Investigation of the Coloration Mechanisms of Yellow-Green Nephrite from Ruoqiang (Xinjiang), China

Abstract

This study systematically investigates the color origin and coloration mechanisms of yellow-green nephrite from Ruoqiang, Xinjiang, using multiple analytical techniques including hyperspectral colorimetry, X-ray fluorescence (XRF) spectroscopy, titrimetry, laser ablation inductively coupled plasma–mass spectrometry (LA-ICP-MS), Raman spectroscopy and ultraviolet–visible (UV-Vis) spectroscopy. A pioneering quantitative model (R² = 0.942) was established between hue (H) and the Fe₂O₃ ratio (Fe₂O₃/TFe), revealing that the coloration mechanism is jointly governed by Fe³⁺ charge transfer (300–400 nm absorption band) and Fe²⁺→Fe³⁺ transitions (600–630 nm absorption band). Furthermore, the intensity variation in the 3651 cm⁻¹ Raman peak serves to further confirm the critical role of Fe³⁺ occupancy in the tremolite lattice for color modulation. In combination with the partition patterns of Rare Earth elements (REEs) (right-leaning LREE distribution with negative Eu anomaly) and trace element characteristics, this study supports the classification of Ruoqiang yellow-green nephrite as a high oxygen fugacity magnesian marble-type deposit. In this type of deposit, the ore-forming environment facilitates Fe³⁺ enrichment and yellow-green hue formation. The findings provide new theoretical insights into the chromatic genesis of yellow-green nephrite and hold significant implications for its identification, quality grading, and research on metallogenic mechanisms.

1. Introduction

Nephrite is an ornamental stone primarily composed of tremolite [Ca₂Mg₅Si₈O₂₂(OH)₂] and actinolite [Ca₂(Mg,Fe)₅Si₈O₂₂(OH)₂], with accessory minerals including zircon, apatite, diopside, epidote, garnet, pyrite, quartz, barite, magnetite and phlogopite [1,2]. Major global deposits occur in Xinjiang region (China) [3,4], Siberia (Russia) [5] and Chuncheon (Republic of Korea) [6,7]. Notably, the West Kunlun magnesian marble-hosted tremolitic nephrite belt in the Hetian region of Xinjiang constitutes the largest metallogenic nephrite province in the world.

The Xinjiang nephrite mineralization zones are distributed across three principal areas: Shache–Yecheng, Hetian–Yutian and Qiemo–Ruoqiang [8]. Most of the deposits are genetically associated with contact metasomatism between Precambrian dolomitic marble and intermediate-acidic magmatic intrusions. Secondary placer deposits of tremolitic nephrite have been identified in the Yurungkash and Karakash River systems, particularly within their terrace sediments [1,9]. The recent discoveries of yellow-green nephrite in Ruoqiang County, commercially termed “Huangkouliao” [8,10], have expanded the chromatic diversity of Xinjiang nephrite.

The coloration mechanisms of nephrite remain a central research topic in gemology. Liu et al. [11] investigated the sugar-colored nephrite from Qiemo in Xinjiang through integrated geochemical, isotopic, and geochronological analyses, elucidating its metallogenic processes and temporal constraints. Subsequent studies by Gao et al. [12] and Bai et al. [13,14] characterized newly discovered nephrite deposits in Heilongjiang (Tieli), Guangxi (Dahua), and Jilin (Panshi). Existing research demonstrates positive correlations between FeO content and cyan/black coloration in nephrite. Recent investigations by Du et al. [10] and Zhang et al. [8] employing LA-ICP-MS, FTIR, and XRF techniques proposed Fe³⁺ as the dominant chromophore in yellow-green nephrite. However, current studies on Ruoqiang yellow-green nephrite remain largely descriptive, lacking quantitative hue-chromophore correlations and systematic mechanistic interpretations, which hinders comprehensive understanding of its coloration genesis. Although previous studies have documented the role of Fe³⁺, the quantitative correlation between lattice occupancy and hue remains unexplored, particularly in systems hosting magnesian marble.

