Gemological Characteristics and Spectral Characteristics of Grossular from Mt. Bozhushan in Southeast Yunnan Province

Abstract

Grossular, a widely recognized colored gemstone in the market, attains premium quality when exhibiting pale-toned orange-yellow to golden-yellow hues with high transparency. Recently, our research team identified abundant gem-grade grossular associated with skarn-type polymetallic deposits in the Mt.Bozhushan area. However, these grossular specimens are currently discarded as mining waste. To promote their sustainable utilization, we conducted a systematic sampling of Mt.Bozhushan grossular and performed comprehensive analyses including fundamental gemological testing, spectroscopic characterization, and compositional profiling through EPMA and LA-ICP-MS. This multidisciplinary approach aims to establish an objective valuation framework for Mt.Bozhushan-derived grossular. The compositional and spectral data obtained in this study provide critical theoretical foundations and technical references for developing the gemstone resources of southeastern Yunnan’s Mt.Bozhushan grossular while establishing an evaluative baseline for grossular specimens from other regional deposits.

1. Introduction

Grossular, a mid-range gemstone, has gained a certain level of market recognition. At present, the most common gem-quality grossulars on the market are iron-bearing grossulars with a brownish–red color, which are of relatively low value. In contrast, high-quality grossulars with pale orange-yellow to vivid orange hues are rarer and more valuable [1]. In southeastern Yunnan, the area of the Mt.Bozhushan ore cluster is characterized by a series of skarn-type polymetallic deposits distributed around the Mt.Bozhushan composite pluton [2]. Along the contact zone between the fine-grained granite of the Dashanjiao unit of the Mt.Bozhushan composite pluton and the Lower Ordovician Dushuke Formation, a series of skarn-type copper–lead–zinc polymetallic deposits have formed, which are associated with a certain quantity of gem-quality grossular. These garnets have significant economic value. However, to date, only the metallic minerals from these deposits have been extracted and utilized, and a large amount of high-quality gem-grade grossular has been discarded, resulting in a significant waste of gemstone resources. Furthermore, although the Mt.Bozhushan ore cluster area is relatively well studied, with numerous research findings published, most of these studies have focused on ore deposit geology, geochemistry, and structural geology. No studies have been dedicated to the gemological and spectroscopic characteristics of grossular from the Mt.Bozhushan area. This study selects grossular samples from skarn-type deposits in the Mt.Bozhushan area, aiming to investigate their gemological and spectroscopic properties. The findings provide a theoretical foundation and data support for the comprehensive development and utilization of grossular resources from the Mt.Bozhushan area.

Geologic Setting

The Mt.Bozhushan Ore Concentration Area (hereafter referred to as the study area) is tectonically situated within the Wenshan–Funing Fault–Fold Belt of the Southeastern Yunnan Fold Zone, South China Fold System (Figure 1a). The region exhibits predominantly NE-SW structural trends, encompassing key geological features including the Mt.Bozhushan Dome, Laohuilong Composite Syncline, Daheishan Syncline, and Bainiuchang Breached Anticline. Extensive Cambrian, Ordovician, and Devonian carbonate formations are developed in the area. Numerous skarn-type W-Sn polymetallic deposits, commonly accompanied by substantial grossular mineralization, are developed at the contact zones between Later Yanshanian granites and the Cambrian, Ordovician, and Devonian carbonate strata widely distributed in the study area. Notably, the highest-quality grossular is associated with the skarn-type W-Sn deposits formed at the contacts between the Dashanjiao granitic unit and Ordovician carbonates.

• Stratigraphy:

The study area exposes Cambrian, Ordovician, Devonian, Carboniferous, and Permian strata (Figure 1b), dominated by argillaceous–silty sedimentary sequences and carbonate rocks. These formations are primarily distributed around the periphery of the Mt.Bozhushan composite granitic pluton, where contact metamorphism has generated skarns, hornfels, marbles, and slates [3].

• Structural Framework:

Located at the southwestern margin of the South China Block near the Yangtze–Indochina Suture Zone (Figure 1a), the study area reflects superimposed effects from two major tectonic regimes [4]:

Tethyan–Himalayan Tectonic Domain: This regime is characterized by NS-trending structures formed through Paleo–Tethyan closure and far-field effects of the India–Eurasia collision.

Western Pacific Tectonic Domain: This regime exhibits reactivated NE-trending structures resulting from Pacific Plate subduction since the Yanshanian period.

The Mt.Bozhushan Dome constitutes the dominant structure, featuring the Mt.Bozhushan composite granitic pluton at its core, surrounded by Paleozoic strata that exhibit outward-younging age progression (Figure 1b). Additionally, the area is dissected by multiple NE-SW-trending faults (Figure 1b) with multiphase reactivation histories. These secondary faults, genetically linked to the Wenshan–Malipo Fault as its splays, serve as critical hydrothermal conduits for mineralization within the study area [5].

• Magmatic Assemblage:

The Later Yanshanian Mt.Bozhushan composite pluton comprises seven intrusive units based on previous studies [6]: Suozuodi, Yangyushu, Dashan, Leidazhan, Fenshuiling, Bozhupo, and Dashanjiao. The main body of the pluton displays an NW-SE orientation within the Mt.Bozhushan Dome, while the Dashanjiao unit occurs as three discrete stocks in the northwestern sector (Figure 1b).

Figure 1. (a,b): Geotectonic location map of south China. (b): Geotectonic location map of study area. Pictures modified according to [4,6,7].

Figure 1. (a,b): Geotectonic location map of south China. (b): Geotectonic location map of study area. Pictures modified according to [4,6,7].

