Gemological and Chemical Characterization of Gem-Grade Peridot from Yiqisong, Jilin Province

Abstract

Peridot has a long history and is deeply loved by people for its unique olive-green color. The Yiqisong peridot deposit in Jilin Province is a newly discovered peridot deposit that still deserves systematic research. In this study, gemological and chemical analyses of thirty-three Yiqisong peridot samples were carried out to investigate the gemological characteristics, as well as the mantle properties and formation conditions of the Yiqisong. In addition, we identified gemological differences in peridot between Yiqisong, Tanzania, and Arizona. The Yiqisong peridot samples have typical peridot gemological characteristics. The UV–visible spectrum indicated that Fe is the chromogenic element. The infrared spectra and Raman spectra of different samples are consistent, which indicates that the Yiqisong peridot belongs to forsterite. The contents of Ni and V in Yiqisong peridot are generally low, distinguishing it from peridot found in Tanzania and Arizona. The major and trace elements of samples show that the Yiqisong peridot is derived from the spinel lherzolite xenoliths with the P–T formation conditions of 813–1087 °C and 21–22 kbar. The Yisqisong peridot samples have relatively high Fo values (up to 91.6), supporting their origin from a moderate refractory lithosphere mantle. Therefore, this study provides gemological, mineralogical, and chemical evidence that fills the research gap in peridot deposit studies and lays the foundation for follow-up investigations of gem-grade peridot deposits.

1. Introduction

Peridot is an ancient gem that was first discovered in ancient Egypt about 3500 years ago, and is also known as “the gem of the sun” [1]. Olivine is one of the most important rock-forming minerals in the mantle; it is the mafic silicate mineral [(Mg,Fe)₂SiO₄]. Peridot is a gem-quality variety of the mineral forsterite, which belongs to the olivine group. Peridot has a narrow range of colors, commonly brownish green, yellowish green, and pure green. The gem-quality peridot is usually transparent to translucent, and is known for its characteristic “olivine color”. Peridot inclusions include black chromite crystals, which surround flat healing planes, mica flakes, and ludwigite crystals. Peridot crystals have a strong birefringence; their internal inclusions often show double shadows, and it is more difficult for them to exhibit chatoyancy or other special optical phenomena. Peridot has a bright and uniform color and fewer internal inclusions, which is considered to be the best quality [1,2,3,4].

Gem-quality peridot is produced all over the world. Peridot was first collected as a gemstone in Western countries. Peridots mainly exist in peridotite xenoliths of alkali basalts as the surrounding rock, which is commonly found in San Carlos (Arizona, SW USA), Vietnam, Italy, Tanzania, Czech Republic, and China [5,6,7,8,9]. Peridot can also exist in serpentinized dunites, which can be found in the western Himalayas in Pakistan and are presumed to be related to hydrothermal activity [10]. The Myanmar peridot has good color, large size, and high quality. Mogok peridot crystals are usually found in veins and cavities within fine-grained serpentinized peridotite [11]. In addition, a small number of peridots can be found in volcanic bombs, which are early crystalline products of lava [4]. Peridot is not only produced internally within the Earth but also can be present in stony-iron meteorites. The differences in inclusions and trace elements make it relatively easy to distinguish between terrestrial and extraterrestrial peridots [1]. Some studies have also focused on artificially synthesized peridots, which can appear purple, blue-green, and yellow-green in color. The characteristic absorption spectra of synthetic peridots are obviously different from natural peridots, providing a basis for differentiation [12,13].

China has the largest peridot mineral reserves in the world: in 1979, the first gem-grade peridot deposit in China was discovered in Zhangjiakou, Hebei Province. Subsequently, another peridot deposit was discovered in Dunhua, Jilin Province, becoming the second-largest peridot-producing area in China [7,14,15]. In 2016, a new gem-grade peridot deposit was discovered in Yiqisong, Dunhua. This gem deposit is one of the few gem-quality peridot ore bodies in China [16]. At present, the study of peridot in this deposit mainly focuses on the geological characteristics and mineralization model, but few studies have simultaneously explored the gemological characteristics of Yiqisong peridot and its chemical characteristics.

Our results supplement the gemological and spectral characteristics of the peridot and discuss the mantle properties and the P–T conditions for Yiqisong peridot formation. This study provides a basis for understanding the formation of gem-grade peridot in Yiqisong. It also effectively distinguishes peridot from the same basaltic xenoliths in Tanzania and Arizona through comparative studies of trace elements in peridot from different regions.