This study investigates the coloration mechanisms of yellow-green nephrite from Ruoqiang (Xinjiang), through an integrated analytical protocol combining hyperspectral colorimetry (quantitative chromatic measurement), X-ray fluorescence spectroscopy (XRF, bulk composition analysis), laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS, trace/Rare Earth element determination), laser Raman spectroscopy, ultraviolet-visible (UV-Vis) spectroscopy, and titrimetry. Multivariate regression modeling of chromatic parameters versus geochemical data elucidated the quantitative relationships between hue variation and iron speciation, revealing the oxygen fugacity-controlled chromogenic processes in this magnesian marble-hosted nephrite system.

2. Geological Setting

The Fugoling mining area is situated within in Ruoqiang County (Xinjiang), China (Figure 1). It lies approximately 11 km south of Milan Bridge and east of Ruoqiang County center, with its central geographic coordinates at 88°48′58.4″ E, 38°54′53.9″ N. The area is accessible via roads, with elevations ranging from 2780 m to 2926 m above sea level. The maximum relative relief within the mining area is approximately 140 m, while the total elevation gains from the valley entrance to the highest point reaches approximately 1600 m. The perennial Milan River, located about 5 km north of the mining area, provides a high-quality water source suitable for drinking. No permanent residents or herders are present in the vicinity. All logistical support for production and daily operations must be supplied from Ruoqiang County town [15].

3. Materials and Methods

3.1. Materials

All studied specimens (RQHN33, RQHN35, RQHN40, RQHN43, RQHN46, RQHN48, RQHN51) were collected in Ruoqiang County (Xinjiang), and the morphologies of the rough gems are reported in Figure 2. To enable quantitative colorimetric analysis, all samples were sectioned and polished to standardized dimensions of 20 mm × 10 mm × 8 mm (length × width × thickness) using a precision diamond saw. The prepared samples exhibit a uniform coloration ranging from yellowish to greenish-yellow hues, as shown in Figure 3. Polishing was performed using 0.25 μm diamond suspension on automated polishing equipment.

3.2. Methods

3.2.1. Chromatometric Analysis

The analyses were conducted at the National Jewelry Testing Center (Chongqing), Chongqing Academy of Metrology and Quality Inspection, using a SHIS-V220 hyperspectral imaging system. The spectral range of the system spans from 420 nm to 750 nm, with a LCTF scanning precision of 1 nm and a full width at FWHM of 10 nm at 550 nm. The spectral acquisition protocol adhered to CIE standard illuminant D₆₅ (correlated color temperature 6504 K) with a 10° standard observer geometry, covering the visible spectrum (450–750 nm) at 2 nm intervals. Color parameter conversion to Munsell notation followed ASTM D1535-14 (2023) specifications [16,17,18], implemented through the “Jewelry Color Intelligent Grading System V1.0” software package (NJTC, Chongqing, China).

3.2.2. UV-Vis Spectroscopy

The analyses were conducted at the Gemmological Institute, China University of Geosciences (Wuhan), using a UV-Visible spectrometer model Gem UV-100 (Tianrui, Pizhou, Jiangsu, China), equipped with an integrating sphere, with a wavelength scanning range of 380–780 nm, using a standard BaSO₄ whiteboard as a reference, and tested by reflection method. The UV-Vis spectrometer was calibrated daily using a reference.

3.2.3. Raman Spectroscopy

The analyses were conducted at the National Jewelry Testing Center (Chongqing), Chongqing Academy of Metrology and Quality Inspection. The analysis of nephrite was carried out using a Renishaw inVia confocal Raman spectrometer (Renishaw, Gloucestershire, UK). The testing conditions included a laser wavelength of 532 nm, L1800 grating. Each scan lasted 10 s with three accumulations, achieving a resolution of 9–15 cm⁻¹ over a spectral range of 100–4000 cm⁻¹. Raman shifts were calibrated using a single-crystal silicon standard (main peak at 520.5 cm⁻¹), and the results were processed using Wire 4.2 software.

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

The analysis of trace elements was performed at the National Jewelry Testing Center (Chongqing), Chongqing Academy of Metrology and Quality Inspection. The NWR 213 laser ablation system was coupled with a Thermo iCAP RQ mass spectrometer (Thermo, Carlsbad, CA, USA). The carrier gas was helium, and each test included approximately 20 s of background collection followed by 30 s of sample collection. The laser spot size was 45 μm, with a frequency of 10 Hz and energy of 90%. The execution of test steps is undertaken in accordance with the protocol delineated in T/CAQI 134-2020 [19]. NIST SRM 610 and NIST SRM 612 were used as a quality control standard to monitor and correct for instrument sensitivity drift, alongside multiple international standards (BHVO-2G, BCR-2Ga, BIR-1G) for multi-standard calibration, and ²⁹Si was chosen as the normalization element to calibrate all elemental concentrations. LA-ICP-MS analyses were conducted at 23 ± 3 °C and 45 ± 10% relative humidity [8,19].

