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Article

Gemology, Mineralogy, and Spectroscopy of Gem-Quality Diopside from Pakistan and Russia: New Insights for the Chromogenic Mechanism and Possible Origin

by
Zixuan Wang
1,*,
Xiaoqing He
2,*,
Bo Xu
1,3,4 and
Jing Ren
1
1
School of Gemology, China University of Geosciences Beijing, 29 Xueyuan Road, Haidian District, Beijing 100083, China
2
Zhongjin Gold Co., Ltd., Beijing 100011, China
3
State Key Laboratory of Geological Processes and Mineral Resources, China University of Geosciences, Beijing 100083, China
4
The Beijing SHRIMP Center, Chinese Academy of Geological Sciences, Beijing 100037, China
*
Authors to whom correspondence should be addressed.
Crystals 2023, 13(5), 727; https://doi.org/10.3390/cryst13050727
Submission received: 2 April 2023 / Revised: 13 April 2023 / Accepted: 18 April 2023 / Published: 25 April 2023
(This article belongs to the Topic In-Situ Study of Mineralogy, Gemology and Progress in Gemology)
(This article belongs to the Section Mineralogical Crystallography and Biomineralization)
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Versions Notes

Abstract

:
Green diopside is currently popular in the jewelry market due to its attractive color and excellent transparency. Gem-quality diopsides are mainly sourced from Pakistan, Italy, Russia, and other places. The color, geographic origin, and formation mechanism are the main factors affecting the value of gemstones, which can be determined by examining their gemology and composition characteristics. This study systematically characterizes the standard gemology of green diopsides from Pakistan and Russia and compares them with the blue diopsides produced within the skarn process and the diopsides from the nearby region in Russia from previous studies by gemological microscopy, spectral testing (infrared, Raman and ultraviolet-visible spectroscopy), and chemical analyses (electron probe and laser ablation inductively coupled plasma mass spectrometry). The results show that the spectral characteristics and phase composition of the green diopside samples from Pakistan and Russia have excellent uniformity and similarity. The high Cr, Fe, V, and Ni contents are the reasons why they appear as green. Meanwhile, the elemental characteristics of the diopside are effective tools for distinguishing different origins of different diopsides. The Russian green diopsides have higher contents of Sr, Sc, Zr, and rare earth elements (REE), indicating that they are related to alkaline ultrabasic rocks, and the source of the diopside sample from Pakistan is metamorphic rock.

1. Introduction

Diopside is commonly found in metamorphic and igneous rocks, and gem-quality varieties are mainly exploited in Myanmar, Canada, China, Pakistan, Italy, South Africa, Russia, the United States, Austria, India, and other places (Figure 1). Diopside is a mineral with a structure of silicate tetrahedra, linked by sharing oxygen as a single chain, belonging to the pyroxene group, with an ideal formula of CaMg[Si2O6]. The general formula of pyroxenes is M2M1T2O6, where M2 structural position is occupied by the cations coordinated by 6–8 oxygen atoms in the generally distorted octahedral coordination. An M1 structural position is occupied by cations with six-fold coordination in the regular octahedral coordination, with Si4+ occupying the tetrahedral position T. Si4+ occupying the T-site can be replaced by Al3+ and Fe3+ in the case of the silica undersaturated end members of the group. The M1 site is occupied by Mg2+, Fe2+, Co2+, Fe3+, Cr3+, etc., while the Ca2+ mainly occupying the M2 site can be replaced by Na+, Mg2+, and Fe2+. Single chains of SiO4 tetrahedra extending parallel to the c axis determine the monoclinic crystal structure of pyroxene in the space group C2/c. The chain structure offers much flexibility in the incorporation of various cations, which is limited by the size of the sites in the structure and the charge of the substituting ions [1,2,3,4,5,6,7,8,9,10,11].
Due to the increasing demand in the jewelry market and the discovery of many jewelry deposits producing diopside worldwide, the appeal of diopside has increased considerably. In the literature, there are far more studies on gem-quality diopside from Russia and Italy than those from other regions. Meanwhile, the present studies of gem-quality diopside are almost limited to gemological basic tests, lacking systematic studies of spectral and compositional characteristics, as well as systematic comparisons of diopsides output from different regions [11,12,13,14].
In this paper, gem-quality diopsides from Pakistan and Russia were selected (Figure 1), and gemological, spectroscopic, and major and trace element analyses were conducted to determine the gemological and compositional characteristics of these crystals. Meanwhile, we also collected the data of gem-quality blue diopsides from Italy and more Russian diopsides from previous studies, aiming to explore the mechanism of coloration and genetic differences of different diopsides [15,16]. This study can enrich the theoretical, gemological, and mineralogical knowledge of diopside and provide a better understanding of the geological background producing these different diopsides.