2. Geological Setting

Northeastern China is tectonically located between the Siberian Craton and the North China Craton (Figure 1a). The Jilin region is located in the Xingmeng Orogenic Belt, belonging to the eastern part of the Central Asian Orogenic Belt, which was formed by the subduction and convergence of several microcontinental blocks during the Paleozoic (Figure 1b). This orogenic belt experienced extensive basaltic magmatic activities in the Cenozoic. Previous studies have divided the Cenozoic volcanic activities in northeastern China into four stages: (1) >29 Ma; (2) 29–16 Ma; (3) 16–4 Ma; and (4) 4–0 Ma, mainly producing alkaline–peralkaline basalts [17,18,19]. The Dunhua is located on the southeastern margin of the Xingmeng Orogenic Belt. The volcanic activities in this region are mainly controlled by the Dunhua–Mishan Fault Zone, and frequent volcanic activities are dominated by fissure-centered eruptions. Regionally exposed strata include Permian, Neogene, and Quaternary, with common Mesozoic granite intrusions [16].

The Yiqisong peridot deposit (43°47′43″ N, 127°55′37″ E) is located on the northern slope and eastern part of Yiqisong. The Yiqisong gem-grade peridot deposit is hosted in the lower strata of the Neogene Chuandishan Formation in the Dunhua–Mishan Fault Zone, which unconformably overlies Paleozoic strata and Mesozoic granites [15,20] (Figure 1c). The Dunhua basalts contain a large amount of mantle peridotite xenoliths, and the peridot gems are produced from the peridotite xenoliths with an approximate east–west distribution in a near-horizontal layer. The peridotite xenoliths containing gem-grade peridot from the Yiqisong deposit are mainly spinel lherzolite xenoliths. The peridotite xenoliths have a light yellow or yellow-green color and are sub-rounded to sub-angular in shape, and were captured by black basalt with clear boundaries. The diameter of the peridotite xenolith is usually 3 to 10 cm, with the larger ones reaching 25 cm. The Yiqisong peridot deposit has been worked by Yanbian Fuli Peridot Mining Industry Co., Ltd. (Dunhua, China), using the room-and-pillar method of underground mining [16,21].

3. Materials and Methods

3.1. Sample Description

In this study, thirty-three raw peridot crystals from the Yiqisong were selected for analysis, which were polished on both sides in parallel. The crystal size ranged from 4 to 8 mm (Figure 2). We collected peridotite xenoliths rich in large-grain peridots distributed in the Yiqisong peridot mine, which are encapsulated in basalts. The Yiqisong peridotite xenoliths are fresh and mainly spinel-bearing lherzolites, with diameters ranging from 4 to 7 cm. Thirty-three peridot crystal samples were mostly derived from the peridotite xenoliths. Peridotite xenolith is mainly composed of olivine (Ol), orthopyroxene (Opx), clinopyroxene (Cpx), and spinel (Sp), which show medium- to coarse-grained granular textures. Olivine is the dominant mineral in the peridotite xenoliths, accounting for 55–65% of the total volume. Some olivine grains exhibit kink bands and triple junction structures. The orthopyroxene (20–25%) and clinopyroxene (10–15%) often occurred in the olivine gaps, with some pyroxenes showing exsolution lamellae on the surfaces. A few irregularly shaped pyroxenes show curvilinear grain boundaries, and some orthopyroxenes partially enclose olivine. Spinel content ranges from 5–8%, mainly appearing reddish-brown in color. Spinel typically occurs as interstitial phases filling the gaps between the main mineral grains, with a diameter smaller than that of olivine and pyroxene minerals.

The peridot samples were observed and numbered YQS–1 to YQS–33: YQS–1 to YQS–7 have a darker green color, YQS–8 to YQS–13 have a brighter green color, YQS–14 to YQS–20 present yellowish-green color, YQS–21 to YQS–26 have a brownish-toned yellowish-green color, and YQS–27 to YQS–33 appear light yellowish-green in color.

3.2. Microscopic Observation and Spectroscopy

Gemological microscope observation, laser Raman spectrum, UV–visible spectrum, infrared spectrum, and other standard gemological tests were conducted at the Gemological Experimental Teaching Center, School of Gemology, China University of Geosciences (Beijing). Other standard gemological tests include measuring the sample’s refractive index (RI) and birefringence using a gem refractometer, determining the specific gravity (SG) of the sample through the hydrostatic method, and observing the pleochroism of the sample by using a dichroscope.

By using a GI–MP22 binocular microscope (Baoguang Technologies, Nanjing, China), the surface and internal features of the samples were observed by changing different light sources under 10× to 40× magnification. Infrared spectra were obtained using a BRUKER TENSOR 27 Fourier transform infrared (FTIR) spectrometer (Bruker, Billerica, Germany). All samples were analyzed by the reflectance method. The experimental test conditions were 220 V voltage, 4 cm⁻¹ resolution, 64 scans, a scanning range of 2000–400 cm⁻¹, and a scanning speed of 10 kHz. The UV–visible spectra were tested by a UV–3600 UV–vis spectrophotometer (Shimadzu Corporation, Kyoto, Japan) using the reflection method with a wavelength range of 300~800 nm and a sampling interval of 1s. The Raman spectra of the sample matrix and internal inclusions were obtained with an HR-Evolution micro-laser confocal Raman spectrometer (CRS) from Horiba(Kyoto, Japan). Before this test, single-crystal silicon was used for Raman calibration. The Raman instrument laser wavelength was 532 nm, the laser output power was 100 mW, the laser spot size was 1–5 µm, and the resolution was 1 cm⁻¹. The exposure time per scan was 20 s and 3 integrations. The collected range was 100–2000 cm⁻¹, and the data were compared with those in the RRUFF databases.