2. Material and Methods

2.1. Sample Materials

Grossular samples were collected from skarn-type polymetallic deposits in the Mt.Bozhushan area. Here, the grossular occurs as discrete single crystals hosted within skarn rocks, with individual crystal dimensions ranging from 0.5 to 3.5 cm. The processing of grossular-bearing skarn through crushing and screening allowed for the recovery of gem-grade grossulars as raw materials. For this study, a total of 27 grossular samples were selected, exhibiting colors ranging from pale orange-yellow to vivid orange. The samples were semi-transparent to transparent, with a vitreous luster and high brightness, representing high-quality grossular of significant value. Some samples exhibited color zoning visible to the naked eye. Of the 27 samples, 3 were cut and polished, resulting in 2 faceted grossular gemstones (samples DSJ-K1 and DSJ-K2) and 1 cabochon (sample DSJ-S). The remaining 24 samples were processed by the Yuneng Company in Langfang, Hebei Province, where 2 thin sections (DSJ-b1 and DSJ-b2), 2 probe sections (DSJ-23-4-C3 and DSJ-23-5-C3), and 1 laser ablation target containing 20 garnet grains (DSJ-23-5-1 to DSJ-23-5-20) were prepared.

2.2. Methods

2.2.1. Basic Gemological Testing

Basic gemological testing was conducted at the Yunnan Research Institute of Gem and Jade Quality Supervision and Inspection.

2.2.2. Electron Probe Microanalysis (EPMA)

Electron probe microanalysis (EPMA) was performed at the Testing Center of Shandong Bureau, China Metallurgical Geology Bureau, using a JXA-8230 electron probe microanalyzer (JOEL, Tokyo, Japan). The testing parameters were as follows: an accelerating voltage of 15 kV, a beam current of 20 nA, and a beam spot diameter of 20 μm. The standards used for calibration included jadeite (SiO₂, Na₂O), rutile (TiO₂), yttrium aluminum garnet (Al₂O₃), olivine (FeO, MgO), rhodonite (MnO), diopside (CaO), sanidine (K₂O), apatite (P₂O₅, F), and tugtupite (Cl), all of which were from SPI Minerals/Metals Standards (West Chester, PA, USA) [8]. The ZAF correction method was applied for data calibration. The compositional data in parentheses were calibrated against the reference material preceding the brackets.

2.2.3. Laser Ablation Inductively Coupled Plasma Mass Spectrometry (LA-ICP-MS)

Laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) was conducted at the Chinese Academy of Geological Sciences. The laser ablation system used was a GeoLasPro 193 nm ArF excimer system (Coherent, New York, NY, USA), coupled with a Thermo Fisher ICAP Q (Thermofisher, Waltham, MA, USA) inductively coupled plasma mass spectrometer. During the laser ablation process, helium was used as the carrier gas, and the aerosol generated from the sample was mixed with argon (used as the carrier, plasma, and makeup gas) via a T-shaped connector before it entered the ICP. The flow rates of helium and argon were optimized to achieve the best signal intensity using NIST SRM 610 (a synthetic silicate glass reference material developed by the National Institute of Standards and Technology, Gaithersburg, MD, USA) [9] as the tuning standard [10]. The laser ablation parameters included a beam spot diameter of 40 μm, a frequency of 6 Hz, and an energy density of approximately 10–12 J/cm². The external standards used for calibration were NIST SRM 610, NIST SRM 612, BCR-2G, and BIR-1G [9,10,11,12], while the monitoring standards included CGSG-1G and CGSG-2G [13,14]. The sampling method involved single-point ablation with peak-hopping acquisition. The time-resolved analysis mode consisted of 25 s of a gas blank, 60 s of sample ablation, and 25 s of washout. After every 10 sample points, a set of reference materials (NIST 610 and NIST 612) was analyzed [9,12]. Elemental concentrations were calculated using the ICPMSDATACAL v9.5 software, with normalization to calcium (Ca) for data correction [15].

2.2.4. Ultraviolet–Visible Absorption Spectroscopy (UV-VIS)

Ultraviolet–visible absorption spectroscopy (UV-VIS) was performed at the Research Center for Analysis and Measurement, Kunming University of Science and Technology, using a Skyray UV100 spectrophotometer (Skyray Instrument, Suzhou, China). The testing conditions were as follows: a spectral range of 200–1000 nm, an integration time of 100 ms, and a smoothing width of 5; the temperature was 20 °C, the humidity was 52%, and the air pressure was 80.735 kpa.

2.2.5. Fourier Transform Infrared Spectroscopy (FTIR)

Fourier transform infrared spectroscopy (FTIR) was conducted at the Research Center for Analysis and Measurement, Kunming University of Science and Technology, using a Thermo NICOLET Is50 spectrometer (Thermofisher, Waltham, MA, USA) equipped with a Pike UpIR diffuse reflectance accessory. The number of scans was 8, the resolution was 4 cm⁻¹, the gain was 2, the moving mirror speed was 0.4747, and the aperture was 80. The collection range of diffuse reflection was 400–4000 cm⁻¹, the temperature was 20 °C, the humidity was 52%, and the air pressure was 80.735 kpa. The analyses were carried out at the Kunming University of Science and Technology Analysis and Testing Research Center.

2.2.6. Laser Raman Spectroscopy (RAMAN)

Laser Raman spectroscopy was performed at the Research Center for Analysis and Measurement, Kunming University of Science and Technology, using a LabRAM HR Evolution Raman spectrometer (HORIBA, Palaiseau, France). The testing conditions were as follows: a laser wavelength of 532 nm, output power of 12.5 mW, wavenumber range of 100–4000 cm⁻¹, and resolution of 1 cm⁻¹, with 5 accumulations, and an acquisition time of 5 min; the temperature was 20 °C, the humidity was 52%, and the air pressure was 80.735 kpa.

