintang Formation in the upper subgroup, and Chenzhuang and Wangjiagou Formations in the lower subgroup. The studied color-change sapphire samples in the paper are collected from the Wangjiagou Formation in the lower subgroup. The Fuping sapphire deposit is generally layered and occurs in migmatized gneisses, and the ore body is 100–800 m in length, with 1 m thickness and tens to hundreds of meters depth. [12].
The grain size of the studied samples varies from 10 × 15 mm to 15 × 20 mm, and their length is generally about 30–40 mm. The six samples were cut and ground into 2.00 mm thick slices along the vertical optical axis, and polished on both sides. In order to compare the composition characteristics, we also studied one color-change sapphire (Figure 1d) from the Mogok gneiss, Myanmar after purchasing from a mineral dealer. The sample is a 2 mm sheet with a diameter of about 8 × 10 mm and was polished on both sides and numbered as MD-9.

2.2. Methods

Gemological characteristic tests, polarized light microscope observations, and ultra-violet-visible absorption spectrum test were carried out completed in Gem Laboratory of the China University of Geosciences (Beijing, China). Six color change sapphires from Fuping deposits and one color-change sapphire from Myanmar were tested using microscopes, ultraviolet fluorescent lamps, refractometers, polarizers, and precision balances. The conventional gemological data of the test samples are shown in Table S1. One sample of the gneiss which is the wall rock of the deposit, was also tested with polarizing microscope. The UV-Vis absorption spectrometer was a model U-3010 UV-Vis spectrophotometer produced by Hitachi, Japan. The samples were analyzed in room temperature of 24 °C under transmitted light conditions with a wavelength ranging from 300–900 nm and resolution of 1 nm. The scanning speed was fast, and the slit width was 2 nm in a room temperature 24 °C. Abs/T% measurement method was adopted. In this study, the UV-Vis spectra of sample (FU-5) from Fuping deposit one color-change sapphire sample (MD-9) from Burma were tested.
The LA-ICP-MS analyses were performed at the State Key Laboratory of Geological Processes and Mineral Resources, China University of Geosciences (Wuhan, China). The instrument used is an Agilent 7500a quadrupole plasma mass spectrometer produced by Agilent, with a power of 1350 W, a plasma flow rate of 14 L/min, an auxiliary gas flow rate of 1 L/min, and a sampling depth of 5.4 mm. The ablation system was an excimer laser ablation system produced by Lambda Physiks, with a wavelength of 193 nm, a pulse width of 15 ns, an energy density of 14 J/cm−2, and a frequency of 8 Hz. Helium (0.7 L/min) was used as the ablation material in the experiment. The carrier gas was argon (0.8 L/min) as the compensation gas. In this paper, 6 samples of Fuping color-change sapphire were tested by LA-ICP-MS. The samples FU-1, FU-2, FU-3 and FU-6 were each tested 3 times while the samples FU-4, FU-5, and MD-9 (Burma sample) were each tested 4 times. The test results are shown in Table 1. The inclusions in the FU-3 sample were also analyzed and results are shown in Table S2.