3.3. Chemical Analysis

The analysis of major elements was conducted at the Key Laboratory of Submarine Geoscience, State Oceanic Administration, Second Institute of Oceanography, Ministry of Natural Administration, using a four-spectrometer Jeol JXA–8100 electron probe microanalyzer (EPMA) from JEOL Ltd. In Tokyo, Japan. The experimental tests were carried out using an accelerating potential of 15 kV, beam current of 20 nA, accounting time of 20 s, and a spot size of 10 μm for mineral phase analysis of the major elements. Data were corrected using a modified ZAF (atomic number, absorption, fluorescence) correction procedure. The following standards were used for quantification: chromium oxide (Cr), diopside (Ca), jadeite (Na), nickel (Ni), olivine (Si, Mg), hematite (Fe), pyrope garnet (Al), rhodonite (Mn), rutile (Ti), and sanidine (K). The limits for individual elements were as follows: Si (339 ppm), Na (330 ppm), K (186 ppm), Fe (468 ppm), Al (309 ppm), Mg (336 ppm), Ca (210 ppm), Mn (441 ppm), P (321 ppm), Cr (825 ppm), Ti (564 ppm), and Ni (888 ppm).

Trace elements were analyzed at the Institute of Geomechanics, Chinese Academy of Geological Sciences Institute of Geology, Beijing, using an Agilent 7900 quadrupole inductively coupled plasma mass spectrometry (ICP–MS) in combination with the GeoLas HD 193 nm ArF excimer laser to compose the laser-ablation inductively coupled plasma mass spectrometry (LA–ICP–MS) test samples. LA–ICP–MS is manufactured by Coherent in Santa Clara, CA, USA. Laser ablation was performed at a repetition frequency of 5 Hz in a helium atmosphere, and NIST SRM612, ARM1, and ARM2 were used as reference materials to standardize all analyzed elements. The concentration errors of most elements measured were less than or equal to 10%. The chemical analysis was conducted on polished sample surfaces.

4. Results

4.1. Gemological Properties

Table 1. Properties of peridots from Yiqisong.

Property	Yiqisong, China
Color	Light yellowish-green	Yellowish-green	Yellowish-green	Light yellowish-green
Pleochroism	Weak: light yellowish-green to brownish-green
Diaphaneity	Transparent
RI	1.660–1.698	1.655–1.694	1.659–1.695	1.655–1.690
Birefringence	0.038	0.039	0.036	0.035
SG	3.44	3.31	3.21	3.37
Internal features	‘Lily pad’ inclusions; partially healed fractures; brown staining; dark solid inclusions

summarizes the gemological properties of the studied samples. The Yiqisong peridot samples had an overall yellowish-green color. All samples were transparent and showed a glassy luster. The RI varied between 1.655 and 1.698, the SG varied between 3.21 and 3.44. They were inert under both long-wave and short-wave UV light.

4.2. Microscopic Characteristics

Microscopic observation of gemstones showed that most of the Yiqisong peridot samples were incomplete, with visible conchoidal fractures on the surface and greasy luster (Figure 3a). Rhombohedral growth etch pits could be seen on some crystal faces (Figure 3b). Some surface cracks were often stained with brown-yellow iron. The Yiqisong peridot crystals contain few impurities. The internal solid inclusions are mainly dark-colored euhedral minerals or anhedral crystals. The euhedral minerals are usually present as isolated grains within the peridot crystals (Figure 3c). Peridots derived from mantle xenoliths commonly contain dark chromite, biotite, and spinel inclusions [7,8]. Diopside inclusions can be found in many regions, such as the United States, China, Mexico, and Korea [5,22]. In addition, the internal features of Yiqisong peridot include negative crystals (Figure 3d) and partially healed–healed cleavages. Some partially healed cleavages show an iridescence effect under reflected light (Figure 3e). The most common inclusions in Yiqisong peridot are “Lily pad” inclusions, with diameters ranging from 500 μm to larger sizes (Figure 3g). The center may be a mineral crystal or negative crystal, surrounded by round healing planes, representing a signature inclusion of peridot (Figure 3f,g). Notably, a green euhedral mineral inclusion surrounded by brown anhedral crystals forms an assemblage inclusion in YQS–14; the green euhedral mineral has a diameter of approximately 100 μm (Figure 3h).