3. Results and Discussion

Gemological Characteristics

In this study, the refractive index of the Mt.Bozhushan grossular samples ranged from 1.73 to 1.74 (the wavelength of the incident light source was 589.5 nm and the air pressure was 80.735 kpa), falling within the theoretical range for grossular. The specific gravity, measured using the hydrostatic method, was 3.4–3.5, which is slightly lower than that of grossular from other localities [16,17,18]. This phenomenon may be attributed to the relatively low iron content and the presence of numerous inclusions. Under a dichroscope, the Mt.Bozhushan grossular exhibited no dichroism. Additionally, their color remained unchanged when viewed through a Chelsea filter. The samples were inert to fluorescence under both long-wave (365 nm) and short-wave (253.7 nm) ultraviolet light.

4. Analysis and Discussion of Test Results

4.1. Microscopic Characteristics

Microscopic observation and photomicrography were conducted at the Analysis and Testing Center of Kunming University of Science and Technology, using a Jiangnan Yongxin NGI6 gemological microscope was used to examine the faceted and cabochon samples of Mt.Bozhushan grossular. Microscopic examination revealed that the Mt.Bozhushan samples contain many clusters of needle-like and fibrous solid inclusions.(Figure 2), similar to those found in grossular from other skarn-type deposits worldwide [19]. In contrast, the number of two-phase (liquid–gas) inclusions is relatively small, and they are significantly smaller than the solid inclusions. Based on their morphology and distribution patterns, the two-phase inclusions can be classified into two types: (1) elliptical two-phase inclusions sporadically distributed sporadically around solid inclusions and (2) fingerprint-like two-phase inclusions aligned along healed fractures. These characteristics indicate significant similarities to grossular of analogous origins worldwide [20].

Polarizing microscope observation was conducted at the Faculty of Land Resources Engineering, Kunming University of Science and Technology, using a SOPTOP CX40P polarizing microscope system to examine and photograph thin sections of Mt.Bozhushan grossular (Figure 3). Thin-section analysis reveals that the grossular is closely associated with minerals such as actinolite and calcite in the host rock. Petrographic observation under cross-polarized light (XPL) reveals distinct zonation patterns in the grossular that remain undetectable under plane-polarized light (PPL). And petrographic analysis under cross-polarized light (XPL) further reveals the presence of actinolite inclusions within grossular crystals. The morphology of these actinolite inclusions is consistent with the fibrous and needle-like inclusions observed under a high magnification. Furthermore, the actinolite inclusions in the thin sections are predominantly clustered, aligning with the distribution patterns of the fibrous and needle-like inclusions observed during microscopic examination. Additionally, alteration features can be identified along fractures and at the edges of the Mt.Bozhushan grossular in the thin sections. Some fractures and crystal edges exhibit replacement textures. The mineral assemblages observed are similar to those found in grossular from other skarn-type deposits worldwide [21].

4.2. Compositional Analysis

The compositional analysis consisted of two parts: electron probe microanalysis (EPMA) and laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS). EPMA was used to obtain backscattered electron (BSE) images of the samples (Figure 4), determine the oxide content of the metallic elements, and calculate the ionic numbers, average end-member compositions, and crystal chemical formulas of the Mt.Bozhushan grossular. LA-ICP-MS was employed to quantitatively measure the elemental composition of the Mt.Bozhushan grossular, with a particular focus on trace elements that may have influenced its color.

4.2.1. Electron Probe Microanalysis (EPMA)

EPMA was used to capture backscattered electron (BSE) images of typical grossular grains from probe slices DSJ-23-4-C3 and DSJ-23-5-C3. The BSE images revealed well-developed zoning in the Mt.Bozhushan grossular, with “embayment” structures formed by alteration observed in areas with fractures and healed fractures. Additionally, small metallic mineral inclusions were identified within the grossular grains in the BSE images.

Electron probe wavelength dispersive spectroscopy (WDS) analysis was performed at 10 points each on typical grossular grains in two probe sections to determine the major element oxide contents of the Mt.Bozhushan grossular. The results are shown in

Table 1. EPMA analytical results for grossular from Mt. Bozhushan.

Points	SiO2	TiO2	Al2O3	Cr2O3	FeOT	MnO	MgO	CaO	Na2O	K2O	Total
DSJ23-4-C3-1	39.123	0.006	21.157	0	2.092	1.62	0.05	35.012	0	0.007	99.067
DSJ23-4-C3-2	39.301	0	20.99	0	2.396	1.565	0.082	34.946	0.017	0	99.297
DSJ23-4-C3-3	39.298	0.009	21.069	0	2.153	1.563	0.059	35.152	0.046	0.002	99.351
DSJ23-4-C3-4	39.313	0	20.88	0.003	2.578	1.547	0.047	34.922	0.006	0	99.296
DSJ23-4-C3-5	38.885	0.109	20.821	0	2.293	1.545	0.064	35.113	0.031	0.018	98.879
DSJ23-4-C3-6	38.938	0.19	20.398	0.004	3.296	1.91	0.039	34.408	0.022	0.007	99.212
DSJ23-4-C3-7	39.282	0	20.839	0.011	2.381	1.63	0.048	34.856	0	0.005	99.052
DSJ23-4-C3-8	39.446	0.011	20.031	0	3.477	1.546	0.033	34.879	0.019	0.018	99.46
DSJ23-4-C3-9	38.873	0.068	19.941	0	3.818	1.766	0.035	34.28	0.029	0.003	98.813
DSJ23-4-C3-10	39.206	0	20.443	0	3.045	1.322	0.022	35.043	0.016	0	99.097
DSJ23-5-C3-1	39.457	0.008	20.783	0.008	2.716	1.675	0.032	35.012	0.024	0	99.715
DSJ23-5-C3-2	39.122	0.202	20.825	0	2.324	1.4	0.096	35.281	0	0	99.25
DSJ23-5-C3-3	39.062	0.107	20.153	0	3.435	1.879	0.041	34.065	0	0	98.742
DSJ23-5-C3-4	39.086	0.006	20.772	0	2.322	1.39	0.083	35.046	0	0.008	98.713
DSJ23-5-C3-5	39.339	0.107	20.718	0	2.607	1.348	0.051	35.125	0.009	0	99.304
DSJ23-5-C3-6	39.314	0.033	20.627	0	2.624	1.524	0.033	34.593	0	0	98.748
DSJ23-5-C3-7	39.195	0.003	19.888	0	3.895	1.475	0.025	34.975	0	0	99.456
DSJ23-5-C3-8	39.254	0	20.347	0	3.14	1.528	0.026	34.851	0	0	99.146
DSJ23-5-C3-9	39.259	0	20.505	0.016	3.127	1.595	0.039	34.513	0.035	0	99.089
DSJ23-5-C3-10	39.523	0.002	20.415	0	2.992	1.311	0.028	35.03	0.004	0	99.305