3. Results

3.1. Conventional Gemological Characteristics

(1) Crystal morphology, color, and luster
Fuping color-change sapphire samples show a high degree of self-shape, with columnar, long columnar, and waist drum-like shapes (Figure 1a–c) with opaque-translucent, vitreous, and inferior to high-quality sapphire luster. The samples display the same color, which is blue with a purplish tinge, and exhibit a color-changing effect, i.e., purple-blue under sunlight (Figure 2b,d), and purple-red under incandescent light (Figure 2a,c). Burma color-change sapphire is translucent, vitreous, and show a deep blue hue with a weak color-changing effect, i.e., purple-blue in sunlight and weak purple-red in incandescent light.
(2) Refractive Indices
Among the refractive indices of the six Fuping color-change sapphires, the No values of the samples FU-4 and FU-6 is the highest (1.772), by contrast, the No values of FU-1 and FU-2 is the lowest (1.765). The Ne values of the samples FU-4 and FU-6 is the highest (1.764), while the Ne values of FU-2 is the lowest (1.760). The samples of FU-4, FU-5 and FU-6 have the largest values of birefringence (0.008) and the sample of FU-1 has the lowest value of 0.003. The No of the color-change sapphire from Burma (MD-9) is 1.766, and Ne is 1.760 with a birefringence of 0.006.
(3) Specific Gravity
The sample FU-4 shows highest specific gravity (4.00) among all the samples of Fuping color-change sapphires, while the sample FU-3 shows lowest value (3.91) of specific gravity. Burmese color-change sapphire (MD-9) has a specific gravity of 4.00.
(4) Fluorescence
All the Fuping color-change sapphire samples are fluorescently inert, while the Burma color-change sapphire shows weak pink fluorescence at long wavelengths and inert fluorescence at short wavelengths.
(5) Micro-examination
Microscopic examination shows that all the sapphire samples show well developed internal inclusions and fissure. In the thin slices, the fissures are obliquely intersected at nearly 120° in two directions, and the fissures are filled with gas-liquid inclusions, and the overall appearance is feathery and planar. Reddish-brown rutile inclusions are found in all the Fuping color-change sapphire samples (Figure 1f). Based on LA-ICP-MS analyses, the content of TiO2 in the rutile is high, followed by FeO, and the contents of rare elements Cu and Zr are also elevated (Table S2). The fissures and disjunction in Burmese color-change sapphire are well developed, including small needle-like and tubular inclusions with twin-crystal flakes.

3.2. Polarizing Microscope Observation

A corundum bearing surrounding rock sample was selected to grind the probe piece, and observed with a polarizing microscope. The associated minerals are mainly biotite, potassium feldspar, and sillimanite (Figure 1e). The biotite content is about 20% and shows obvious pleochroism, perfect cleavage, and parallel extinction. The K-feldspar is about 70% showing turbid and kaolinized surface. The sillimanite content is about 15% and found in the form of fibrous aggregates or columns, with a set of diagonal cleavage. Positive high protrusions show weak pleochroism, parallel extinction. The surrounding rock shows scaly granular metamorphic structure based on polarizing microscopic observations, and named as sillimanite biotite gneiss.

3.3. UV-Vis Spectral Characteristics

UV-Vis absorption spectra were carried out on the color-change sapphire sample FU-5 from Fuping, China and the sample MD-9 from Mogok, Myanmar. The results are shown in Figure 3. The absorption peaks at 377 nm, 385 nm, 450 nm, 694 nm and 741 nm and the absorption bands of 558–570 nm can be obviously observed in the Fuping color-change sapphires. The sample MD-9 also presents absorption peaks at 390 nm, 450 nm, 565 nm, 693 nm and 741 nm.

3.4. Chemical Compositions

The chemical compositions of Fuping color-change sapphires are shown in Table 1. The main component is Al2O3, and its content ranges from 97.77 wt.% to 99.14 wt.%, with an average of 98.89 wt.%. Among the trace elements, Fe content is the highest, ranging from 4848 ppm to 6742 ppm, with an average of 5625 ppm. While Cr content range from 1250 ppm to 160 ppm, showing a wide range with an average of 552 ppm. In addition, Ga, Ti and V contents are in the range of 85–159 ppm, 21–188 ppm and 36–146 ppm with an average value of 132 ppm, 52 ppm and 99 ppm, respectively. It is noted that the Ti content in one analytical spot (FU-4-4) is 188 ppm which is abnormally higher as compared to the values of other analytical points. However, this abnormal value of Ti shows no effect on the color, and also there no inclusions observed and the content of other trace elements are also normal at this analytical spot.
The major component of Burmese color-change sapphire is Al2O3 with a mean value of 99.30 wt.%. The average content of Ti is 786 ppm which is higher than that of the Fuping color-change sapphire. In contrast the Fe, V, Cr, and Ga contents are lower than those of Fuping color-change sapphire.