The peridot crystals were cut to expose the inclusions to the surface, and the types and morphology of the two inclusions were determined by combining Raman spectra with microscopic features (Figure 4). The main Raman spectral bands of the first green euhedral mineral were located at 329 cm⁻¹, 395 cm⁻¹, 668 cm⁻¹, and 1012 cm⁻¹, indicating that the mineral is diopside. The absorption peaks at 668 cm⁻¹ and 1012 cm⁻¹ are related to [Si₂O₆]⁴⁻ tetrahedral groups of diopside, and those at 329 cm⁻¹ and 395 cm⁻¹ are attributed to ionic M–O vibrations (Figure 4a) [23]. The main spectral bands of the second mineral around the diopside were located at 587 cm⁻¹ and 705 cm⁻¹. The absorption peaks of 705 cm⁻¹ are due to the symmetric stretching vibration of Mg-O, while the one at 587 cm⁻¹ is attributed to the bending vibration between metal cations and oxygen ions, indicating that the mineral is spinel with Raman peak shifts caused by element substitutions (Figure 4b) [24]. The euhedral granular dark inclusion exhibited characteristic Raman shifts at 593 cm⁻¹ and 704 cm⁻¹, indicating that it is spinel (Figure 4c).

4.3. Absorption Spectra

4.3.1. UV–vis–NIR Spectra

The ultraviolet–visible–infrared (UV–Vis–NIR) absorption spectra of peridot samples from Yiqisong display similar patterns, as shown in Figure 5. Absorption occurs in Yiqisong peridot at 450 and 490 nm, and there is a broad absorption band centered at 635 nm. Peridot is colored by iron as an autochromatic mineral [25]. The representative absorption spectra of the samples are similar to those of peridot from Vietnam, Pakistan, China, Myanmar, and Italy, and this spectral feature is mainly caused by Fe²⁺ [5,6,10,22,26]. During the testing process, we found that there were no significant differences in the UV–Vis–NIR spectra of light yellow-green and yellow-green peridot.

4.3.2. FTIR Spectra

A representative FTIR spectrum of Yiqisong peridot samples is shown in Figure 6. There are characteristic absorption peaks and some shoulder absorptions in the three spectral regions of 1100–800 cm⁻¹, 600–450 cm⁻¹, and 450–400 cm⁻¹. The 1100–800 cm⁻¹ absorption band is attributed to symmetrical stretching vibrations of the Si–O–Si group. The 650–450 cm⁻¹ absorption is caused by bending vibrations of the Si–O group. The absorption between 450 and 400 cm⁻¹ is attributed to crystal lattice vibrations and internal vibrations [27,28]. The characteristic peak frequencies of the peridot infrared spectrum can move with the change in Fe content. When the iron content increases, all of the absorption peaks on the forsterite–fayalite series move to lower frequencies [29].

4.3.3. Raman Spectra

Peridot has 84 vibration modes, and the vibration modes with Raman activity are ¹¹A_g + ¹¹B_1g + ⁷B_2g + ⁷B_3g, totaling 36 kinds [30]. The fingerprint region absorption peaks of Yiqisong peridot samples show consistency (Figure 7): 222, 303, 326, 371, 544, 587, 824, 856, 919, and 961 cm⁻¹, belonging to forsterite. The absorption peaks with the strongest intensities of 824, 856, 919, and 961 cm⁻¹ are caused by the symmetric (ν₁) and antisymmetric (ν₃) stretching vibrations of Si—O. In addition, there are also weak–medium-intensity Raman shifts at 544 and 587 cm⁻¹ caused by the antisymmetric bending vibration (ν₄) of Si—O. The Raman shifts at 222, 303, 326, and 371 cm⁻¹ belong to metal–oxygen translational vibration modes (T) and silicon–oxygen tetrahedra rotational vibration modes (R) [31,32].

4.4. Chemical Compositions

Table 2. Major element composition (wt %) of Yiqisong peridot samples by EPMA.

Sample	YQS–9	YQS–12	YQS–13	YQS–15	YQS–17	YQS–18
SiO2	41.52	41.79	41.18	41.70	41.26	41.46
FeO	8.14	8.27	8.67	8.52	8.91	8.55
MgO	49.83	50.44	50.38	50.29	50.86	50.39
CaO	0.03	0.03	0.04	0.05	0.03	0.04
MnO	0.12	0.12	0.14	0.15	0.10	0.11
NiO	0.50	0.42	0.33	0.44	0.38	0.42
Total	100.14	101.07	100.74	101.15	101.54	100.97
Cations per 4 oxygen
Si	1.01	1.01	1.00	1.00	0.99	1.00
Fe	0.17	0.17	0.18	0.17	0.18	0.17
Mg	1.80	1.81	1.82	1.81	1.82	1.81
Ca
Mn
Ni	0.01	0.01	0.01	0.01	0.01	0.01
Fo	91.60	91.58	91.20	91.33	91.05	91.31

lists the major element compositions of Yiqisong peridot samples measured by EPMA. The results show that the MgO content of the samples ranged from 49.83 wt % to 50.86 wt %. The range of SiO₂ content was 41.18–41.79 wt %, and the FeO content was 8.14–8.91 wt %. Using the oxygen atom method, taking the number of oxygen atoms per formula unit as four, the atomic numbers of the major elements in the peridot formula unit were calculated. The Fo values of peridots were calculated according to Fo = Mg/(Mg + Fe) × 100. The Fo values of Yiqisong peridot are in the range of 90–100 (Table 2). The CaO content of Yiqisong peridot is in the range of 0.03–0.05 wt %, less than 0.1 wt %, indicating that the peridots do not originate from basalt phenocrysts [33,34]. Nickel is one of the major elements enriched in the Earth’s core and mantle. In peridots of the upper mantle with Fo of about 90, the NiO content is stable at 0.4 wt %. The NiO content of Yiqisong peridot samples was between 0.33 and 0.50 wt %, with an average of 0.42 wt %, indicating that the Yiqisong peridots are derived from mantle peridotite xenoliths [35].