3.2.5. X-Ray Fluorescence Spectroscopy (XRF) and Titrimetric Analysis

Bulk composition analysis and FeO determination were performed at the National Jewelry Testing Center (Chongqing), Chongqing Academy of Metrology and Quality Inspection and Wuhan SampleSolution Analytical Technology Co., Ltd. (Wuhan, China) For bulk composition analysis, powdered samples were mixed with lithium tetraborate-lithium nitrate flux (8:1 flux-to-sample ratio) in platinum crucibles and fused at 1150–1250 °C for 15 min in a high-temperature furnace. The resulting glass beads were analyzed using a Rigaku ZSX Primus II wavelength-dispersive XRF spectrometer (Rigaku, Tokyo, Japan).

The quantitative determination of FeO was carried out by the following method. Firstly, the powdered sample was eliminated using hydrofluoric acid and sulphuric acid. Secondly, the remaining fluorine in the solution was removed by adding boric acid complexation. Thirdly, the Fe²⁺ content was titrated with a standard solution of potassium dichromate using sodium dianiline sulfonate as an indicator. Finally, the FeO was calculated.

4. Results

4.1. Chromaticity of Yellow-Green Nephrite

The samples were analyzed using a hyperspectral imaging system (SHIS-V220, spectral range 450–750 nm, resolution 2 nm). The measured CIE 1931 XYZ tristimulus values were converted to Munsell color system (Hue-Value-Chroma, HVC). The sRGB color values were calculated using Equation (1) [20]. The sRGB is a color standard that achieves a wide range of colors through variations in the three color channels of red (R), green (G), and blue (B), as well as their overlaps. sRGB represents the colors of these three channels. This standard encompasses almost all colors perceivable by human vision and is one of the most widely used color systems.

$$\left[\begin{array}{c}\mathrm{sR}\\ \mathrm{sG}\\ \mathrm{sB}\end{array}\right]=\left[\begin{array}{ccc}3.2404542& -1.5371385& -0.4985314\\ -0.9692660& 1.8760108& 0.0415560\\ 0.0556434& -0.2040259& 1.0572252\end{array}\right]\left[\begin{array}{c}X\\ Y\\ Z\end{array}\right]$$

(1)

The resulting chromatic parameters are summarized in

Table 1. Chromatometric information of the polished yellow-green nephrite from Ruoqiang County (Xinjiang).

N.O	X	Y	Z	H	V	C
RQHN33	29.22	32.63	19.11	2.4 GY	6.36	3.71
RQHN35	24.72	27.48	13.26	1.2 GY	5.92	4.42
RQHN40	29.21	31.79	18.58	10.0 Y	6.29	3.53
RQHN43	25.59	27.79	14.22	9.4 Y	5.95	4.05
RQHN46	30.98	32.90	15.71	7.1 Y	6.41	4.62
RQHN48	29.92	32.09	15.10	8.1 Y	6.33	4.65
RQHN51	21.73	22.29	10.03	4.5 Y	5.42	4.43

. Normalized color coordinates were plotted on the CIE 1931 chromaticity diagram (Figure 3), revealing that all seven samples cluster within the yellow-green region. As can be seen from Figure 3, the hue decreases linearly from 32.4 to 24.5 as the hue changes from green to yellow.

In the Munsell color system, H denotes hue, V represents value (lightness), and C indicates chroma (saturation). To facilitate quantitative analysis of coloration mechanisms, the hue values were digitized according to ASTM D1535-14 (2023) specifications [18], enabling simulation of sample-specific chromatic profiles (

Table 2. Digital forms of hue and fitted colors.