. The analytical results from Table 1 reveal no significant compositional distinction between the bright and dark growth zones observed within the grossular samples.

Electron probe microanalysis (EPMA) provided the oxide weight percentages of the Mt.Bozhushan grossular samples. The values of Fe₂O₃ and FeO were calculated using the charge balance method [22]. The raw EPMA data were processed using the geochemical data analysis software GeokitPro(build20250210) [23]. The ionic numbers of the samples were calculated based on 12 oxygen atoms and 8 cations [23], and the results are presented in

Table 2. Ions of grossular from Mt. Bozhushan. n_B (number of ions calculated on the basis of n (O) = 12).

Points	Si	Ti	AlⅣ	Ti*	AlⅥ	Cr	Fe3+	Fe2+	Mn	Mg	Ca
DSJ23-4-C3-line001	2.9871	0.00	0.00	0.0003	1.9039	0.0000	0.1212	0.01	0.10	0.01	2.86
DSJ23-4-C3-line002	2.9958	0.00	0.00	0.0000	1.8857	0.0000	0.1227	0.03	0.10	0.01	2.86
DSJ23-4-C3-line003	2.9925	0.00	0.00	0.0005	1.8909	0.0000	0.1231	0.01	0.10	0.01	2.87
DSJ23-4-C3-line004	2.9986	0.00	0.00	0.0000	1.8770	0.0002	0.1256	0.04	0.10	0.01	2.85
DSJ23-4-C3-line005	2.9771	0.00	0.00	0.0063	1.8787	0.0000	0.1468	0.00	0.10	0.01	2.88
DSJ23-4-C3-line006	2.9823	0.00	0.00	0.0109	1.8413	0.0002	0.1719	0.04	0.12	0.00	2.83
DSJ23-4-C3-line007	3.0031	0.00	0.00	0.0000	1.8776	0.0007	0.1155	0.04	0.11	0.01	2.86
DSJ23-4-C3-line008	3.0132	0.00	0.00	0.0006	1.8033	0.0000	0.1691	0.05	0.10	0.00	2.86
DSJ23-4-C3-line009	2.9923	0.00	0.00	0.0039	1.8091	0.0000	0.1983	0.05	0.12	0.00	2.83
DSJ23-4-C3-line010	2.9997	0.00	0.00	0.0000	1.8434	0.0000	0.1573	0.04	0.09	0.00	2.87
DSJ23-5-C3-line001	2.9994	0.00	0.00	0.0005	1.8620	0.0005	0.1377	0.03	0.11	0.00	2.85
DSJ23-5-C3-line002	2.9838	0.00	0.00	0.0116	1.8719	0.0000	0.1373	0.01	0.09	0.01	2.88
DSJ23-5-C3-line003	3.0071	0.00	0.00	0.0062	1.8285	0.0000	0.1449	0.08	0.12	0.00	2.81
DSJ23-5-C3-line004	2.9960	0.00	0.00	0.0003	1.8765	0.0000	0.1309	0.02	0.09	0.01	2.88
DSJ23-5-C3-line005	3.0007	0.00	0.00	0.0061	1.8625	0.0000	0.1237	0.04	0.09	0.01	2.87
DSJ23-5-C3-line006	3.0169	0.00	0.00	0.0019	1.8656	0.0000	0.0967	0.07	0.10	0.00	2.84
DSJ23-5-C3-line007	2.9961	0.00	0.00	0.0002	1.7918	0.0000	0.2156	0.03	0.10	0.00	2.86
DSJ23-5-C3-line008	3.0041	0.00	0.00	0.0000	1.8352	0.0000	0.1566	0.04	0.10	0.00	2.86
DSJ23-5-C3-line009	3.0062	0.00	0.00	0.0000	1.8505	0.0010	0.1362	0.06	0.10	0.00	2.83
DSJ23-5-C3-line010	3.0177	0.00	0.00	0.0001	1.8371	0.0000	0.1273	0.06	0.08	0.00	2.87

Note: The general formula of garnet is A₃B₂(SiO₃)₄. The ion calculation was based on 12 oxygen atoms and 8 cations, with Na and K (divided by 2) incorporated into Ca ions. Ti* represents the titanium atom occupying the B position.

. The crystal chemical formula of the samples was derived from the calculated ionic numbers. The results show that the crystal chemical formula of the Mt.Bozhushan grossular is (Ca_2.81–2.88, Mn_0.08–0.12, Fe_0.00–0.18, Mg_0.00–0.01)₃(Al_{1.7918–1.9039}, Fe_{0.0967–0.2156}, Cr_{0.0000–0.0010})₂Si_{2.9771–3.0177}O₁₂, which is very close to the theoretical chemical formula of grossular, Ca₃Al₂(SiO₄)₃.