2. Geological Setting

The Shigar valley, located about 32 km north of Skardu, is one of the most famous valleys of the Gilgit-Baltistan region of Pakistan, as it is the gateway for most of the expeditions to the K-2, the second highest peak of the world. This valley comprises the Himalayan collision zone, illustrated by the Karakoram mountain ranges (KMR) and the Kohistan-Ladakh Island Arc (KLIA), which possessed a complex history of crustal evolution. The KMR exposed in the Shigar Valley comprises predominantly granitoids, amphibolite, locally granulite-grade polyphase deformed sedimentary rocks, and orthogneiss [17,18,19,20]. A large number of pegmatites are also produced in the Shigar valley. Diopside has been reported mainly in the pegmatites and also in the metamorphic rocks of the Shigar valley [18,21,22,23,24,25,26,27]. The diopside sample from Pakistan in this study was obtained from skarn formed by metamorphism of limestone.
The Inagli deposit is located within the Inagli massif of alkaline ultramafic rocks of the potassic series, 30 km west of the city of Aldan, Yakutia, Russia [12,28]. The massif is topographically manifested as a cupola structure with a central caldera. The central part of the massif is made up of forsterite dunites, surrounded by alkali gabbroids and pulaskites. The gabbroids are mainly shonkinites grading into mica-bearing pyroxenites, and the pulaskites are accompanied by alkaline pegmatites. Sheet intrusions of syenite porphyry occur at the periphery of the massif within the Cambrian carbonate sequence. The major units of the geologic structure of the deposit are the country dunites, hosting numerous pegmatite veins composed of early diopside-bearing and late amphibole–feldspar varieties. The deposit contains ten chrome diopside-bearing vein zones, among which only one is of economic importance [12,29,30,31,32]. Alkaline pegmatite veins of dunite and the feldspar–diopside metasomatic rocks are the primary sources of diopside in this region [12,13,14,29,33].

3. Materials and Methods

3.1. Samples Description

Standard gemological tests were performed on two diopside samples (PKD and RUS) from Pakistan and Russia, respectively. These samples were taken from the market, being all rough stones with a green color. They were transparent, with a bright glass luster (Figure 2 and Figure 3). The crystal morphology of the diopside sample from Pakistani (PKD) was short prismatic, with an idiomorphic grain, and the colors of the individual crystal columns were uneven due to varying levels of local transparency (Figure 2A,B). The crystal surface displayed stepped crystal patterns and raised growth mounds, with the cross sections of the growth mounds being square (Figure 3A,B). The cleavage and fracture in the sample are relatively well developed. When observed under reflected light, distinct internal cleavage flash could be seen from the crystal surface. The healing cracks contained groups of gas–liquid inclusions (Figure 3C). In contrast, the Russian sample (RUS) had a uniform color (Figure 2C,D), with a bright color and high saturation. The crystal morphology was irregular, with conchoidal fractures (Figure 3D,E). The interior of the sample was relatively clean, with fewer visible crystalline inclusions. However, there were more gas–liquid inclusions and cracks in the shape of the beads or fingerprints (Figure 3F).

3.2. Microscopic Analysis and Spectroscopy

The gemstone microscopic observation, infrared spectrum, UV-visible spectrum, and laser Raman spectrum were conducted in the Gemological Experimental Teaching Center of School of Gemology, China University of Geosciences (Beijing). Internal and external features were observed with a GI-MP22 gemological photographic microscope using a dark field, bright field, and top illumination.
The infrared spectra were conducted with a Tensor 27 Fourier-transform infrared spectrometer (Bruker, Billerica, MA, USA) using a reflection method. The reflection method conditions were as follows: humidity of less than 70%, scanning voltage of 85–265 V, resolution of 4 cm−1, grating of 6 mm, test range of 400–2000 cm−1, and scanning signal accumulation of 32 times. The UV-visible (UV-VIS) spectroscopy tests were collected from 300 to 800 nm with a UV-3600 UV-VIS spectrophotometer (Shimadzu Corporation, Kyoto, Japan) using a reflection method. A data interval of 2 nm and a scan speed of 405 nm/min were used. An HR-Evolution-type micro-Raman spectrometer (HORIBA, Shimadzu, Kyoto, Japan) was used to perform Raman spectroscopy tests with a 532 nm laser. Raman spectra were collected from 400 to 4000 cm−1, with 50 mW of laser power and a resolution of 4 cm−1. The slit width was set to 100 µm, with a grating of 600 gr/mm. The scanning time was 4 s, and the integration time was 3 s.

3.3. SEM Analyses

The two diopside samples were first mechanically crushed, after which the pure parts of the samples were selected under a binocular microscope and placed in an epoxy block to polish it to the largest surface. The polished portions were surface carbon blasted before testing. The backscattered electron (BSE) imaging was performed to observe the structure of diopside samples at the Beijing Gaonianlinghang Company Limited (Beijing, China). The XL30-SFEG electron beam from the company FEI (Eindhoven, The Netherlands) was used for micrographic analysis of diopside samples.

3.4. Chemical Analysis

The electron microprobe analysis was completed in the Experimental Center of the Research Institute of China University of Geosciences (Beijing). The laser ablation–inductively coupled plasma–mass spectrometry (LA-ICP-MS) analysis was carried out at the Institute of Geomechanics, Chinese Academy of Geological Sciences. All the data results are shown in Table 1 and Table 2.
Major-element compositions were analyzed by an electron probe micro-analyzer (EPMA), JEOL model JXA-8100 (Tokyo, Japan). The analyses were carried out with an accelerating voltage of 15 kV and an electron beam current of 20 nA. Measurement times were set at 10 s for the peak of all analyzed elements and 5 s for the background. The standards used for calibration were NaAlSi2O3 (Na), KAlSi3O8 (K), Cr2O3 (Cr), diopside (Si, Mg, Ca), Fe2O3 (Fe), pyrope (Al), Mn2O3 (Mn), NiO (Ni), ZnO (Zn), TiO2 (Ti), and Ca5P3F (P) [35,36].
LA-ICP-MS was applied to analyze the trace element concentrations, using a 193 nm excimer laser ablation system (GeoLas HD; Coherent, Santa Clara, CA, USA), and a four-stage rod mass spectrometer (Agilent 7900, Agilent Technologies, Tokyo, Japan) was used for inline testing, with Ar and He as the carrier gases. The laser employed a 10 Hz pulse rate and a 35 μm diameter spot size. The energy density was about 9 J/cm2. NIST-SRM 610 and 612 glasses reference materials were used as external calibration standards. Data reduction was carried out using the software Iolite to analyze the following elements: Li, Be, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, and Zn [37].