The EPMA test results obtained from previous studies [14] show that the major element composition of Yiqisong peridot was SiO₂ (40.96–41.33 wt %), FeO (7.80–8.99 wt %), and MgO (48.26–49.74 wt %). The CaO content was between 0.01 and 0.04 wt %, the NiO content ranged from 0.36 to 0.44 wt %, and MnO was in the range of 0.06 to 0.17 wt %, which is consistent with the main component contents of peridot obtained in this study.

The trace element compositions of Yiqisong peridot samples are shown in

Table 3. Trace element composition (ppm) of Yiqisong peridots and those from other regions.

Chemical Composition	Li	Na	Al	Ca	Sc	Ti	V	Cr	Mn	Co	Ni	Zn	Zr
YQS–30–1	1.46	13.09	26.24	236.38	2.07	2.04	2.17	58.79	953.08	119.85	2797.65	48.53	0.00
YQS–30–2	1.33	16.10	25.62	177.42	2.05	2.44	2.07	59.98	979.61	115.89	2746.05	48.47	0.01
YQS–30–3	1.22	13.83	21.03	117.41	1.56	2.04	1.69	48.42	812.47	99.47	2295.68	40.05	0.01
YQS–30–4	1.24	26.36	19.98	117.96	1.59	1.94	1.78	52.03	801.65	101.44	2279.97	40.85	0.00
YQS–30–5	1.14	56.90	23.41	609.42	1.58	4.98	1.90	70.27	855.89	105.31	2481.38	46.16	0.08
YQS–25–1	1.62	19.99	35.49	224.23	1.94	5.54	2.37	82.59	916.72	112.19	2577.93	45.46	0.01
YQS–25–2	1.30	29.40	36.68	239.99	1.76	7.59	2.40	81.33	916.35	116.91	2661.27	45.70	0.01
YQS–25–3	1.49	26.66	40.66	243.69	1.93	6.91	2.43	83.66	946.46	124.07	2763.86	46.81	0.01
YQS–25–4	1.74	27.98	44.93	265.62	2.16	8.42	2.67	88.72	1011.60	128.86	3050.41	48.26	0.01
YQS–25–5	1.90	32.33	46.01	312.00	2.30	9.17	2.97	94.13	1051.41	132.90	3139.02	51.41	0.02
YQS–23–1	2.25	81.43	73.85	309.41	2.35	25.53	1.87	169.15	1200.67	121.13	2505.48	77.68	0.07
YQS–23–2	2.73	90.85	71.92	327.23	2.31	25.48	2.02	177.26	1244.29	126.40	2557.99	77.49	0.05
YQS–23–3	3.00	86.21	78.92	328.45	2.39	25.19	2.14	177.17	1227.54	127.58	2509.95	75.47	0.06
YQS–23–4	3.14	85.81	75.79	399.27	2.47	23.98	2.23	183.98	1287.55	130.95	2662.17	77.97	0.07
YQS–23–5	2.71	90.02	73.88	311.04	2.63	23.02	2.24	189.77	1301.20	132.98	2666.46	76.21	0.04
Vietnam [5]	1.24–1.71		46.40–63.30	38.60–782.00		7.19–13.50	1.70–2.58	86.70–124.00	869.00–1121.00	137.00–150.00	2602.00–3062.00	50.4–64.4	0.04–0.15
Pyaung–Gaung, Myanmar [10]	1.45–15.77	23.04–97.75	19.50–103.14		8.19–15.92	6.35–57.94	1.01–7.69	12.83–68.97	788.54–1097.55	123.46–167.77	2761.62–3965.21	40.86–119.95	bdl–0.15
Pakistan [26]	14.45–49.92				10.85–29.31		0.27–1.70					10.37–26.58

bdl: below detection limit.

, measured by LA–ICP–MS, compared with peridots from Vietnam [5], Myanmar Mogok [10], and Pakistan [26]. In general, nickel is the richest trace element, with nickel contents in Yiqisong peridot samples ranging from 2280 to 3139 ppm (average 2646 ppm), followed in order of abundance by Mn, Ca, Co, Cr, and Na. The Cr content of Yiqisong peridots is higher than that of Mogok peridots. While the Co content is lower than those of Vietnam and Mogok peridots. The Li content of Yiqisong peridots is 1.14–3.14 ppm, which is similar to peridots from Vietnam but much lower than those of Mogok and Pakistani peridots. The V content ranged from 1.69 ppm to 2.97 ppm, which is close to Vietnam peridot and slightly higher than that of Pakistani peridots. Previous studies have shown that Yiqisong peridots are derived from spinel lherzolite xenoliths, which are similar to peridots from Vietnam [10,36], Mogok peridots and Pakistani peridots have similar origins [37].