4. Discussion

4.1. Gemological Feature Comparisons

The Fe ion content inside natural corundum crystals is directly proportional to its refractive index and inversely proportional to its fluorescence [15]. The highest refractive index of the Fuping color-change sapphires is 1.764–1.772, slightly higher than 1.760–1.768, that of the Burmese color-change sapphires. This is mainly related to its internal higher Fe content of 5158 ppm, while the Burmese color-change sapphires have lower Fe content inside [16]. Similarly, the higher Fe content of Fuping color-changing sapphire also contributed to its fluorescence inertness, while the low Fe content of the Burmese color-changing sapphire showed weak pink fluorescence at long wavelengths and is fluorescently inert at short wavelengths. Fissures, disjunctions, and inclusions are developed in the Fuping color-change sapphires, so the transparency of higher-quality sapphires is slightly lower due to the prevalence of rutile inclusions inside, which is in accordance with previous studies [10,12]. Rutile inclusions are also found in the Burmese color-change sapphires [16] and Tanzania color-change sapphires [14].

4.2. Color Genesis

Previous discoveries have found that pure corundum displays no color [17], but after incorporation of cations impurities (such as Fe, Mg, Ti, Cr, B), substituting in to the crystal lattice for Al and the energy levels of the impurities are introduced into the energy bands, causing electron transitions, absorbing energy within the visible light capacity, and thus producing color [18,19]. The cations such as Fe2+, Fe3+, Ti4+, and V3+ play a crucial role in sapphire color production [20,21].
The UV-Vis spectrum of the sample FU-5 of Fuping color-change sapphire with Fe (5158 ppm), Cr (504 ppm) and V (55 ppm) is shown in Figure 3a. There are 377 nm, 385 nm, 450 nm, 694 nm, 741 nm absorption peaks, and 558–570 nm absorption bands in the spectrum. The absorption peaks located in the violet-blue-light region and visible region is formed by O2−-Fe3+ [22,23,24]. The absorption peaks generated at 377 nm, 385 nm, and 450 nm can be assigned as Fe3+ spectral transitions of 6A1-4E(D), 6A1-4T2(D), 6A1-4E+4A(G) and 6A1-4T2(G) [17,25,26]. The absorption peaks at 377 and 450 nm are due to the displacement of two Al3+ cations adjacent to the middle by Fe3+-Fe3+ ion pairs [27,28,29]. The absorption peak at 385 nm is thought to be generated because of the substitution of Al3+ by Fe3+ in corundum [27]. It is worth noting that it has been documented that Co2+ in sapphire can also produce absorption peaks near 450 nm and in the range of 600–700 nm [30,31]. Therefore, the influence of Co2+ on sapphire color can’t be excluded, which requires further study.
The generation of absorption band of 558–570 nm is attributed to the Cr3+, Fe2+-Ti4+ pairs, and V3+ may also be absorbed at this location [19,32,33]. These elements together contribute to the creation of a color-change effect [3]. According to the LA-ICP-MS analyses results of FU-5 sample (Table 2), the average Cr3+ is 504 ppm, and the Ti4+ average is 30 ppm, which is relatively low. A single Fe2+-Ti4+ pair does not cause a color-change effect [5]. Therefore, the absorption band of 558 nm–570 nm is mainly generated by Cr3+, but the absorption peak is offset from the theoretical value of 550 nm in the orange-yellow region. Due to the presence of the V element with an average content of 54 ppm, the V3+ electron transition promoted the absorption of orange-yellow light [6]. Therefore, the absorption band of 558 nm–570 nm is formed by the combination of Cr3+ and V3+.
The absorption peak at 694 nm is the result of a Cr3+d electron transition. Combined with the LA-ICP-MS chemical composition data of FU-5 sample (Table 2), the Fe element content of the Fuping color-change sapphires is up to 5158 ppm, and the absorption peak at 385 nm is produced by Fe3+ which is much higher than that of other absorption peaks and absorption bands. Therefore, according to the comprehensive analysis of UV-Vis absorption spectrum and LA-ICP-MS data, the main coloring element of the Fuping color-change sapphires is Fe3+, and the secondary coloring elements are Fe2+, Cr3+ and V3+.
According to the theory of gem color-change effects, the color change effect of gems is due to the fact that the absorption bands create two transparent regions in the visible light region on both sides of the absorption bands. Because different light sources have different power distributions and different power transmission, as a result, gems appear in varying colors under different light sources [34,35]. As shown in Figure 3a, a wide slow absorption band centered at 558 nm–570 nm in the visible region of the Fuping sample cause the development of Transmission Window A and Transmission Window B on both sides. Since sunlight has more blue-to-green waves than incandescent light sources, the gem passes more blue light [6], and thus it appears blue. The A light source is enriched with orange-to-red waves, so under this light source, the gem passes more of these waves and appears purple. Combined with the previous ultraviolet spectroscopic analyses results, the absorption band at 558–570 nm is formed by the combination of Cr3+ and V3+. Therefore, the color-change effect of the Fuping color-change sapphire is caused by a combination of Cr3+ and V3+ transition metals ions.
Comparing the UV-Vis spectra of the Fuping color-change sapphires with those of samples from other regions and synthetic color-change sapphires, it is found that the absorption peaks at 377 nm, 385 nm, 450 nm and 560 nm appeared in Fuping, Myanmar, Tanzania and Xinjiang color change sapphires which further emphasize the fact that Fe element is found in all samples of the above mentioned four regions. In addition, the 560 nm absorption peaks of the Fuping color-change sapphire and Burmese color-change sapphire are slightly offset towards the orange-yellow region when compared with same absorption peaks (560 nm) of the both Tanzania and Xinjiang color-change sapphires. The spectrum of synthetic color-change sapphires is the color spectrum caused by a single V3+ element, showing an absorption band of 580 nm [6], therefore, the 580 nm absorption band is caused by the V3+ cation. As shown in Table 2, V3+ content is low in in the Tanzania and Xinjiang color-change sapphires, therefore, its impact is also low as result, the 560 nm absorption peak is developed. While, the V3+ contents of the Fuping and Burmese color-change sapphires are 55 ppm and 93 ppm, respectively. The V3+ absorbed part of the orange-yellow light, which caused the absorption peak at 560 nm to shift towards the orange-yellow region, forming a 558–570 nm absorption band and a 570 nm absorption peak. The 741 nm absorption peak appeared in the Fuping and Burmese color-change sapphires, but did not appear in the Xinjiang and Tanzania color-change sapphires, indicating that Fe2+-Fe3+ charge transfers appeared in the Fuping and Burmese color-change sapphires, but did not appear in the Xinjiang and Tanzania color-change sapphires. The 694 nm absorption peak produced by Cr3+ appeared in color-change sapphires of Fuping, Myanmar, and Xinjiang, but not in Tanzania color-change sapphires. Combine with trace elements contents we argue that the Cr has high impact on coloration of the Fuping and Burmese color change sapphire.