N.O	Munsell/H		sRGB
			sR	sG	sB	Fitted Colors
RQHN33	2.4 GY	32.4	160	157	109
RQHN35	1.2 GY	31.2	152	145	88
RQHN40	10.0 Y	30.0	163	154	107
RQHN43	9.4 Y	29.4	156	144	92
RQHN48	8.1 Y	28.1	173	154	96
RQHN46	7.1 Y	27.1	170	153	94
RQHN51	4.5 Y	24.5	151	127	76

). The studied samples exhibit distinct yellow-green hues, with hue values uniformly distributed between 24.5 and 32.5 (Figure 4, Table 2). Statistical analysis reveals relatively constant value (V_ave = 6.10, σ = 0.36) and chroma (C_ave = 4.20, σ = 0.46) values across the sample set, while hue variation accounts for 78.3% of total chromatic variance (principal component analysis with Varimax rotation, λ₁ = 0.783). This predominant hue variation provides an optimal basis for establishing quantitative chromophore-correlation models through multivariate regression analysis.

4.2. Spectroscopic Characteristics of Yellow-Green Nephrite

The UV-Vis absorption spectra of the samples are shown in Figure 5. The samples RQHN33 and RQHN35 exhibit a broad absorption band between 600 and 650 nm, while all samples display prominent absorption in the 300–400 nm range, along with sharp peaks at 730 nm and 950 nm. These spectral features indicate that the yellow-green coloration arises from selective absorption in the blue-violet (300–400 nm) and red (600–650 nm) regions, consistent with subtractive color theory.

The broad 600–630 nm absorption band is attributed to Fe²⁺→Fe³⁺ intervalence charge transfer (IVCT) occurring between adjacent octahedral sites [21], and the absorption bands at wavelengths of 650 nm are associated with Cr³⁺ [22]. The 300–400 nm absorption corresponds to Fe³⁺ ligand-to-metal charge transfer (LMCT) between Fe³⁺ and O²⁻ ligands. Specifically, the absorption peaks at 367 nm and 380 nm are assigned to crystal field transitions of Fe³⁺: ⁶A₁ → ⁴E(D) and ⁶A₁ → ⁴T₂(D), respectively [23,24].

All samples exhibit Fe³⁺-O²⁻ LMCT features, confirming the ubiquitous presence of Fe³⁺ in the tremolite lattice. Notably, the greenish samples (RQHN33, RQHN35) demonstrate enhanced Fe²⁺→Fe³⁺ IVCT intensity (IVCT/LMCT ratio = 1.25 ± 0.15). This suggests that the greenish hue arises from increased Fe²⁺ content, facilitating stronger IVCT interactions, while the dominant yellow component reflects Fe³⁺-dominated LMCT processes.

Raman spectroscopic analysis was conducted on polished yellow-green nephrite samples. While nephrite exhibits characteristic Raman peaks in two distinct regions (150–1200 cm⁻¹ and 3500–3800 cm⁻¹), this study focused on the high-frequency region (3500–3800 cm⁻¹) to investigate the chromophore-related M–OH stretching vibrations, as the low-frequency region (150–1200 cm⁻¹) primarily corresponds to SiO₄ tetrahedral vibrations [8,25].

In the tremolite structure (ideal formula: Ca₂Mg₅Si₈O₂₂(OH)₂), the high-frequency Raman shifts are attributed to M–OH stretching modes, where M denotes octahedrally coordinated cations at the M(1) and M(3) sites. Under ideal Mg²⁺ occupancy, these sites are fully occupied by Mg²⁺. However, partial substitution by Fe²⁺ and Fe³⁺ at these sites induces distinct spectral changes (Figure 6,

Table 3. Raman shift and attribution caused by M-OH stretching vibration in nephrite.

Raman Shift	M-OH
3675 cm−1	Mg2+, Mg2+, Mg2+
3661 cm−1	Mg2+, Mg2+, Fe2+
3651 cm−1	Mg2+, Mg2+, Fe3+

):

• Fe²⁺ substitution at M(1) and M(3)sites enhances green hues through crystal field effects [26];
• Fe³⁺ substitution at M(1) and M(3) sites intensifies yellow coloration via charge transfer mechanisms [26].

The Raman spectra (Figure 6) reveal a prominent 3661 cm⁻¹ peak in sample RQHN33, attributed to ν(OH)-Mg²⁺ stretching. Quantitative analysis of the 3651 cm⁻¹ peak (assigned to ν(OH)-Fe³⁺) demonstrates a systematic increase in its relative intensity (

Table 4. The strength of M-OH stretching vibration of yellow-green nephrite samples from Ruoqiang County (Xinjiang).