4. Results

4.1. Visual Appearance and Gemological Properties of Diopside

The two diopside samples from Pakistan and Russia were a green uniform color, with a transparent and glassy luster (Figure 3). The PKD diopside sample showed a stepped cleavage with two well-defined cleavage planes (Figure 3A). The RUS diopside sample had greasy and shiny conchoidal fractures (Figure 3D,E). Gemstone microscopic observation showed that there were inclusion groups and fractures in both of the diopside samples. They all had lamellar gas–liquid groups and partially healed fissures (Figure 3C,F).

4.2. Spectral Characteristics

4.2.1. FTIR Spectrum

The representative FTIR spectrums of the diopside samples from Pakistan and Russia are shown in Figure 4. The infrared spectra of the two diopside samples in the range of 4–1200 cm−1 are relatively similar. In general, no reflectance bands are observed above 1500 cm−1, which suggests the absence of OH groups or water molecules in these crystalline phases.
According to the literature on diopside, the infrared spectrum vibration of diopside is mainly manifested in the characteristic Si–O stretching modes, non-bridging stretching modes of Si–O, and the non-bridging bending vibrations of the O–Si–O of a SiO4 tetrahedron [38,39,40,41,42]. All the detected bands were in good agreement with the vibrations of the functional groups of diopside. Seven weak absorption peaks at around 472 cm−1, 505 cm−1, 534 cm−1, 550 cm−1 625 cm−1, 656 cm−1, and 671 cm−1 were assigned to the bending vibrations of the non-bridging bending vibrations of the O–Si–O of a SiO4 tetrahedron. The broad absorption band around 862 cm−1 and the broad absorption band in the range of 900–1100 cm−1 (including 916 cm−1, 935 cm−1, 957 cm−1 and 1100 cm−1) were assigned to the non-bridging stretching modes of Si–O and the characteristic Si–O stretching modes of the SiO4 tetrahedron, respectively. The infrared spectra of the diopside samples from Pakistan and Russia tended to be consistent, and only some absorption peaks were slightly offset, which could be due to different orientations of the diopside crystals within the gem or to variations in the elemental compositions of non-stoichiometric diopside phases.

4.2.2. Raman Spectra

The Raman spectra of pyroxene silicates are characterized by four types of vibration bands: (1) Si–O nonbridging stretching (Si–Onbr); (2) Si–O bridging stretching (Si–Obr); (3) O–Si–O bending; and (4) cation M1- and M2-oxygen vibration modes (M–O). The stretching modes of Si–Onbr are observed in the spectra at frequencies higher than the modes of Si–Obr due to a stronger force constant, i.e., the coefficient that links the displacement of atoms from their equilibrium position with the force that such displacement induces on adjacent atoms [43,44,45,46,47].
The Raman spectra of the diopside samples from Pakistan and Russia showed sharp spectrum peaks and high signal-to-noise ratios (Figure 5). The Raman spectra of the diopside samples from Pakistan and Russia exhibited remarkable similarities to each other, as well as to those previously reported on in the literature [45,47], and only the intensity and wavenumber of each Raman scattering peak feature were slightly different. There were four evident peaks within the range of 100–1200 cm−1, respectively, 322 cm−1, 389 cm−1, 666 cm−1, and 1012 cm−1. The 322 cm−1 and 389 cm−1 figures were caused by the cation–oxygen vibrations. The 666 cm−1 figure was the stretching of Si–Obr bonds, and 1012 cm−1 corresponded to the Si–Onbr stretching mode [45,48]. The Pakistan diopside sample showed a high Raman scattering peak intensity of 141 cm−1, while the diopside sample from Russia was relatively weak. The Raman scattering peaks of the diopside samples from Pakistan and Russia showed the same positions but different intensities, which may be caused by the variation in the cation occupancy in the M sites. In other words, the lattice vibration involving Mg should also affect Raman spectrum characteristics of the diopside samples in the range of 326–393 cm−1, leading to the difference in the intensities of the Raman scattering peaks [45].

4.2.3. UV-Visible Spectra

The UV-VIS spectra of the diopside samples from Pakistan and Russia showed similar patterns, as illustrated in Figure 6. The UV-VIS spectrum revealed that the diopside samples had significant reflections at 460–470 nm and 655–665 nm. The 460–470 nm and 655–665 nm reflections were attributed to Cr3+ in octahedral coordination [41,49]. The diopside samples from Pakistan and Russia had weak reflections between 230 nm and 285 nm, which were caused by the intervalence Fe2+–Fe3+ charge transfer [40,41,49].