4.3. Compositional Characteristics of Fuping Color-Change Sapphire

In the jewelry trading market, the prices of ruby and sapphire from different origins vary greatly. Therefore, the study of the origins of ruby and sapphire is also a hot topic in gemology [35]. It was found in previous studies that rutile has been found in many sapphires, however rutile cannot be considered a possible indicator of origin, as it is thought to be found in corundum of different sources [36].
In origin traceability, the type and concentration of trace elements can reflect the source of the elements and genesis of deposit, so trace elements are generally used as the main indicator to trace out the origin [7]. In this context, some researchers argue that trace element contents in natural corundum can be employed to discriminate magmatic and metamorphic corundum [6,37,38,39].
The chemical characteristics, such as high Cr, low Ga, Ga2O3 content <0.01 wt%, and Cr2O3/Ga2O3 > 3 represent metamorphic sapphires. In contrast, high Ga, low Cr, Cr2O3/Ga2O3 < 1 are the chemical characteristics of basalt sapphires [4]. Metamorphic blue sapphires typically contain low Fe and Ga than those of basalt sapphires [40]. Studies of the trace elements of sapphires from different regions show that Cr content in metamorphic sapphires from the Sutara placer mines in the Russian Far East is higher than 200 ppm, with less Ga content. [41]. Fe, V, and Ga concentrations can be used to better distinguish rubies from Mozambique and Myanmar [15]. Burmese color-change sapphires have the characteristics of low Fe and high V [16].
In this study, the trace element analytical data of the six Fuping color-change sapphires are marked with red circles on Cr2O3/Ga2O3 vs. Fe2O3/TiO2 diagram [39], Figure 4, which shows the data of the Fuping color-change sapphires plotted on the field of non-basalt sapphires above the dotted line which discriminates basalt sapphires and non-basalt sapphires. The majority of the Fuping color-change sapphires show a much higher ratio of Fe2O3/TiO2 than that of the Sri Lankan and Burmese sapphires, while four analytical data fall in the field of Madagascar sapphires, and only 1 analytical data is consistent with that of Sri Lankan and Burmese sapphires (Figure 4).
The trace element (Fe, Ti, Cr and Ga) contents of sapphires from four regions were compared and shown in Figure 5. The highest and lowest Fe values of the Fuping color-change sapphires are about 6 times and 54 times higher those of the Madagascar sapphires, 6 times and 27 times higher as compared with those of the Sri Lankan sapphires, and 4 times and 5 times higher than those of the Burmese sapphires, respectively. The Ti content of the Fuping color-change sapphires coincided with in the concentration range of Ti of the other three producing regions and did not show obvious characteristics. The much higher Fe contents of the Fuping color-change sapphire led to the elevated Fe2O3/TiO2 ratios than those of the Madagascar, Sri Lankan and Burmese sapphires.
On the other hand, the Cr2O3/Ga2O3 ratios of the Fuping color-change sapphire are similar to those of the Madagascar, Sri Lankan and Burmese sapphires. As shown in Figure 5c, the highest and lowest Cr contents of the Fuping color change sapphire are 19 times and 80 times higher than those of the Madagascar sapphires, 10 times and 53 times higher than those of the Sri Lanka sapphires, and 40 times and 40 times higher than those of the Burmese sapphires, respectively. While the highest and lowest values of Ga are 2.5 times and 12 times higher than those of the Madagascar sapphires, 4 times and 12 times higher than those of the Sri Lanka sapphires, and 3.7 times and 10 times higher than those of the Burmese sapphires, respectively. The Cr content of the Fuping color change sapphire is much higher while Ga content is not much high, therefore Cr2O3/Ga2O3 ratios are almost similar for the sapphires from the four regions.