5. Discussion

5.1. Mineral Characteristics and Chromogenesis of Yiqisong Peridot

The FTIR spectral distributions of Yiqisong peridots were in the three spectral regions of 1100–800 cm⁻¹, 600–450 cm⁻¹, and 450–400 cm⁻¹, which is similar to peridots from Myanmar and Sardinia, Italy [6,10]. The Raman characteristic peaks of peridot were caused by vibrations of Si-O bonds [31,38]. The positions of the two strong absorption peaks of Raman spectra for Yiqisong peridot were both at 824 and 856 cm⁻¹ (Figure 7). Infrared and Raman spectral analyses indicated that the peridot is forsterite, representing a typical gem-grade peridot.

The olivine group minerals can be divided into three sub-groups: liebenbergite Ni₂SiO₄, olivine (Mg, Fe)₂[SiO₄], and tephroite Mn₂SiO₄. Gem-grade peridot usually belongs to the olivine sub-group, with chemical compositions in the solid solution series of Mg₂[SiO₄] –Fe₂[SiO₄]. There is a complete isomorphic substitution between Mg²⁺ and Fe²⁺ in the structure [39]. The crystal structure of olivine is an independent silicon–oxygen tetrahedral island structure. The O^2- is used as an approximate hexagonal closest packing, and Si⁴⁺ fills the tetrahedral space. Mg²⁺ and Fe²⁺ ions occupy the octahedral sites (M1 and M2). M1 is a distorted octahedron, and M2 is a regular octahedron [40]. When elements have suitable ionic valences and radius ranges, ionic substitution can be carried out. Therefore, only a few elements (Ni²⁺, Mn²⁺) have relatively high contents, while ions such as Zr⁴⁺, Li⁺, and Sc³⁺ with similar radii but different valences require to be paired with other valent ions for substitution [41,42,43]. Based on EPMA data of six Yiqisong peridot samples, the crystal chemical formula of the peridot was deduced, indicating it belongs to the olivine series (

Table 4. Crystal chemical formulas of Yiqisong peridot samples.

Samples	Crystal Chemical Formula	Fo
YQS–9	(Mg1.80Fe0.17Ni0.01)2Si1.01O4	91.60
YQS–12	(Mg1.81Fe0.17Ni0.01)2Si1.01O4	91.58
YQS–13	(Mg1.82Fe0.18Ni0.01)2Si1.00O4	91.20
YQS–15	(Mg1.81Fe0.17Ni0.01)2Si1.00O4	91.33
YQS–17	(Mg1.82Fe0.18Ni0.01)2Si0.99O4	91.05
YQS–18	(Mg1.81Fe0.17Ni0.01)2Si1.00O4	91.31

).

Peridot is an autochromic gemstone. The typical UV–Vis–NIR absorption spectra of Yiqisong peridots were located at 450, 490, and 635 nm (Figure 5), caused by Fe²⁺. Both the oxidation state and Fe content have an effect on the color of peridots. The higher the Fe²⁺ content in peridot, the yellower its color [16,44,45,46]. In the visible light range, absorption of orange light at 635 nm formed a blue-green hue; absorption of green-blue light at 490 nm formed orange, and the mixing of the two hues formed the unique olivine-green color, confirming that the green color of peridot was caused by the iron element.

5.2. Origin Determination

The measured RI, DR, and SG values of Yiqisong peridot were similar to peridot samples from Arizona, Tanzania, and Italy [6,8,44], within the theoretical range for peridot (

Table 5. Gemological properties of peridot from various origins.

Property	Yiqisong, China	San Carlos, Arizona [8]	Tanzania [44]	Sardinia, Italy [6]
Color	Light yellowish-green to yellowish-green	Very dark brown to brownish-green to bright lime green	Light yellowish-green	Yellowish-green
Pleochroism	Weak: light yellowish-green to brownish-green			Weak to moderate α, β = green; γ = yellow–green
Diaphaneity	Transparent		Translucent
RI	1.655–1.698	1.649–1.691	1.650–1.684	1.650–1.690
Birefringence	0.035–0.039	0.037–0.038		0.038–0.039
SG	3.21–3.44	3.28–3.38		3.32–3.36
Internal features	“Lily pad” inclusions; partially healed fractures; brown staining;	“Lily pad” cleavages; Chromite, chromian spinel; negative crystals; glass blebs; chrome diopside; biotite; smoke-like veiling	“Lily pad” inclusions, chromite inclusions, healed secondary fractures, and hercynite (rarely)	Partially healed fractures, liquid inclusions, “Lily pad” inclusions, crystals, growth planes, traces of parallel twinning

). Their common gemological characteristics could not effectively distinguish the origin of peridot. Internal features of gemstone inclusions can often indicate their specific origin. Pakistani peridot can be distinguished from that of other origins by black Vonsenite–Ludwigite needle inclusions [26]. Internal inclusions in Yiqisong peridot lacked distinguishing features, but trace element chemical analysis methods often play a major role in tracing the origin of gemstones.