The end-member components of the Mt.Bozhushan grossular were calculated using the geochemical data processing software GeokitPro(build20250210) [23], with the results shown in

Table 3. Average end-member components of grossular from Mt. Bozhushan.

	End-Member	Uva	Pyr	Spe	And	Gro	Alm	Ski	Other
Points
DSJ23-4-C3-line001		0.00	0.19	3.48	6.04	89.21	0.41	0.00	0.67
DSJ23-4-C3-line002		0.00	0.31	3.37	6.13	88.97	1.00	0.00	0.22
DSJ23-4-C3-line003		0.00	0.22	3.36	6.15	89.43	0.46	0.00	0.37
DSJ23-4-C3-line004		0.01	0.18	3.33	6.28	88.84	1.30	0.00	0.07
DSJ23-4-C3-line005		0.00	0.24	3.33	7.32	88.48	0.00	0.00	0.63
DSJ23-4-C3-line006		0.01	0.15	4.12	8.58	85.47	1.31	0.00	0.36
DSJ23-4-C3-line007		0.03	0.18	3.52	5.78	89.01	1.22	0.00	0.25
DSJ23-4-C3-line008		0.00	0.13	3.34	8.48	85.17	1.77	0.00	1.11
DSJ23-4-C3-line009		0.00	0.13	3.84	9.91	84.34	1.58	0.00	0.20
DSJ23-4-C3-line010		0.00	0.08	2.86	7.86	87.93	1.25	0.00	0.02
DSJ23-5-C3-line001		0.02	0.12	3.59	6.89	88.20	1.16	0.00	0.01
DSJ23-5-C3-line002		0.00	0.36	3.01	6.86	89.16	0.36	0.00	0.25
DSJ23-5-C3-line003		0.00	0.16	4.09	7.26	84.87	2.55	0.00	1.07
DSJ23-5-C3-line004		0.00	0.32	3.01	6.54	89.34	0.60	0.00	0.20
DSJ23-5-C3-line005		0.00	0.19	2.91	6.19	88.73	1.42	0.00	0.55
DSJ23-5-C3-line006		0.00	0.13	3.31	4.85	87.79	2.40	0.00	1.51
DSJ23-5-C3-line007		0.00	0.09	3.18	10.77	84.64	1.11	0.00	0.20
DSJ23-5-C3-line008		0.00	0.10	3.30	7.84	86.95	1.48	0.00	0.33
DSJ23-5-C3-line009		0.05	0.15	3.45	6.82	86.90	2.14	0.00	0.49
DSJ23-5-C3-line010		0.00	0.11	2.84	6.39	87.11	2.13	0.00	1.43

Abbreviations: Uva: uvarovite; Pyr: pyrope; Spe: spessartine; And: andradite; Alm: almandine; Gro: grossular; Ski: skiagite. Note: Ti* is not included in the end-member components. Excess trivalent cations (Al) or divalent cations (Ca) were divided by 5 and assigned to the “Other” category, while still contributing to the calculation of end-member molecular proportions.

. The calculations reveal that the analyzed samples are primarily composed of grossular, and contain minor components of andradite, spessartine, almandine, and pyrope. Trace amounts of uvarovite were also detected at some measurement points.

During microscopic observation, we initially identified the inclusions in Mt.Bozhushan grossular garnet as actinolite. To determine the specific mineral species of these inclusions, we conducted wavelength dispersive spectroscopy (WDS) analysis using EPMA (electron probe microanalysis) on one inclusion from sample DSJ-1 and two inclusions from sample DSJ-B2. Electron probe microanalysis (EPMA) provided oxide weight percentages of the inclusions within the Mt.Bozhushan grossular garnet samples. The raw EPMA data were processed using the geochemical data analysis software GeokitPro(build20250210) [23]. The general chemical formula of actinolite inclusions in grossular garnet can be expressed as A₀–₁B₂C₅[T₈O₂₂](OH,F,Cl)₂. The electron probe data for garnet-hosted inclusions were normalized to 23 oxygen atoms and 16 cations [23], while the chemical analyses were adjusted to (O+OH+F+Cl) = 24 as the calculation basis. The EPMA test results are presented in

Table 4. EPMA analytical results of actinolite inclusions in grossular of Mt. Bozhushan.

Points	SiO2	TiO2	Al2O3	Cr2O3	FeO	MnO	MgO	CaO	Na2O	K2O	H2O	F	Cl	Total
DSJ-B1-C1	57.2671	0.0593	0.0023	0.0223	10.7479	0.0278	18.0524	11.4794	0.0028	0.008	1.710	0.020	0.309	99.7083
DSJ-B2-C1	55.557	0.0691	0.1802	0.0102	11.7553	0	18.2106	11.2193	0.0272	0.0407	1.819	0	0.253	99.1416
DSJ-B2-C2	56.973	0.0901	0.0765	0.1666	12.3796	0	17.4873	10.6217	0.0929	0.0071	1.832	0	0.261	99.9878

, and the analytical outcomes from GeoKit calculations are presented in

Table 5. Ions of actinolite inclusions in grossular of Mt. Bozhushan.

Points	T				C						B				A
	Si	Al	Ti	Fe3+	Al	Ti	Cr3+	Fe3+	Mg	Fe	Fe	Mn	Ca	Na	K	OH	F	Cl
DSJ-B1-C1	8	0	0	0	0	0.006	0.002	0.573	3.759	0.658	0.024	0.003	1.718	0.001	0.001	1.489	0.009	0.073
DSJ-B2-C1	7.866	0.03	0.007	0.097	0	0	0.001	0.473	3.844	0.682	0.14	0	1.702	0.007	0.007	1.615	0	0.046
DSJ-B2-C2	8	0	0	0	0.013	0.01	0.018	0.469	3.661	0.829	0.155	0	1.598	0.025	0.001	1.62	0	0.052

.