5. Conclusions

A series of analysis and observations on raw sapphires such as gemological characteristic tests, LA-ICP-MS, and UV-Vis spectra conducted to discuss the compositional characteristics, color genesis, and color changing mechanism of the Fuping color-change sapphire. Based on the discussion above we conclude that;
The refractive index of Fuping color-change sapphires is between No = 1.772–1.765; Ne = 1.764–1.760; the relative density is between 3.91–4.00; the fluorescence is inert; the color bands and disjunctions are highly developed, with rutile inclusions inside, which shows that the disjunctions in the two directions is intersected at 120° obliquely.
The main coloring element of the Fuping color-change sapphires is Fe3+, while the secondary coloring elements are Fe2+, Cr3+, and V3+. The color-change effect is mainly generated due to the combine role of Cr3+ and V3+, of which the role of Cr3+ is dominant in creating color changing effect in the Fuping sapphires.
The chemical composition of Fuping color-change sapphires represents the genesis characteristics of metamorphic rock types. Compared the chemical composition with those of Madagascar Isalo, Myanmar Mogok and Sri Lanka Ratnapura sapphires, the Fe and Cr contents are far high with higher Ga contents in the Fuping sapphires color-change sapphires.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/cryst12040463/s1, Table S1. Conventional gemological characteristics of the color-changed sapphires from the Fuping deposit in China and from Mogok deposit in Myanmar. Table S2. Composion data of rutile inclusions in Fuping color-change sapphires analyzed by the LA-ICP-MS.

Author Contributions

Conceptualization, H.W., X.-Y.Y. and F.L.; methodology, H.W., X.-Y.Y.; software, H.W., F.L.; validation, H.W., X.-Y.Y. and F.L.; resources, X.-Y.Y.; data curation, H.W., X.-Y.Y., F.L., G.-C.W.; writing—original draft preparation, H.W., X.-Y.Y. and F.L.; writing—review and editing, H.W., X.-Y.Y., F.L. and M.A.; supervision, X.-Y.Y. and F.L.; project administration, X.-Y.Y. and F.L.; funding acquisition, X.-Y.Y. and F.L. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by grants from the Key Special Project for Introduced Talents Team of Southern Marine Science and Engineering Guangdong Laboratory (Guangzhou) (GML2019ZD0201), State Key Laboratory of Geological Processes and Mineral Resources, China University of Geosciences (GPMR202115), National Science Foundation of China (41720104009; 92062215), China Geological Survey (DD20190379-88; DD20190060), 7th Institute of Geology & Mineral Exploration of Shandong Province (QDKY202007) and Key Laboratory of Deep-Earth Dynamics of the Ministry of Natural Resources (J1901-32).