4.3. Major and Trace Elements Characteristics of Diopside

The compositions of major elements and trace elements of the diopside samples from Pakistan and Russia are presented in Table 1 and Table 2, respectively. The diopside samples contained <5 mol.% ferrosilite (Fs). In the En–Wo–Fs classification diagram [50], they are all localized in the diopside corner (Figure 7A,B). We analyzed the major and trace elements of the diopside samples at the same positions, including the core and rim of the diopside crystals. There was no evident difference in the elements’ characteristics between the core and rim of the same diopside sample. Meanwhile, the backscattered electron images of the diopside samples from Pakistan and Russia also showed no zoned features (Figure 7C,D).
The diopside sample from Russia had higher Mg# (91–93; Mg# = molar Mg/(Mg + FeT) × 100), SiO2 (53.5–54.9 wt.%), Na2O (0.33–0.52 wt.%), and lower CaO (24.0–24.7 wt.%), while the diopside sample from Pakistan had relatively lower Mg# (81–86), SiO2 (52.9–54.2 wt.%), Na2O (0.12–0.39 wt.%), and higher CaO (24.4–25.1 wt.%). The major elements of the diopsides with different geographic origins showed great differences (Figure 8; Table 1).
The diopside sample from Russia displayed significantly higher concentrations of Cr, Zr, Sr, and Sc compared with the Pakistan diopside sample (Table 2). Investigating the distribution patterns of rare earth elements (REEs) can be used as a very valuable tool to determine phyrochemical conditions and the origin of minerals and rocks [51]. Notably, the chondrite-normalized REE diagrams revealed discernible differences between the Pakistan and Russia diopside samples (Figure 9). The diopside sample from Russia had a greater abundance of REE, was particularly enriched in light rare earth elements (LREEs), with no Eu anomalies, and had low levels of heavy rare earth element (HREE) contents. Conversely, the diopside sample from Pakistan displayed extremely low REE contents (Figure 9; Table 2).

5. Discussion

5.1. Gemological Characteristics

The diopside sample from Russia (RUS) exhibited remarkable characteristics of high transparency, a strong luster, and a bright green color with yellow tones. The transparency of the Pakistan diopside sample (PKD) was relatively modest, with shades of emerald-green that bore a resemblance to the Russian diopside sample. The Pakistani diopside sample had been subjected to an irregular configuration of crystal particles, alongside impurities that impeded its transparency levels, giving it a semi-translucent, micro-transparent appearance. Notably, the luster of diopside samples from Pakistan and Russia exhibited little variation.
The diopside samples from Pakistan and Russia exhibited color stabilities under the Charles filter. The Russian sample, however, displayed a visible red absorption band of Cr3+ under spectroscopic examination. Conversely, the Pakistan sample showed only a combined absorption band of Fe2+/Fe3+ or Fe2+ + Fe3+. Additionally, both samples displayed almost no reaction under ultraviolet fluorescence, which was likely due to the presence of Fe content within the samples. The refractive indexes of the diopside samples from Pakistan and Russia, after being polished and ground, were measured using a refractometer. The results revealed that the refractive indexes for the two diopside samples ranged between 1.68 and 1.69. The average refractive index for the diopside sample from Pakistan was slightly higher than that of the Russian diopside sample. This variance in refractive index was attributed to the Fe content in the diopside samples, as the Russian samples possessed lower Fe content than that of the Pakistan sample (Table 1).

5.2. Chromogenic Mechanism of Different Diopsides

The chemical composition of diopside is CaMg[Si2O6]. Ca2+ and Mg2+ could be often replaced by Fe3+, Fe3+, V3+, Cr3+, Mn2+, and other transition metal ions, which are known as “chromophores” and are considered to play a significant role on the color characteristics. In this study, we give a certain explanation for the chromogenic mechanism of diopsides of different colors, according to the characteristics of their chemical compositions.
We collected the chemical compositions of the blue diopsides from Saint Marcel (Val D’Aosta, Italy) to compare with the diopside samples from Pakistan and Russia in this study [15,16]. The collected diopside data from the Zagadochnaya kimberlite (Yakutia, Russia) had no color information, and they also were not gem-quality diopside; thus, these data were excluded from the study of chromogenic mechanism [15].
The transition elements contained in these blue and green diopsides were mainly Cr, Mn, Fe, Co, Ni, and V. The Ti and REE contents of the diopsides in different colors cannot be distinguished, suggesting they had no decisive effect on chromogenesis. As shown in Figure 10, the green hue observed in the diopside samples could be explained by their enrichment in Cr, Fe, V, and Ni, with respect to the blue diopside samples. Mn and Co contents could be related to the blue hue of the diopside; the blue diopside samples from Italy had higher Mn and Co contents compared with the green diopsides (Figure 10).
In summary, our new data showed that the behavior of major and trace elements played a significant role in the color differences of diopside crystals. The diopside was colored blue to green by Cr and Fe, respectively, both substituting for Al and Mn in the structure (Figure 11). Meanwhile, the enrichment of V and Ni instead of Co in the trace elements made the diopside present green rather than blue (Figure 10).