N.O	3651 cm−1	3661 cm−1	3651 cm−1/(3661 cm−1 + 3651 cm−1)
RQHN33	450	2500	0.15
RQHN35	322	1415	0.19
RQHN40	213	550	0.28
RQHN43	285	620	0.31
RQHN48	500	833	0.38
RQHN46	764	1100	0.41
RQHN51	540	632	0.46

) with yellow hue enhancement. The 3651 cm⁻¹/(3661 cm⁻¹ + 3651 cm⁻¹) intensity ratio correlates positively with Fe³⁺ content.

These results confirm that the yellow-green coloration arises from competitive occupancy of Fe²⁺ and Fe³⁺ at the M(1) and M(3) sites, with Fe³⁺ proportion governing yellow intensity through electron-hole recombination processes.

4.3. Bulk Composition and Trace Element Analysis of Yellow-Green Nephrite

4.3.1. Bulk Composition Analysis

The major element composition of yellow-green nephrite is presented in

Table 5. Bulk composition analysis of yellow-green nephrite samples (%).

N.O	RQHN33	RQHN35	RQHN40	RQHN43	RQHN48	RQHN46	RQHN51
SiO2	58.29	52.51	58.35	58.39	57.81	58.34	57.31
TiO2	0.011	0.006	0.009	0.012	0.009	0.006	0.005
Al2O3	0.75	0.63	0.86	0.67	0.81	0.91	0.90
TFe	0.57	0.33	0.21	0.34	0.13	0.16	0.17
MnO	0.031	0.021	0.015	0.019	0.016	0.014	0.018
MgO	24.67	20.62	24.59	24.65	24.75	24.72	23.13
CaO	13.46	21.03	13.65	13.70	13.70	13.48	16.48
Na2O	0.16	0.20	0.16	0.14	0.16	0.16	0.19
K2O	0.18	0.05	0.07	0.06	0.06	0.22	0.05
P2O5	0.008	0.016	0.010	0.013	0.012	0.018	0.011
LOI	2.03	5.21	2.51	2.53	2.68	2.88	2.05
SUM	100.18	100.62	100.43	100.52	100.67	100.37	100.32
FeO	0.47	0.26	0.17	0.26	0.12	0.10	0.12
Fe2O3	0.05	0.04	0.03	0.05	0.03	0.03	0.04
Fe2O3/TFe	0.117	0.122	0.139	0.158	0.190	0.213	0.233

. The samples are predominantly composed of SiO₂ (52.51–58.39 wt.%), MgO (20.62–24.75 wt.%), and CaO (13.46–21.03 wt.%), consistent with the theoretical composition of tremolite (SiO₂ 59.17 wt.%, MgO 24.81 wt.%, CaO 13.81 wt.%). The low total iron content (TFe = 0.13–0.57 wt.%) is partitioned between Fe₂O₃ (0.03–0.05 wt.%) and FeO (0.10–0.47 wt.%), yielding an Fe₂O₃ ratio (Fe₂O₃/TFe) of 0.117–0.233.

We established a linear regression model between hue and Fe₂O₃ ratio, with the former as the independent variable (x-axis) and the latter as the dependent variable (y-axis). The fitting results (Figure 7, Table 6) demonstrate a high correlation coefficient (R² > 0.90), indicating a good linear relationship. The proposed chromatic model is expressed as Equations (2) and (3).

Figure 7. Fe₂O₃ ratio diagram of yellow-green nephrite samples from Ruoqiang County (Xinjiang).

Figure 7. Fe₂O₃ ratio diagram of yellow-green nephrite samples from Ruoqiang County (Xinjiang).

Table 6. Linear fitting results of hue and Fe₂O₃ ratio of yellow-green nephrite samples from Ruoqiang County (Xinjiang).

	Value	Standard Error	Adj. R-Square
Intercept	0.651	0.0489	0.942
Slope	−0.017	0.0017

Table 6. Linear fitting results of hue and Fe₂O₃ ratio of yellow-green nephrite samples from Ruoqiang County (Xinjiang).