The analytical results reveal that the actinolite inclusions in Mt.Bozhushan grossular garnet exhibit low magnesium (Mg) and high iron (Fe) contents. In accordance with the IMA 1997 amphibole nomenclature, these inclusions should be classified as ferro-actinolite.

4.2.2. LA-ICP-MS Analysis

LA-ICP-MS was used to conduct single-spot ablation tests on the grossular samples from laser ablation targets DSJ23-5-1 to DSJ23-5-20, aiming to characterize the elemental composition of the Mt.Bozhushan grossular. The results are presented in

Table 6. LA-ICP-MS test data of grossular from Mt. Bozhushan (μg·g⁻¹).

Points	Si	Li	Na	Mg	Al	K	Ca	Rb	Ti	V	Cr	Mn	Fe	Cs
DSJ23-5-1	4,495,000	1.72	bdl	360.4	111,000	bdl	234,500	bdl	105.2	0.58	bdl	13,500	17,510	bdl
DSJ23-5-2	4,607,000	1.104	3.08	335.9	112,300	bdl	234,500	0.0024	99.4	0.373	bdl	13,370	17,290	bdl
DSJ23-5-3	4,589,000	1.164	3.71	319.1	111,500	bdl	233,500	bdl	144.6	0.381	bdl	14,150	17,550	bdl
DSJ23-5-4	4,401,000	0.28	4.59	85.6	115,800	5.1	238,400	0.097	411.3	1.068	bdl	14,890	10,480	0.197
DSJ23-5-5	4,566,000	1.46	3.62	240.2	109,800	bdl	232,300	bdl	216.4	1.173	bdl	15,110	18,050	bdl
DSJ23-5-6	4,552,000	3.42	3.12	428.7	111,500	bdl	231,400	bdl	183.4	1.399	bdl	14,020	17,940	bdl
DSJ23-5-7	4,683,000	0.688	5.7	209.3	106,800	1.7	233,500	0.094	64.6	1.084	bdl	14,380	25,970	0.15
DSJ23-5-8	4,644,000	1.66	48	186.4	109,900	75	235,300	1.59	171.9	2.47	40	13,800	25,150	2.13
DSJ23-5-9	4,595,000	1.2	bdl	198.6	110,900	bdl	234,200	0.0084	19.2	0.293	bdl	13,520	21,900	0.0078
DSJ23-5-10	4,730,000	0.614	4.01	207.4	106,600	1.8	231,900	bdl	56.2	0.963	bdl	13,960	26,680	0.0033
DSJ23-5-11	4,437,000	5.39	10.17	434.4	110,300	bdl	232,700	bdl	2516	2.3	bdl	13,120	17,660	bdl
DSJ23-5-12	4,523,000	2.62	7.96	455.6	110,600	2.41	234,100	bdl	1212	1.072	bdl	11,920	20,070	bdl
DSJ23-5-13	4,527,000	1.81	5.45	408.3	112,100	2.71	237,600	0.053	276.6	0.363	bdl	12,700	20,040	0.083
DSJ23-5-14	4,591,000	1.64	4.91	386.4	112,000	1.14	238,700	0.044	202.6	0.309	bdl	12,590	20,540	0.075
DSJ23-5-15	4,603,000	1.62	5.46	352.4	112,000	bdl	236,100	0.047	55	0.192	bdl	12,860	20,730	0.111
DSJ23-5-16	4,482,000	1.75	3.41	382.9	111,500	bdl	235,900	bdl	533	0.454	bdl	12,410	19,460	bdl
DSJ23-5-17	4,615,000	10.49	18.92	406.2	108,700	bdl	231,800	0.0208	3976	4.08	bdl	13,390	17,560	bdl
DSJ23-5-18	4,566,000	2.42	5.33	455	112,200	bdl	236,700	0.0101	773	0.956	bdl	12,990	20,030	0.0226
DSJ23-5-19	4,578,000	1.94	8.89	422.7	111,600	5.7	235,600	0.096	457	0.527	0.86	11,810	19,810	0.209
DSJ23-5-20	4,628,000	1.74	4.33	401	114,200	bdl	238,400	bdl	122	0.277	bdl	13,490	20,320	bdl

Abbreviations: bdl, the element content was below the detection limit.

. The Mt.Bozhushan grossular exhibits the following elemental concentrations: Si—440,100–473,000 μg/g (average 457,060 μg/g); Ca—231,400–238,700 μg/g (average 234,855 μg/g); Al—106,600–115,800 μg/g (average 111,065 μg/g); Mn—11,810–15,110 μg/g (average 13,399 μg/g); Fe—10,480–26,680 μg/g (average 19,737 μg/g); and Mg—85.6–455.6 μg/g (average 333.8 μg/g). The results indicate that the Mt.Bozhushan grossular is compositionally pure, with few impurities. The presence of Fe (which imparts reddish hues and reduces lightness [16,17]) and Mn (which contributes to orange-yellow tones and decreases lightness [16,17,18,24,25,26]) accounts for the observed color variations. Notably, Fe content fluctuates significantly, leading to a color distribution ranging from pale orange-yellow to vivid orange. Due to the relatively low concentrations of both Fe and Mn, the garnet exhibits excellent lightness. Additionally, V and Cr (elements associated with green hues [1,25]) are present in negligible amounts, confirming the absence of green tones in all the analyzed samples.