5.3. Origin of Different Diopsides: Elemental Constraints

Combining the collected diopside data from Saint Marcel (Val D’Aosta, Italy) and the same region in Russia, as well as the data in this study, we can see the distinct characteristics of the contrasting trace element patterns and the chondrite-normalized REE (Figure 9 and Figure 12) [15,16]. The diopside samples from Russia exhibited higher Sc, Zr, Sr, and REE contents than those of the Pakistan and Italy diopside samples (Figure 9 and Figure 12), which could be well explained by the distinct formation processes of the two categories of diopside samples with different origins. The diopside samples from Russia formed from the magmatic evolution process of ultrabasic rocks and crystallized as a basic mineral at the early stages. The overall compositions of the diopsides from Russia remained relatively uniform, indicating their crystallization in a similar magma system. They had higher Cr, Sc, Zr, Sr, and REE, which were close to the composition of volcanic clinopyroxene in basalt magma [52,53].
Clinopyroxene is commonly present in igneous rocks and exhibits large compositional variations. Numerous thermometers that utilize clinopyroxene composition have been established and widely applied in magmatic systems, due to their compositional sensitivity to changes in temperature [54,55,56,57]. The clinopyroxene-only thermometer is one of the most practical tools to reconstruct crystallization temperatures of clinopyroxenes because it does not require any information of coexisting melts or other co-crystallized mineral phases. Thus, an updated clinopyroxene-only thermometer was used for the diopside sample from Russia in this study and the collected diopside data from Russia, which yielded uncertainties in the estimated temperature (±37 °C) [15,34], returning crystallization temperature conditions of 1107–1188 °C and 1108–1166 °C, respectively (Figure 13). These similar crystallization temperatures also confirmed that they formed at similar magmatic crystallization stages, similar to the early stage of alkaline ultrabasic magma.
The collected diopside data from Russia exhibited comparably low Mg# in contrast to those of the diopside sample from Russia in this study, indicating the relatively latter crystallization process of the collected Russian diopsides, which was further proved by their lower crystallization temperatures (Figure 8 and Figure 13). The Sc and Zr, as incompatible elements within the magma evolution process, showed an upward trend in conjunction with the decrease in Mg# (Figure 12B,D). Conversely, the Sr content of diopsides from Russia gradually declined as the Mg# decreased (Figure 12C), suggesting that the crystallization of plagioclase, which is rich in Sr, had occurred with diopside crystallization, resulting in a significant depletion of Sr in the residual melt [58,59,60,61]. The relatively high concentration of the REE in alkaline ultrabasic magma rendered the diopside from Russia to have a notable REE contents, with comparable REE distribution characteristics (Figure 9).
The diopside sample from Pakistan shared similar composition characteristics with the diopsides from Italy, produced in the skarn metamorphism process. They all had very low Sc, Zr, Sr, and REE contents (Figure 12) and exhibited similar REE characteristics, which were significantly different from the Russian diopside produced during magmatic processes (Figure 9) [60,61,62,63,64,65]. These suggested that the diopside sample from Pakistan also crystallized within the skarn metamorphism process.
In summary, the elemental characteristics of the diopside samples could well distinguish the distinct formation processes of different diopsides from Russia and Pakistan (Figure 9 and Figure 12). The diopsides from Russia crystallized in the early stages of the ultrabasic magma evolution process, while the diopsides from Pakistan crystallized during the process of skarn metamorphism.

6. Conclusions

We have investigated the gemological characteristics of diopside samples from Pakistan and Russia and analyzed their infrared spectra, Raman spectra, ultraviolet spectra, and chemical compositions. The results were used to present a comprehensive report on their systematic mineralogical characteristics and to discuss the differences in the chromogenic mechanisms and origins of the diopsides. The diopsides from Russia and Pakistan had similar gemological characteristics. The green hue of the diopside was due to chromophore elements such as Cr, Fe, V, and Ni, while the blue color could be attributed to the Al, Mn, and Co elements. The green diopside sample from Pakistan crystallized during the skarn metamorphic process, and the Russian green diopside crystallized at an early stage of the ultrabasic magma evolution process. These findings provided valuable insights into how the diopsides were formed, along with the geological histories and mineralogies of these two regions. These diopside samples showed well-developed crystals without any zoning in BSE images, which enhanced their scientific value and made them suitable as standard samples for further research.

Author Contributions

Writing—original draft, Z.W.; writing—review and editing, Z.W., X.H. and B.X.; data curation, Z.W. and J.R.; methodology, Z.W. and X.H. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by the National Natural Science Foundation of China (42222304, 42073038, 41803045, 42202081), the Young Talent Support Project of CAST, the Fundamental Research Funds for the Central Universities (Grant No. 265QZ2021012), and IGCP-662.

Data Availability Statement

The data presented in this study are available within the article.

Acknowledgments

We thank the editor and reviewers for their constructive comments, which helped in improving our paper. This is the 14th contribution of B.X. from the National Mineral Rock and Fossil Specimens Resource Center.

Conflicts of Interest

The authors declare no conflict of interest.