	Value	Standard Error	Adj. R-Square
Intercept	0.651	0.0489	0.942
Slope	−0.017	0.0017

$$\mathrm{f}\left({\mathrm{F}\mathrm{e}}_2{\mathrm{O}}_3\right)=-0.017\mathrm{H}+0.651$$

(2)

$$\mathrm{f}\left({\mathrm{F}\mathrm{e}}_2{\mathrm{O}}_3\right)={\displaystyle \frac{{\omega }_{{Fe}_2{O}_3}}{{\omega }_{TFe}}}$$

(3)

In the equation:

ω_Fe2O3→Content of Fe₂O₃, %;

ω_TFe→Content of TFe, %;

H→Hue.

This model provides a petrogenetic framework for predicting hue variations in yellow-green nephrite based on Fe³⁺ content, with implications for gemological quality assessment and metallogenic studies.

4.3.2. Trace Element Analysis

Trace element compositions of the yellow-green nephrite samples are presented in

Table 7. Trace element compositions of yellow-green nephrite samples from Ruoqiang County (Xinjiang).

Rare Element	Content (μg/g)
	RQHN33	RQHN35	RQHN40	RQHN43	RQHN48	RQHN46	RQHN51
Li	2.784	5.504	2.061	3.352	0.871	2.646	1.988
Be	1.643	2.613	10.154	5.995	7.454	11.167	4.467
Sc	5.025	6.347	7.968	4.054	6.509	8.218	9.571
Ti	32.559	12.316	47.775	63.387	48.387	35.234	39.577
V	5.898	1.242	<0.335	12.704	5.837	8.085	8.558
Cr	5.461	<0.100	7.159	<0.100	0.293	56.676	9.588
Mn	242.816	118.559	106.807	153.171	120.949	122.369	121.918
Co	1.975	1.128	0.636	0.667	0.503	0.513	1.339
Ni	1.520	4.548	0.905	3.866	2.417	1.736	1.557
Cu	0.771	<0.230	<0.230	2.904	<0.230	2.623	4.127
Zn	93.609	76.790	38.488	47.299	49.106	54.954	25.779
Ga	0.624	0.625	0.769	0.781	0.995	0.989	1.515
Rb	4.077	0.233	1.282	0.921	1.129	0.184	0.773
Sr	9.401	3.925	5.488	4.195	5.203	5.252	4.761
Y	2.202	2.262	2.205	3.507	2.847	2.057	3.071
Zr	1.162	1.727	<0.001	1.156	0.649	3.918	6.791
Nb	0.377	0.047	0.282	0.202	0.321	0.164	0.470
Mo	<0.001	<0.001	<0.001	<0.001	<0.001	<0.001	0.227
Ag	0.128	<0.027	0.104	0.156	0.079	0.079	0.259
Cd	<0.001	<0.001	0.196	<0.001	<0.001	<0.001	0.250
Sn	0.560	0.948	3.451	1.980	1.072	3.105	3.874
Sb	0.022	0.256	0.236	0.294	0.229	0.278	0.384
Cs	0.176	0.066	0.233	0.082	0.150	0.079	0.140
Ba	4.061	0.646	1.264	5.424	0.988	1.505	1.333
La	0.313	1.022	1.232	0.844	1.045	0.979	0.740
Ce	1.009	1.260	1.837	1.201	1.522	1.465	1.116
Pr	0.289	0.122	0.215	0.136	0.185	0.206	0.109
Nd	0.982	0.446	0.731	1.125	1.086	0.686	0.850
Sm	0.456	0.493	0.055	0.226	0.210	0.203	0.365
Eu	0.018	0.015	<0.001	0.016	<0.001	<0.001	<0.001
Gd	0.319	0.306	0.167	0.451	0.181	0.200	0.221
Tb	0.051	0.050	0.038	0.075	0.064	0.059	<0.001
Dy	0.330	0.241	0.170	0.493	0.325	0.198	0.114
Ho	0.076	0.052	0.078	0.111	0.100	0.055	0.055
Er	0.105	0.368	0.207	0.262	0.261	0.267	0.189
Tm	0.035	0.026	0.019	<0.001	0.007	0.028	0.008
Yb	0.097	0.072	0.068	0.406	0.092	0.294	0.035
Lu	0.024	0.025	0.007	0.027	<0.001	0.015	<0.001
Hf	0.042	0.094	<0.001	<0.001	0.040	<0.001	0.030
Ta	0.059	<0.001	<0.001	0.052	0.055	0.009	0.046
W	0.043	<0.001	0.662	0.361	0.645	0.881	0.435
Tl	0.021	<0.001	0.017	0.011	0.003	0.009	<0.001
Bi	0.018	<0.001	0.073	0.035	0.009	0.040	0.052
Pb	0.345	0.509	19.514	1.830	1.216	1.298	0.940
Th	0.036	0.124	0.179	0.664	0.145	0.258	0.198
U	0.314	0.165	0.772	0.667	0.641	0.834	0.832
LREE	3.049	3.342	4.071	3.532	4.047	3.538	3.180
HREE	1.038	1.140	0.754	1.827	1.030	1.115	0.622
ΣREE	4.105	4.497	4.825	5.375	5.077	4.653	3.802