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

Not applicable.

Acknowledgments

We are very grateful to Ye Yuan and Chenglu Li for their help and valuable comments. We also thank Xinguo Liu, Chaofan Zhang, Guangya Wang for their valuable help.

Conflicts of Interest

The authors declare no conflict of any interest in this paper.

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Figure 1. Occurrence features of the Fuping color-change sapphires. (a,b) Fuping sapphires occurring in granitic gneiss; (c) Fuping sapphire crystals exposed in biotite sillimanite gneiss; (d) Sapphire samples from the Mogok gneiss in Burma; (e) Microscopic picture of Fuping color-change sapphire (sample FU-3) showing the mineral assemblage of sapphire (Sap), sillimanite (Sil), biotite (Bi) and potassium feldspar (Kf); (f) A reddish-brown rutile (Rul) inclusion within Fuping color-change sapphire, sample FU-3.
Figure 1. Occurrence features of the Fuping color-change sapphires. (a,b) Fuping sapphires occurring in granitic gneiss; (c) Fuping sapphire crystals exposed in biotite sillimanite gneiss; (d) Sapphire samples from the Mogok gneiss in Burma; (e) Microscopic picture of Fuping color-change sapphire (sample FU-3) showing the mineral assemblage of sapphire (Sap), sillimanite (Sil), biotite (Bi) and potassium feldspar (Kf); (f) A reddish-brown rutile (Rul) inclusion within Fuping color-change sapphire, sample FU-3.
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Figure 2. Color-change characteristics of Fuping sapphires, showing purplish blue color under sunlight (a,c), and light purplish red color under incandescent lamp light (b,d).
Figure 2. Color-change characteristics of Fuping sapphires, showing purplish blue color under sunlight (a,c), and light purplish red color under incandescent lamp light (b,d).
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Figure 3. Ultraviolet and visible absorption spectra of color-change sapphires from different regions. (a) The absorption peaks at 377 nm, 385 nm, 450 nm, 694 nm and 741 nm and the absorption bands of 558–570 nm can be obviously observed in the sample FU-5 of the Fuping color-change sapphires; The absorption peaks of 390 nm, 450 nm, 694 nm, 741 nm and the wide and slow absorption band of 565 nm can be observed in the color-change sapphire sample MD-9 from Mogok, Myanmar; The absorption peaks at 450 nm, 562 nm, 694 nm can be observed in Xinjiang color-change sapphires, the data cited from [13]. (b) Tanzanian Umba color-change sapphire exhibits absorption peaks of 377 nm, 388 nm, 450 nm and 560 nm wide and slow absorption band, the data cited from [14]; The synthetic color-change sapphire has absorption peaks at 400 nm and 580 nm, the data cited from [6].
Figure 3. Ultraviolet and visible absorption spectra of color-change sapphires from different regions. (a) The absorption peaks at 377 nm, 385 nm, 450 nm, 694 nm and 741 nm and the absorption bands of 558–570 nm can be obviously observed in the sample FU-5 of the Fuping color-change sapphires; The absorption peaks of 390 nm, 450 nm, 694 nm, 741 nm and the wide and slow absorption band of 565 nm can be observed in the color-change sapphire sample MD-9 from Mogok, Myanmar; The absorption peaks at 450 nm, 562 nm, 694 nm can be observed in Xinjiang color-change sapphires, the data cited from [13]. (b) Tanzanian Umba color-change sapphire exhibits absorption peaks of 377 nm, 388 nm, 450 nm and 560 nm wide and slow absorption band, the data cited from [14]; The synthetic color-change sapphire has absorption peaks at 400 nm and 580 nm, the data cited from [6].
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Figure 4. Cr2O3/Ga2O3 versus Fe2O3/TiO2 ratios of the Fuping color-change sapphires as well as blue sapphires from other major gem-producing countries [39]. The data of the Fuping color-change sapphires in China are shown with red circles. The dashed line is the boundary between basalt-type and non-basalt-type sapphires.