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Crystals 13 00727 g001 550
Figure 1. Statistical diagram of the global gem-diopside origins. The gray shaded areas are the countries of gem-diopside origins. The red dots are the locations of the diopside samples from Pakistan and Russia.
Figure 1. Statistical diagram of the global gem-diopside origins. The gray shaded areas are the countries of gem-diopside origins. The red dots are the locations of the diopside samples from Pakistan and Russia.
Crystals 13 00727 g001
Crystals 13 00727 g002 550
Figure 2. The two diopside rough samples collected and examined for this study: (A,B) Diopside rough sample PKD from Pakistan; (C,D) Diopside rough sample RUS from Russia.
Figure 2. The two diopside rough samples collected and examined for this study: (A,B) Diopside rough sample PKD from Pakistan; (C,D) Diopside rough sample RUS from Russia.
Crystals 13 00727 g002
Crystals 13 00727 g003 550
Figure 3. The micrographs of diopside sample PKD (A–C) and RUS (D–F); (A) stepped cleavage formed by two well-defined cleavage planes of PKD diopside sample; (B) fracture perpendicular to c-axis of PKD diopside sample; (C) inclusion group within PKD diopside sample; (D,E) conchoidal fractures of RUS diopside sample; (F) inclusion group within RUS diopside sample.
Figure 3. The micrographs of diopside sample PKD (A–C) and RUS (D–F); (A) stepped cleavage formed by two well-defined cleavage planes of PKD diopside sample; (B) fracture perpendicular to c-axis of PKD diopside sample; (C) inclusion group within PKD diopside sample; (D,E) conchoidal fractures of RUS diopside sample; (F) inclusion group within RUS diopside sample.
Crystals 13 00727 g003
Crystals 13 00727 g004 550
Figure 4. FTIR spectrum for the 400–1200 cm−1 frequency range, obtained from PKD and RUS diopside samples.
Figure 4. FTIR spectrum for the 400–1200 cm−1 frequency range, obtained from PKD and RUS diopside samples.
Crystals 13 00727 g004
Crystals 13 00727 g005 550
Figure 5. The representative Raman spectrum of the diopside samples.
Figure 5. The representative Raman spectrum of the diopside samples.
Crystals 13 00727 g005
Crystals 13 00727 g006 550
Figure 6. The representative UV-Vis-NIR spectrum of the diopside samples.
Figure 6. The representative UV-Vis-NIR spectrum of the diopside samples.
Crystals 13 00727 g006
Crystals 13 00727 g007 550
Figure 7. (A) The diagram of pyroxene classification based on En–Wo–Fs compositions for the diopside samples; (B) partial Enlargement of the En–Wo–Fs composition diagram (A); (C,D) backscattered electron images of the Pakistan and Russia diopsides.
Figure 7. (A) The diagram of pyroxene classification based on En–Wo–Fs compositions for the diopside samples; (B) partial Enlargement of the En–Wo–Fs composition diagram (A); (C,D) backscattered electron images of the Pakistan and Russia diopsides.
Crystals 13 00727 g007
Crystals 13 00727 g008 550
Figure 8. Variation diagrams of Mg# vs. (A) SiO2; (B) Al2O3; (C) Na2O; (D) CaO for the diopside samples.
Figure 8. Variation diagrams of Mg# vs. (A) SiO2; (B) Al2O3; (C) Na2O; (D) CaO for the diopside samples.
Crystals 13 00727 g008
Crystals 13 00727 g009 550
Figure 9. Chondrite-normalized REE patterns for the diopside samples. The grey and blue areas represent the diopsides from magmatic and skarn processes, respectively.
Figure 9. Chondrite-normalized REE patterns for the diopside samples. The grey and blue areas represent the diopsides from magmatic and skarn processes, respectively.
Crystals 13 00727 g009
Crystals 13 00727 g010 550
Figure 10. Variation diagrams of (A) Cr2O3 vs. MnO; (B) FeOT vs. Al2O3; (C) V vs. Ni; (D) Co vs. Cr for the diopside samples.
Figure 10. Variation diagrams of (A) Cr2O3 vs. MnO; (B) FeOT vs. Al2O3; (C) V vs. Ni; (D) Co vs. Cr for the diopside samples.
Crystals 13 00727 g010
Crystals 13 00727 g011 550
Figure 11. Variation diagrams of Cr2O3 + FeOT vs. MnO + Al2O3 for the diopside samples.
Figure 11. Variation diagrams of Cr2O3 + FeOT vs. MnO + Al2O3 for the diopside samples.
Crystals 13 00727 g011
Crystals 13 00727 g012 550
Figure 12. Variation diagrams of Mg# vs. (A) REE, (B) Sc, (C) Sr, and (D) Zr for the diopside samples.
Figure 12. Variation diagrams of Mg# vs. (A) REE, (B) Sc, (C) Sr, and (D) Zr for the diopside samples.
Crystals 13 00727 g012
Crystals 13 00727 g013 550
Figure 13. Temperature results for the diopside samples from Russia.