. The concentrations of Cr (<0.100–56.676 μg/g), Co (0.513–1.975 μg/g), and Ni (0.905–4.548 μg/g) align with typical values for nephrite deposits [25,26,27]. The concentrations of Ti (12.316–63.387 μg/g), Mn (106.807–242.816 μg/g), and Zn (25.779–93.609 μg/g) is relatively high. The main reason for this is that it was formed in the contact metasomatic zone between magnesium marble and intermediate-acidic magmatic rocks, where mineral-rich hydrothermal activity brought in these elements. These elements are mainly hosted by replacing Mg in the tremolite lattice through isomorphism. The total Rare Earth element (ΣREE) content ranges from 3.802 to 5.375 μg/g, consistent with magnesium marble-hosted nephrite systems (2.84–84.81 μg/g) [28]. The chondrite-normalized REE patterns (Figure 8a) exhibit a pronounced right-leaning trend for light REE (LREE: La–Sm), while heavy REE (HREE: Gd–Lu) remain relatively flat. In addition, a distinct negative anomaly of Eu is also observed.

The primitive mantle-normalized spider diagram (Figure 8b) reveals enrichment in large-ion lithophile elements (Rb, U) and depletion in high-field-strength elements (Ba, Th, Sr, Zr, Hf).

• Low ΣREE content (<10 μg/g);
• LREE-enriched patterns with negative Eu anomalies;

These geochemical signatures are diagnostic of magnesian marble-type nephrite [29,30,31].

Figure 8. (a) Chondrite-normalized trace element diagram of yellow-green nephrite samples from Ruoqiang County (Xinjiang); (b) Normalized trace element diagram of primitive mantle of yellow-green nephrite samples from Ruoqiang County (Xinjiang). Normalizing values of Rare Earth and trace elements are from Sun and McDonough [32].

Figure 8. (a) Chondrite-normalized trace element diagram of yellow-green nephrite samples from Ruoqiang County (Xinjiang); (b) Normalized trace element diagram of primitive mantle of yellow-green nephrite samples from Ruoqiang County (Xinjiang). Normalizing values of Rare Earth and trace elements are from Sun and McDonough [32].

5. Discussion

5.1. Coloration Mechanism Analysis

The coloration mechanism, primarily determined by Fe³⁺, as revealed in this study, is consistent with the hypotheses proposed by Du et al. [10] and Zhang et al. [8]. On this basis, a color model incorporating hue and the Fe₂O₃/TFe ratio was developed, allowing for the quantitative determination of the hue of yellow-green nephrite through fitting methods. This evidence contrasts with the coloration mechanism determined by Bai et al. [13] in Guangxi nephrites, which instead appears to be Fe²⁺; this difference may result from regional environmental variations that form the minerals: the highly oxidized conditions of the magnesian marble rock in the Ruoqiang deposit favor the enrichment of Fe³⁺, while the fluid metamorphic environment in southern China may retain more Fe²⁺ [29]. Notably, although the TFe content in our nephrites (0.13%–0.57%) is significantly lower than that in traditional green nephrite (>1.5%) [12], even slight changes in the Fe₂O₃ ratio (0.117–0.233) can trigger marked hue shifts (H = 24.5–32.5), implying that the content of Fe₂O₃ in tremolite has a certain influence on the yellow hue. The intensity variations in the M-OH stretching vibration peak (3651 cm⁻¹) in Raman spectra provide structural evidence for this, as the Fe³⁺ substitution at the M1/M3 sites may alter the vibration modes of the hydroxyl [25].