4.3. UV-Vis Absorption Spectroscopy Analysis

As Fourier transform infrared spectroscopy (FTIR) requires well-polished surfaces and specific curvature, the cut and polished samples DSJ-S, DSJ-K1, and DSJ-K2 were selected for testing. The wavelength range of 360–780 nm, which is the most relevant to the coloration of grossular, was analyzed. The results (Figure 5) indicate that Mt.Bozhushan grossular exhibits strong absorption in the ultraviolet (UV), blue–violet, and green regions, and transmits more light in the yellow–red regions, resulting in its pale orange-yellow to vivid orange hues. The UV-Vis absorption spectrum of the Mt.Bozhushan grossular displays peaks at 370 nm, 398 nm, 410 nm, 420 nm, 432 nm, 437 nm, 460 nm, 527 nm, 660 nm, 686 nm, and 699 nm. Based on previous studies, the peaks at 370 nm, 398 nm, 432 nm, 437 nm, and 527 nm can be attributed to Fe³⁺ occupying octahedral sites, corresponding to electronic transitions between the ⁶A_1g→⁴E₁⁴A_g energy levels [26]. The 460 nm absorption arises from charge transfer between Fe²⁺ and Fe³⁺. The 686 nm peak is assigned to Fe²⁺in dodecahedral sites, which is caused by ⁵E_g→³T_1g(3H) electronic transitions [27]. The peaks at 660 nm and 699 nm are linked to Cr³⁺ in octahedral coordination, corresponding to ⁴A_2g(4F)→⁴T_2g(4F) transitions [28]. The absorptions near 410 nm and 420 nm are attributed to Mn²⁺ in dodecahedral sites, resulting from d-d transitions within its 3d⁵ electronic configuration [29,30]. Combined with the electron probe microanalysis (EPMA) and LA-ICP-MS results, this study reveals that, although the Mt.Bozhushan grossular is compositionally pure, its trace elements—particularly Fe and Mn—significantly influence its color [31]. The strong absorption of blue–violet light and higher transmittance in the yellow–red regions due to Fe and Mn [32,33] result in the garnet’s pale orange-yellow to vivid orange coloration, which is highly valued in gemology [34].

4.4. FTIR Spectroscopy Analysis

Since FTIR analysis requires well-polished surfaces, samples DSJ-S, DSJ-K1, and DSJ-K2 were selected for testing. The results (Figure 6) show a valid data range of 100–1300 cm⁻¹. The infrared spectra of grossular, a member of the garnet group, are primarily attributed to vibrations of the [SiO₄]⁴⁻ group and cation lattice vibrations [28]. Theoretically, the four absorption peaks in the 800–1100 cm⁻¹ range arise from the triply degenerate splitting of asymmetric stretching vibrations (ν₃) of the [SiO₄]⁴⁻ group. The three peaks in the 500–700 cm⁻¹ range correspond to doubly degenerate symmetric bending vibrations (ν₂) or triply degenerate asymmetric bending vibrations (ν₄) of the [SiO₄]⁴⁻ group. The peaks below 500 cm⁻¹ are caused by lattice vibrations associated with cations other than Si⁴⁺ [16]. The FTIR absorption peaks of Mt.Bozhushan grossular are as follows:

• A: 964 ± 4 cm⁻¹.
• C: 866 ± 3 cm⁻¹.
• D: 841 ± 5 cm⁻¹.
• E: 616 ± 1 cm⁻¹.
• F: 553 ± 3 cm⁻¹.
• G: 510 ± 3 cm⁻¹.
• H: 488 ± 3 cm⁻¹.
• I: 455 ± 5 cm⁻¹.

According to previous research [35], the infrared spectra of grossular have B-site absorption peaks that are expected near 945 cm⁻¹, but this was not detected in any of the tested samples, and likely merged with Peak A.

Figure 6. FTIR spectra of samples.

Figure 6. FTIR spectra of samples.

4.5. Raman Spectroscopy Analysis

According to previous studies, garnet theoretically exhibits 25 active Raman modes represented by the irreducible Γ = 3A_1g + 5A_2g + 8E_g + 14F_1g + 14F_2g + 5A_1u + 5A_2u + 10E_u + 17F_1u + 16F_2u, with A_1g, E_g, and F_2g being Raman-active [36,37]. Grossular, specifically, has 24 theoretically active modes [36]. Samples DSJ-K1, DSJ-K2, DSJ-S, and DSJ-B1, with high surface flatness and cleanliness, were selected for Raman spectroscopy analysis. Although the initial aim was to analyze both the Mt.Bozhushan grossular and its inclusions, the depth of the inclusions exceeded the capability of the equipment, preventing the acquisition of their Raman spectra. Due to strong background fluorescence in the samples, the effective data range was limited to 0–1600 cm⁻¹. A total of 12 Raman-active modes were detected (Figure 7), with missing peaks attributed to overlapping signals, low-intensity peaks, or fluorescence interference. The observed Raman shifts were as follows: 1027 ± 1 cm⁻¹, 894 ± 2 cm⁻¹, 838 ± 1 cm⁻¹, 646 ± 1 cm⁻¹, 561 ± 1 cm⁻¹, 524 ± 2 cm⁻¹, 429 ± 1 cm⁻¹, 388 ± 2 cm⁻¹, 344 ± 2 cm⁻¹, 292 ± 1 cm⁻¹, 261 ± 1 cm⁻¹, and 195 ± 1 cm⁻¹. The peak assignments were as follows: The Raman bands at 1027 ± 1 cm⁻¹, 894 ± 2 cm⁻¹, and 838 ± 1 cm⁻¹ correspond to the symmetric stretching vibrations (ν₁) of non-bridging oxygens in the [SiO₄]⁴⁻ tetrahedron of the F_2g symmetry mode—(Si-O)n_ᵦsing–O stretching [36,37]. The peak at 561 ± 1 cm⁻¹ corresponds to the bending vibration (ν₂) of bridging oxygens (BO) in the [SiO₄]⁴⁻ tetrahedron under the A_1g symmetry mode—(Si-O) bending [36,37,38,39]. The peaks at 524 ± 2 cm⁻¹ and 429 ± 1 cm⁻¹ are attributed to the bending vibrations of bridging oxygens in the [SiO₄]⁴⁻ tetrahedron within the E_g symmetry mode—(Si-O) bending. The peak at 646 ± 1 cm⁻¹ is assigned to the bending vibration of bridging oxygens in the [SiO₄]⁴⁻ tetrahedron associated with the F_2g symmetry mode—(Si-O) bending [37,38,39]. The peaks at 388 ± 2 cm⁻¹ and 344 ± 2 cm⁻¹ are assigned to the F_2g-symmetry librational modes of the [SiO₄]⁴⁻ tetrahedron (R(SiO₄)⁴⁻) [29,30,31,32]. The peak at 195 ± 1 cm⁻¹ corresponds to the F_2g-symmetry translational mode of the [SiO₄]⁴⁻ tetrahedron (T[SiO₄]⁴⁻). The peaks at 292 ± 1 cm⁻¹ and 261 ± 1 cm⁻¹ are attributed to the F_2g-symmetry translational modes of divalent cations (T(X²⁺)) [36,37,38,39].