Figure 13. Temperature results for the diopside samples from Russia.
Crystals 13 00727 g013
Table
Table 1. Major element composition (wt.%) of diopside samples from Pakistan and Russia by EPMA.
Table 1. Major element composition (wt.%) of diopside samples from Pakistan and Russia by EPMA.
Sample SpotsSiO2TiO2Al2O3FeOMnOMgOCaONa2OK2OCr2O3NiOTotal
RUS-0154.290.060.421.250.1417.3724.200.390.030.670.0698.88
RUS-0254.120.170.291.150.0217.8124.180.410.010.640.0498.84
RUS-0354.700.160.241.050.0518.2524.650.390.010.590.09100.19
RUS-0454.630.090.251.050.0718.1924.520.420.020.720.0299.98
RUS-0554.900.040.221.110.0018.2024.660.520.040.640.06100.37
RUS-0654.790.080.231.100.0118.3024.440.440.020.690.00100.09
RUS-0754.510.160.241.090.0618.1024.440.420.010.700.1099.82
RUS-0854.540.070.231.060.0617.8523.990.350.010.670.1198.92
RUS-0953.520.070.251.100.0518.3324.440.330.020.670.0998.85
RUS-1053.830.090.231.090.0018.4724.110.390.010.610.0798.88
RUS-1154.050.160.331.130.1117.9724.100.390.020.690.0298.97
RUS-1253.460.070.321.180.1018.1924.320.420.030.830.0098.93
PKD-0154.220.000.492.170.1616.9524.870.290.000.390.2499.78
PKD-0253.360.000.522.340.1417.0524.410.350.010.460.2998.91
PKD-0353.370.000.412.210.1617.3324.490.220.010.190.2498.63
PKD-0453.500.000.442.330.2316.9624.440.290.000.260.2498.70
PKD-0553.500.020.442.580.1717.3224.420.270.010.210.2799.20
PKD-0653.470.030.472.530.0917.3824.870.310.000.240.0999.47
PKD-0753.020.000.492.710.1617.2624.640.340.000.190.2199.02
PKD-0853.470.000.462.640.1517.3324.890.390.030.260.1899.81
PKD-0953.200.000.682.370.1217.0824.550.340.010.360.3499.04
PKD-1052.930.000.163.070.2217.4125.100.150.010.060.1299.23
PKD-1153.330.000.162.670.1217.3724.720.120.010.000.1498.65
PKD-1253.510.000.142.430.2017.6125.060.130.020.080.1599.31
Cations on the Basis of 6 Oxygens
Sample SpotsSiTiAlIVAlVIAltotalFeMnMgCaNaKCrSumWoEnFsMg#T (°C) [34]
RUS-011.9910.0020.0090.0090.0180.0380.0040.9500.9510.0280.0010.0204.00348.9348.872.200.911161
RUS-021.9850.0050.0150.0000.0130.0350.0010.9740.9500.0290.0000.0194.01048.4849.691.830.921159
RUS-031.9810.0040.0190.0000.0100.0320.0020.9850.9560.0280.0010.0174.01548.4449.881.690.931148
RUS-041.9810.0020.0190.0000.0110.0320.0020.9830.9530.0300.0010.0214.01648.3649.921.730.931153
RUS-051.9840.0010.0160.0000.0090.0330.0000.9810.9550.0370.0020.0184.02048.5049.801.700.931107
RUS-061.9830.0020.0170.0000.0100.0330.0000.9880.9480.0310.0010.0204.01648.1350.161.700.931163
RUS-071.9820.0040.0180.0000.0100.0330.0020.9810.9520.0300.0000.0204.01448.3749.851.770.931155
RUS-081.9960.0020.0040.0060.0100.0320.0020.9740.9400.0250.0000.0194.00048.2749.981.750.931188
RUS-091.9680.0020.0320.0000.0110.0340.0021.0050.9630.0230.0010.0194.02748.0750.161.770.931141
RUS-101.9750.0020.0250.0000.0100.0330.0001.0100.9480.0280.0000.0184.02347.6050.721.680.931171
RUS-111.9800.0040.0200.0000.0140.0350.0030.9810.9460.0280.0010.0204.01348.1449.931.930.921171
RUS-121.9650.0020.0350.0000.0140.0360.0030.9970.9580.0300.0020.0244.03048.0349.991.970.921130
PKD-011.9850.0000.0150.0060.0210.0660.0050.9250.9750.0200.0000.0114.00949.4746.923.620.86
PKD-021.9740.0000.0260.0000.0230.0720.0040.9400.9680.0250.0010.0134.02048.7647.383.860.85
PKD-031.9770.0000.0230.0000.0180.0690.0050.9570.9720.0160.0000.0064.01948.5347.803.670.86
PKD-041.9820.0000.0180.0010.0190.0720.0070.9370.9700.0210.0000.0084.01548.8547.173.980.85
PKD-051.9740.0000.0260.0000.0190.0790.0050.9530.9650.0200.0010.0064.02348.1947.574.240.84
PKD-061.9670.0010.0330.0000.0200.0780.0030.9530.9800.0220.0000.0074.03048.6747.324.000.84
PKD-071.9640.0000.0360.0000.0210.0840.0050.9530.9780.0250.0000.0054.03548.4047.194.410.83
PKD-081.9650.0000.0350.0000.0200.0810.0050.9490.9800.0280.0010.0084.03648.6347.114.260.84
PKD-091.9680.0000.0320.0000.0300.0730.0040.9420.9730.0240.0010.0104.02548.8547.293.860.85
PKD-101.9610.0000.0390.0000.0070.0950.0070.9610.9960.0110.0010.0024.04048.3746.684.950.81
PKD-111.9780.0000.0220.0000.0070.0830.0040.9600.9820.0090.0000.0004.02348.4247.334.260.83
PKD-121.9720.0000.0280.0000.0060.0750.0060.9680.9900.0090.0010.0024.02948.5547.483.970.85
Wo, Ca2Si2O6; En: Mg2Si2O6; Fs: Fe2Si2O6; Mg# = molar Mg/(Mg + FeT) × 100.
Table
Table 2. Trace element concentrations (ppm) of diopside samples from Pakistan and Russia by LA-ICP-MS.
Table 2. Trace element concentrations (ppm) of diopside samples from Pakistan and Russia by LA-ICP-MS.