Ultraviolet-visible spectroscopy analysis shows that the yellow-green color of nephrite is determined mainly by electronic transition processes. The 300–400 nm absorption band is attributed to the electronic transitions of ⁶A₁→⁴E(D) and ⁶A₁→⁴T₂(D) of Fe³⁺, reflecting charge transfer between Fe³⁺ and O²⁻. This absorption band causes the absorption of blue-violet light, enhancing the yellow-green hue. The 600–630 nm absorption band corresponds to Fe²⁺→Fe³⁺ charge transfer, mainly in greener samples (RQHN33, RQHN35). This transition may be related to the coexistence of Fe²⁺ and Fe³⁺ in adjacent octahedral sites [21]. Tremolite, a monoclinic crystal, has M1 and M3 sites in its structure that can be occupied by Fe²⁺ or Fe³⁺. According to crystal field theory, Fe³⁺ has low d-d transition energy in an octahedral field, producing absorption bands in the visible light range that affect color. In this study, Raman spectroscopy revealed that as Fe³⁺ proportion increases, the intensity of the 3651 cm⁻¹ peak significantly strengthens, indicating that the Fe³⁺ occupancy at the M1/M3 sites greatly influences the M-OH vibration modes, providing crystallochemical evidence for the Fe³⁺ coloration mechanism.

5.2. Relationship Between Ore-Forming Environment and Color Genesis

In this study, the Rare Earth element distribution pattern and trace element abundances (enriched in Rb, U, and depleted in Ba, Th, Sr, Zr, Hf), further confirmed that the Ruoqiang yellow-green nephrite belongs to magnesian marble-type deposits [29,30,31], and formed through contact metasomatism between basic rocks and carbonate rocks [33]. Furthermore, this ore-forming environment may influence the genesis of color through the following pathways:

• Oxidation-reduction conditions: The ore-forming fluid in Ruoqiang, Xinjiang, has a high oxygen fugacity, which favors the oxidation of Fe²⁺ to Fe³⁺, thereby increasing the proportion of this ion. The oxidation may result from regional metamorphism or magma-fluid interaction; under high-temperature and high-pressure conditions, Fe²⁺ and Fe³⁺ distribution is controlled by oxygen fugacity and fluid composition, and Fe³⁺ is more stable and color-determining under high oxygen fugacity conditions.
• Fluid composition and temperature-pressure conditions: High SiO₂ activity in ore-forming fluid may affect Fe site selection during tremolite crystallization. In Si-rich conditions, Fe³⁺ prefers the M1 site, altering M-OH vibration modes. Regional metamorphism may change crystal field splitting energy, affecting Fe³⁺ d-d transition energy. Higher temperatures may enhance Fe³⁺ charge transfer intensity, deepening the yellow hue.
• Influence of country rock properties: The country rock of Ruoqiang in Xinjiang is mainly composed of magnesian marble with high MgO content, which can inhibit the entry of Fe²⁺ and promote Fe³⁺ enrichment. This interaction between the properties of the country rocks and the ore-forming fluid is a key factor in yellow-green hue formation. Additionally, intermediate-acidic igneous rocks in the contact zone can provide additional Fe inputs into the ore-forming system through hydrothermal activity.

6. Conclusions

This study, combining colorimetry, spectrometry, and major/trace element analysis, has established for the first time a quantitative model linking hue to the amount of Fe3⁺ in yellow-green nephrite from Ruoqiang County (Xinjiang). Results show that the color is mainly due to Fe³⁺ charge transfer (300–400 nm absorption band) and Fe²⁺→Fe³⁺ transitions (600–630 nm absorption band). Raman spectroscopy analysis, which determined the intensity of the M-OH vibration peak (3651 cm⁻¹) and quantitative analysis of FeO and Fe₂O_3, confirmed the key role of Fe³⁺ occupancy in the tremolitic lattice for color regulation. Furthermore, the formation environment of magnesian marble ore in Ruoqiang, characterized by high oxygen fugacity, was found to be the main factor controlling Fe³⁺ enrichment and the formation of the yellow-green hue, offering new theoretical insights into the color genesis of yellow-green nephrite.