Peridot from Tanzania [47] and Arizona [48] are both produced in peridotite xenoliths. Different trace element characteristic data can effectively distinguish them. It was found that the contents of Ni and V from Yiqisong peridots were significantly lower than those from Tanzania and Arizona peridots. In contrast, the Sc content in Arizona peridot was significantly higher than in that from Yiqisong. The Sc content of the Yiqisong peridots partially overlaps with the Sc range of the Tanzanian peridot but is overall higher than that of the Tanzanian peridot. The V content of Tanzania peridots was generally higher than that of Yiqisong and Arizona peridots. Its Co content was lower than that from Arizona, but higher than that of Yiqisong. Therefore, peridots from Yiqisong, Arizona, and Tanzania can be distinguished based on their Ni and V contents. The Sc content can better distinguish Arizona peridot from the other two origins. Tanzanian peridot can be effectively distinguished from the other two origins based on V and Sc contents. By plotting bivariate diagrams of Co–Cr, Sc–Cr, Ni–Sc, V–Mn, Mn–Co, and Sc–V contents (Figure 8), peridots from Yiqisong, Tanzania, and Arizona can be basically identified.

5.3. Mantle Properties and P–T Conditions of Yiqisong Peridot

Olivine, as an early crystallizing mineral in basaltic magma, is commonly produced in the forms of mantle xenolith, xenocryst, and phenocryst. It is an important information carrier of the magma source [49,50]. Olivine from mantle xenolith has a relatively high Fo value and low CaO content (<0.1 wt %). Olivine xenocryst typically has poor morphological development and obvious compositional zoning with Mg-rich cores and Fe-rich rims; olivine phenocrysts have relatively good morphological development, lower Fo content, and relatively higher CaO content [51]. The CaO content of Yiqisong peridot is within the range of 0.025–0.05 wt %, less than 0.1 wt %, indicating that it was produced from mantle xenoliths. The major element contents are close to the compositional ranges of peridots from mantle xenoliths in Vietnam [5] and Italy [6] (

Table 6. Chemical compositions of peridot from Yiqisong and other countries.

Chemical Composition	Yiqisong, China	Vietnam [5]	Sardinia, Italy [6]
SiO2	41.18–41.79	39.82–42.64	40.83–40.69
FeO	8.14–8.91	8.24–9.90	8.98–8.77
MgO	49.83–50.86	47.22–52.25	50.15–50.17
CaO	0.03–0.05	0.03–0.10
MnO	0.10–0.15	0.03–0.24	0.11–0.15
NiO	0.33–0.50	0.16–0.55	0.38–0.39

).

Previous studies found that the geochemical characteristics of peridotites can effectively reflect mantle properties. Olivines in refractory mantle typically have high Fo values of between 92 and 94, while olivines in fertile mantle have a low Fo content of below 90. The Fo values of Yiqisong peridots are between 91.05 and 91.60, which is similar to the Jiaohe olivines [52]. This indicated that Yiqisong peridots are derived from a moderate refractory lithosphere mantle (Figure 9).

Trace elements can effectively distinguish the genesis of olivine. The contents of Ca, Al, Ti, and Ni in olivine from mantle peridotites are generally relatively narrow, while the elemental contents of olivine phenocrysts are relatively broader. The Ti (2–26 ppm), Ni (2280–3139 ppm), Al (20–79 ppm), and Ca (117–609 ppm) contents of Yiqisong peridots are close to those of olivines derived from mantle peridotites. They have low contents of Ca and Ti, which is consistent with mantle peridotite characteristics [48]. Olivines in spinel lherzolites have high Sc content and low Zr content, while olivines in garnet lherzolites [41] are the opposite (Figure 10). Yiqisong peridots have high Sc content and low Zr content, showing a clear distinction from garnet lherzolites, and directly indicating that they may be derived from spinel lherzolites.

The geothermometers widely used for estimating the temperatures of spinel lherzolites include the two-pyroxene thermometer and Cpx thermobarometry. This study selected the following geobarometers to estimate the P–T conditions of the peridotite xenolith: (1) garnet-calculation of Cpx thermobarometry, also used to calculate the temperature of spinel lherzolites [55,56] (

Table 7. Equilibration temperatures and pressure of Yiqisong spinel peridotites; method by Nimis and Taylor.

Calculation Method	Calculation of Cpx Thermobarometry [56]
T (°C)	813	815
P (kbar)	21.72	21.32

); (2) Taylor’s [57] two-pyroxene thermometer (

Table 8. Equilibration temperatures of Yiqisong spinel peridotites; method by Taylor.