Figure 4. Cr2O3/Ga2O3 versus Fe2O3/TiO2 ratios of the Fuping color-change sapphires as well as blue sapphires from other major gem-producing countries [39]. The data of the Fuping color-change sapphires in China are shown with red circles. The dashed line is the boundary between basalt-type and non-basalt-type sapphires.
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Figure 5. Comparison of trace element compositions of non-basalt-type sapphires. (a) Comparison of Fe element content; (b) Comparison of Ti element content; (c) Comparison of Cr and Ga contents. Note: Data of Fe, Ti, Cr and Ga elements of sapphires from Madagascar, Sri Lankan and Myanmar are taken from [39].
Figure 5. Comparison of trace element compositions of non-basalt-type sapphires. (a) Comparison of Fe element content; (b) Comparison of Ti element content; (c) Comparison of Cr and Ga contents. Note: Data of Fe, Ti, Cr and Ga elements of sapphires from Madagascar, Sri Lankan and Myanmar are taken from [39].
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Table 1. Major and trace element compositions of color-change sapphires from the Fuping, China and Mogok, Myanmar.
Table 1. Major and trace element compositions of color-change sapphires from the Fuping, China and Mogok, Myanmar.
Sample IDAl2O3MeanFeMeanTiMeanVMeanCrMeanGaMean
FU-1-199.0398.95619365425150118115288294129131
FU-1-298.87674241113435127
FU-1-398.94669159114160137
FU-2-199.0598.9353765709344785100439533142140
FU-2-298.82559555108600132
FU-2-398.91615751108560147
FU-3-199.1498.61548355464042106112473525149144
FU-3-298.91546346112565142
FU-3-397.77569140118537141
FU-4-199.1298.955318568547838194180336144137
FU-4-298.9456354394446145
FU-4-398.9554945591486147
FU-4-498.806291188109231112
FU-5-199.0698.97516251582630365546150498102
FU-5-298.9353813867558113
FU-5-398.9652423756521111
FU-5-498.934848216047785
FU-6-198.7998.9055675245525314113712501207159150
FU-6-298.815303611461246147
FU-6-399.094866481251126144
MD-9-199.3599.301762 1863252 78687 93314 35667 85
MD-9-299.531480 206 74 381 66
MD-9-399.212079 1355 108 317 104
MD-9-499.112132 1329 105 412 104
Note: The Al2O3 data unit is wt.%, and the other trace elements data units are ppm.
Table 2. Trace element compositions of color-change sapphire from Fuping and Xinjiang in China, Mogok in Myanmar and Tanzania deposits.
Table 2. Trace element compositions of color-change sapphire from Fuping and Xinjiang in China, Mogok in Myanmar and Tanzania deposits.
Number of SamplesFeTiVCrReference
Average value of FU-551583055504This paper
Average value of MD-9186378693356This paper
Tanzania color-change sapphire134020.38.6515.9[14]
Xinjiang color-change sapphire5141.1154.44bdl25.81[13]
The unit is ppm; bdl stands for below detection limit.
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Wang, H.; Yu, X.-Y.; Liu, F.; Alam, M.; Wu, G.-C. Color Genesis and Compositional Characteristics of Color-Change Sapphire from Fuping, China. Crystals 2022, 12, 463. https://doi.org/10.3390/cryst12040463

AMA Style

Wang H, Yu X-Y, Liu F, Alam M, Wu G-C. Color Genesis and Compositional Characteristics of Color-Change Sapphire from Fuping, China. Crystals. 2022; 12(4):463. https://doi.org/10.3390/cryst12040463

Chicago/Turabian Style

Wang, Hui, Xiao-Yan Yu, Fei Liu, Masroor Alam, and Gai-Chao Wu. 2022. "Color Genesis and Compositional Characteristics of Color-Change Sapphire from Fuping, China" Crystals 12, no. 4: 463. https://doi.org/10.3390/cryst12040463

APA Style

Wang, H., Yu, X. -Y., Liu, F., Alam, M., & Wu, G. -C. (2022). Color Genesis and Compositional Characteristics of Color-Change Sapphire from Fuping, China. Crystals, 12(4), 463. https://doi.org/10.3390/cryst12040463

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