In the grossular–andradite solid solution system, Raman shifts migrate toward shorter wavelengths (blue shift) with an increasing andradite content and toward longer wavelengths (red shift) with a higher grossular content [38]. The observed red shifts in the tested samples suggest a dominant grossular composition with minimal andradite content. This trend may also reflect the influence of trace components, including spessartine (Mn-Al garnet), almandine (Fe-Al garnet), pyrope (Mg-Al garnet), and negligible uvarovite (Ca-Cr garnet).

4.6. Discussion

In this study, the color of the Mt.Bozhushan grossular primarily ranges from pale orange-yellow to vivid orange. Based on the experimental results, this coloration is significantly influenced by trace elements, despite the garnet’s high compositional purity, while inclusions have negligible impact on its color. According to UV-Vis spectroscopy, the key trace elements affecting the color are Fe, Mn, and Cr. EPMA results confirm the presence of Fe²⁺ and Fe³⁺ in the garnet, and UV-Vis analysis reveals that their contribution to coloration arises from electronic transitions (Fe³⁺) and charge transfer between Fe²⁺ and Fe³⁺ [31]. Similarly, Mn²⁺ influences color through electronic transitions. The UV-Vis spectra demonstrate the strong absorption of blue-violet light and the high transmittance of yellow-red light due to Fe and Mn [34]. Although weak Cr-related absorption peaks were detected in the UV-Vis spectra, their minimal intensity suggests that Cr has little effect on the garnet’s color. This aligns with the EPMA and LA-ICP-MS data, which indicate extremely low Cr concentrations in the Mt.Bozhushan grossular.

5. Conclusions

(1) The Mt.Bozhushan grossular exhibits pale orange-yellow to vivid orange hues with high lightness, indicating a high-value variety of grossular. Its specific gravity is slightly lower than the theoretical value, likely due to its low iron content and abundant inclusions. Based on these characteristics, we propose that the Mt.Bozhushan grossular holds significant potential for development as gem-quality material and should be comprehensively recovered and utilized rather than being discarded as mining waste.

(2) The garnet contains many clustered acicular and fibrous solid inclusions, which are predominantly composed of actinolite. The rare gas–liquid two-phase inclusions are smaller than the solid inclusions and display two distinct morphologies: elliptical inclusions scattered around solid inclusions and fingerprint-like inclusions distributed along healed fractures, consistent with the characteristics of grossular samples of similar origins worldwide.

(3) Backscattered electron (BSE) imaging reveals well-developed growth zoning in the Mt.Bozhushan grossular, with alteration features observed along fractures and healed fractures. Electron probe microanalysis (EPMA) yields the following crystallochemical formula:

(Ca_2.81–2.88, Mn₀._08–0.12, Fe_0.00–0.18, Mg_0.00–0.01)₃(Al_{1.7918–1.9039}, Fe_{0.0967–0.2156}, Cr_{0.0000–0.0010})₂Si_{2.9771–3.0177}O₁₂.

End-member calculations confirm that the garnet is predominantly grossular (Ca-Al garnet), with minor andradite (Ca-Fe), spessartine (Mn-Al), almandine (Fe-Al), and pyrope (Mg-Al), as well as trace uvarovite (Ca-Cr) components.

(4) The LA-ICP-MS results indicate the high compositional purity of Mt.Bozhushan grossular. The low concentrations of Fe and Mn (which affect lightness and color) and negligible concentrations of Cr and V (which impart green tones) contribute to its pale orange-yellow to vivid orange coloration and excellent lightness.

(5) Fourier transform infrared (FTIR) spectroscopy of the grossular theoretically exhibits nine peaks in the 100–1300 cm⁻¹ range. However, only eight peaks were detected in the Mt.Bozhushan samples, suggesting the potential merging of the theoretical B-site peak with Peak A.

(6) Raman spectroscopy of the Mt.Bozhushan grossular revealed 12 Raman-active modes within the effective range of 0–1600 cm⁻¹, fewer than the theoretical 24 modes. This discrepancy was attributed to overlapping peaks, low-intensity signals, and strong fluorescence interference.

(7) In the future, we will be able to establish a quality evaluation method for grossular gemstones produced in the Mt. Bozhushan area based on this research. This methodology can be extended to other metal deposits associated with grossular in the region, aiming to assess the value of accompanying grossular resources and promote the comprehensive utilization of these gemstone resources within the Mt. Bozhushan mining area.