Sample SpotsLiBeBScVCrNiZnGaGeAsRbSrYZrCdSn
PKD-0119.61.716.311.6717.8265823951070.5371.46 10.5 0.406 0.250
PKD-0221.12.056.361.7918.2255224061170.4711.430.2210.28210.8 0.5640.02730.368
PKD-0321.21.907.362.2418.9233022371150.5390.6050.0581 11.2 0.2030.09560.532
PKD-0421.41.988.161.7918.3282425581210.6760.855 0.30111.40.02330.2260.2460.250
PKD-0516.81.447.562.1319.319622214129 1.250.605 9.63 0.4070.137
PKD-0612.21.327.442.2120.515692074135 0.576 11.00.02350.3170.2190.222
PKD-0712.11.357.632.3920.315472036148 1.210.1970.17711.0 0.360 0.263
PKD-0812.11.217.382.2819.7150720011390.6330.774 9.25 0.3140.2710.191
PKD-0915.21.927.591.4020.0180323061210.4631.470.3590.33612.00.01160.3570.2590.163
PKD-107.830.3213.266.518.82235805112 0.5800.1970.38811.50.1280.400 0.325
PKD-1110.60.8703.473.4211.5150123199.10.03251.130.0462 14.420.03470.111 0.255
PKD-129.180.9813.212.8310.315711881080.3231.530.01150.22913.40.03460.1100.2090.0416
RUS-010.5700.0722.7068.57.19507925710.80.9784.450.358 21495.378.59 0.386
RUS-020.3720.0292.9968.07.3349932508.760.9214.280.637 21505.458.810.1020.439
RUS-031.310.1302.5557.05.91406024910.90.6303.390.6030.053120653.733.910.08680.479
RUS-041.410.1303.1567.66.9045852579.461.103.650.476 20424.995.650.1580.564
RUS-051.650.1302.5468.66.81464426010.60.8434.510.721 21835.627.020.1560.445
RUS-061.420.1881.5869.97.31487726010.60.5442.620.781 22296.107.770.08410.279
RUS-072.210.1752.3261.96.43509525110.41.062.660.2100.44623074.255.65 0.283
RUS-081.970.1612.6963.56.7251902609.121.224.300.432 23124.546.55 0.622
RUS-092.420.1462.8758.55.8140512269.120.6083.800.304 22404.335.900.08280.151
RUS-101.380.1032.3563.46.51447925711.91.282.83 18974.195.830.1100.227
RUS-111.130.1473.2566.77.42468523810.81.582.160.105 21205.959.57 0.278
RUS-121.26 2.8670.27.7247862489.541.243.410.2340.16019165.759.760.09650.504
Sample SpotsBaLaCePrNdSmEuGdTbDyHoErTmYbLuHfPb (Total)
PKD-010.101 0.0108 0.0315 0.1553 00.545
PKD-02 0.01280.0123 0.06160.0732 0.01080.04280.0112 0.0210.05220.011900.389
PKD-03 0.01290.0248 0.0737 0.01090.0432 0.0106 0.12060.416
PKD-040.2050 0.0203 0.0741 0.0719 0.04030.475
PKD-05 0.03920.0126 0.0746 0.03220.0107 00.373
PKD-06 0.0207 0.011 0.01150.0324 00.465
PKD-07 0.0126 0.0888 0.03220.02150.0536 0.04080.35
PKD-08 0.013 0.0626 0.0413 0.0532 00.373
PKD-09 0.05 0.0206 0.03420.03180.0107 0.01210.04070.444
PKD-100.103 0.01240.0306 0.0731 0.04350.0341 0.01070.10480.0120.08130.459
PKD-110.309 0.0124 0.0715 0.0240.04060.393
PKD-12 0.0429 0.0107 0.04060.343
RUS-011.937.1919.13.0215.22.881.022.940.3190.8120.2470.3750.1070.4630.08370.770.897
RUS-021.216.4719.62.7115.64.051.222.720.221.330.2810.5320.07450.4650.09570.4860.824
RUS-031.816.3716.72.615.22.830.8772.570.3290.8130.1910.3760.04260.4140.09580.2830.823
RUS-040.86.819.23.1714.83.561.083.110.2191.030.2810.3140.09580.5720.09580.6060.838
RUS-052.096.6520.32.6514.72.411.212.340.1530.9430.315 0.04260.4690.07190.4040.727
RUS-061.097.421.32.9315.93.660.9613.50.3381.330.1690.6310.1280.5750.060.7260.797
RUS-071.687.0618.82.9313.13.440.8991.910.1191.150.2460.3780.02110.5730.04760.320.775
RUS-081.896.68182.5313.33.140.8982.730.2581.580.1670.3140.07380.6230.08290.4380.835
RUS-091.995.9516.42.3410.72.990.6731.840.2460.510.1770.2510.0420.4140.0590.2780.763
RUS-101.66.718.92.5414.23.351.242.530.2451.140.1990.4060.04190.3090.0470.2770.714
RUS-111.36.8519.8314.54.581.022.120.2541.650.1980.4680.0730.2050.1290.6690.75
RUS-122.117.2820.22.7816.241.123.280.2641.310.2860.4350.06240.460.1160.5880.771
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MDPI and ACS Style

Wang, Z.; He, X.; Xu, B.; Ren, J. Gemology, Mineralogy, and Spectroscopy of Gem-Quality Diopside from Pakistan and Russia: New Insights for the Chromogenic Mechanism and Possible Origin. Crystals 2023, 13, 727. https://doi.org/10.3390/cryst13050727

AMA Style

Wang Z, He X, Xu B, Ren J. Gemology, Mineralogy, and Spectroscopy of Gem-Quality Diopside from Pakistan and Russia: New Insights for the Chromogenic Mechanism and Possible Origin. Crystals. 2023; 13(5):727. https://doi.org/10.3390/cryst13050727

Chicago/Turabian Style

Wang, Zixuan, Xiaoqing He, Bo Xu, and Jing Ren. 2023. "Gemology, Mineralogy, and Spectroscopy of Gem-Quality Diopside from Pakistan and Russia: New Insights for the Chromogenic Mechanism and Possible Origin" Crystals 13, no. 5: 727. https://doi.org/10.3390/cryst13050727

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