Calculation Method	Two-Pyroxene Thermometer [57]
T (°C)	822	822	822	823	825	826

); (3) Al–in–olivine (

Table 9. Equilibration temperatures of Yiqisong spinel peridotites; method by De Hoog.

Calculation Method	Al–in–Olivine Thermometer [41]
T (°C)	869	871	892	893	894	932	934	944	959	987	1027	1028	1033	1034	1037

), and (4) the Ca content of olivine thermometer [41] (

Table 10. Equilibration temperatures of Yiqisong spinel peridotites; method by De Hoog.

Calculation Method	Ca–in–Olivine Thermometer [41]
T (°C)	877	878	924	952	958	960	962	973	992	993	994	1000	1027	1087

). The garnet-calculation of Cpx thermobarometry can be used to calculate pressure. A two-pyroxene thermometer is based on the Ca–Mg equilibrium between orthopyroxene and clinopyroxene. It is generally considered to be the most reliable estimate. This study selected the EPMA data of clinopyroxene and orthopyroxene in equilibrium state to calculate the P–T conditions using these two thermometers. Intergrown clinopyroxene and orthopyroxene with complete mineral crystal shapes and clear surfaces were selected for testing (Figure 11).

Table 11. Major element compositions (wt %) of pyroxenes and spinel in the peridotite by EPMA.

Sample	Opx–1	Opx–2	Opx–3	Opx–4	Opx–5	Opx–6	Cpx–1	Cpx–2	Sp–1	Sp–2
SiO2	56.82	57.09	56.79	57.01	57.17	56.90	53.95	53.27	0.05	0.08
FeO	5.48	5.5	5.45	5.43	5.49	5.46	2.03	2.12	13.01	12.99
Al2O3	2.58	2.33	2.65	2.38	2.35	2.38	3.12	3.28	36.95	36.55
MgO	34.45	34.72	34.47	34.63	34.74	34.45	16.80	16.70	17.62	17.46
CaO	0.55	0.56	0.53	0.56	0.52	0.54	22.62	22.40	0.01	0.01
MnO	0.14	0.10	0.12	0.10	0.09	0.11	0.06	0.10	0.15	0.21
Cr2O3	0.49	0.44	0.57	0.50	0.49	0.49	1.13	1.29	32.80	34.29
NiO	0.06	0.12	0.07	0.05	0.10	0.04	0.07	0.04	0.25	0.22

lists the major chemical compositions of pyroxene and spinel minerals in the peridotite xenoliths; the experimental results show a total of six orthopyroxenes (Opx), two clinopyroxenes (Cpx), and two spinels (Sp). The Al-in-olivine thermometer primarily calculates the temperature of peridotite mantle sources based on the Al content, Cr# value, and pressure in olivine. The Ca content of the olivine thermometer is mainly based on the Ca content and pressure in olivine, showing excellent results for spinel lherzolite temperatures. Both Al–in–olivine and Ca–in–olivine thermometers require LA–ICP–MS data of peridot for calculation [41,55,56,57] (Table 3).

The temperature results from the four methods are generally consistent, all within the range of 813–1087 °C, close to the temperature range of spinel lherzolite xenoliths from the Jiaohe area (756–1077 °C). A similar range of lherzolites from Huinan is between 850 and 1050 °C, while the Shuangliao peridotites show a range of 833–1068 °C [52,58,59].

The temperature of the spinel lherzolite xenoliths in the study area is between 813 and 1087 °C, possibly due to heterogeneity in the origin depth of the xenoliths or the equilibrium temperature of the upper mantle in the region. The P–T condition of Yiqisong peridotites was plotted against those from the Dashihe spinel lherzolites [60]. They fall into the region of transition from spinel to garnet phase (Figure 12). Combined with the contents of Sc and Zr, it is inferred that the Yiqisong peridots were mainly derived from spinel lherzolite xenoliths, which may have coexisted with the garnet phase during their formation.

6. Conclusions

The Yiqisong peridots exhibit typical gemological characteristics, with RI ranging from 1.655 to 1.698, SG from 3.21 to 3.44, relatively clean internally, and various inclusions (“Lily pad”, euhedral and anhedral spinel, and diopside inclusions). The UV–vis absorption spectrum exhibits Fe absorption, indicating that the yellowish-green color of the Yiqisong peridot is mainly due to Fe. The infrared spectrum and Raman spectrum show that they are forsterite. Major and trace elements reveal that the peridots are forsterite with moderate Fo values (~91), derived from spinel lherzolite xenoliths with moderately refractory mantle properties. The temperature of Yiqisong peridotites was roughly in the range of 813–1087 °C, and the pressure was generally within 21–22 kbar, corresponding to a depth of 70–72 km. In addition, the use of Co–Cr, Sc–Cr, Ni–Sc, V–Mn, Mn–Co, and Sc–V binary diagrams is an effective method of identifying the origin of peridots from Yiqisong, Tanzania, and Arizona.