A New Type of White Nephrite from Limestone Replacement along the Kunlun–Altyn Tagh Mountains: A Case from the Mida Deposit, Qiemo County, Xinjiang, China

Abstract

The recently discovered Mida nephrite deposit, located in the East Kunlun Mountains, Qiemo County, Xinjiang, Northwest China, contains new types of white and greenish white nephrite formed by limestone replacement, which shows microstructures, macroscopic features and country rocks typologies that are quite different from those of the other deposits along the Kunlun–Altyn Tagh Mountains. The gemological and mineralogical characteristics of Mida nephrite are presented here. These nephrites show an ivory white color and a porcelain-like appearance, with semitranslucent-to-opaque transparency and a porcelain-to-greasy luster. Petrographic study, electron probe microanalysis (EPMA) data and scanning electron microscopy (SEM) images have indicated that the nephrite is composed of tremolite, accompanied by minor quartz, calcite and diopside. Tremolite aggregates have shown different textures, like flaky, granular, fibrous–felted, bundle, radial and metasomatic relict textures. Quartz has appeared in granular or disseminated form, dispersed in the tremolite matrix. Calcite has shown a metasomatic relict texture in the white nephrite samples. Diopside has shown euhedral grains, with some distributed with a certain geometric appearance. Based on our observations, it is suggested that the quartz in the nephrite originated from Si-rich hydrothermal fluids. We propose that the substantial size difference of mineral grains, together with uncompacted grains with inter-particle pores, are the main reasons for the internal reflection and refraction under transmitted light, which allow less transmitted light to pass through the nephrite body and generate the appearance of a semitranslucent-to-opaque transparency, ivory white color and porcelain luster. Our study has unveiled that the Mida nephrite is not typical of the two known types (D-type: dolomite-related; S-type: serpentinite-related) and is overlapped by quartz grains dispersed throughout the less compact tremolite matrix. These observations would help set it apart from the majority of nephrite jades found in the Kunlun Mountains region and provide valuable insights for enhancing comprehension of the diversity of the nephrite deposits.

1. Introduction

Nephrite is a beautiful jade composed mainly of microcrystalline amphibole of the tremolite–actinolite series [1], along with several secondary minerals, such as carbonate minerals, the epidote group, the chlorite group, quartz, rutile, other amphibole minerals, pyrite, and others [2,3,4,5,6,7]. Nephrite deposits are widespread all over the world and are mainly located in China in the regions of Xinjiang, Guangxi, Qinghai, Liaoning, Guizhou and Taiwan [8,9,10,11,12,13,14,15]. In the other parts of the world, nephrite is found in Russia [16,17,18,19,20], South Korea [1,21], Australia [1], Canada [22], the United States [1], New Zealand [23,24], Somaliland [25], Afghanistan [26], Vietnam [1], Italy [27], Pakistan [28] and Poland [29].

Based on variations in mineral content and composition, as well as differences in granular morphology and structure, nephrite can exhibit a wide range of colors, transparencies and lusters. White, green, yellow, gray, brown and black nephrites can be found with greasy, waxy or vitreous lusters and fine-to-coarse consistencies [2,6,30]. Tremolites in nephrite usually show distinctive features in terms of grain size, morphology and structure, resulting in notable macroscopic variations. Based on the grain size of the tremolite, nephrite can be divided into three main types: coarse-grained crystalline, microcrystalline and cryptocrystalline [31]. Furthermore, the internal morphological structure of nephrites can be further classified into various types based on the different morphologies and compact textures of the tremolite grains. These include but are not limited to the felt-like, leaf-like, bladed, fibrous, fibrous–cryptocrystalline and radial microscopic blastic textures. Tremolite in nephrite may exhibit metasomatic relict textures like corona, helicitic, diablastic, ring, harbor and pseudomorphic textures [32,33,34]. In general, nephrite with fine and evenly sized tremolite grains shows a compact microstructure and a delicate and greasy macroscopic appearance, and these characteristics can be observed in nephrites from the regions of Xinjiang, Qinghai and Liaoning.

A major deposit of white and greenish white nephrites (Mida deposit) was recently discovered in the eastern Kunlun Mountains in the region of Qiemo County in Northwest China. The nephrites here have attracted much attention due to their atypical characteristics. Most Mida nephrites show a typical ivory white color, fine texture, semitranslucent-to-opaque transparency and greasy luster, while a smaller portion shows less transparency than the former and a porcelain-like luster. Mida nephrites show distinctive differences from Xinjiang white nephrite, which is known for its fine-grained texture, semitransparent-to-translucent appearance and greasy luster, as well as Qinghai white nephrite, which shows higher transparency and a vitreous luster. Being aware of the potential significance of understanding the formation of nephrite deposits, field investigation and analyses were conducted on this deposit, with a focus on determining the composition and structural characteristics of its component minerals, its microstructure, its macroscopic appearance, the country rock and the connection between the microstructure and the macroscopic appearance.

2. Geological Setting

The Mida nephrite deposit (GPS position: 37°08′51.94″ N, 85°24′23.55″ E, 4069 m) is located in the East Kunlun Mountains (Figure 1).

The Mida nephrite deposit occurs in the Tokuzidaban Formation in Karamiran in the Late Paleozoic trench–arc–basin system between the Altyn fault and Muztag–Jingyu Lake fault (Figure 2). Zircon U–Pb dating of the volcanic rocks in the Tokuzidaban Formation in the Aksu River region indicates the formation age ranges from 364 to 343 Ma [39]. The Tokuzidaban Formation is located in the South Kunlun terrane, which is part of the East Kunlun Orogenic Belt. The East Kunlun Orogenic Belt is divided into North Kunlun, South Kunlun and Bayan Har terranes based on the boundary between the Central Kunlun fault and the South Kunlun fault [40,41].

Field investigations have revealed that the nephrite orebodies occur as blocks, veins and irregular banded bodies within gray-white or gray-black carbonate rocks composed mainly of marble and marbleized limestone. The main minerals in the carbonate rocks are calcite, with small quantities of tremolite and graphite, along with carbonaceous bands in the center. The carbonate rocks exhibit a large scale, measuring approximately 30 m in horizontal width and over 300 m in length and extending vertically for about 500 m in the Mida nephrite deposit (Figure 3).

The Lower Carboniferous Tokuzidaban Group, mainly composed of limestone, siltstone, silty mudstone and tuff, is found outcropping from the marble. The Tokuzidaban Group consists of sedimentary sea fragments and volcanic fragments with intermediate-basic volcanic rock and partly carbonate rocks. It overlies the Middle Proterozoic strata with a supra-unconformity and has a thickness ranging from 881 to 2788 m. In addition, a high number of fossils occur in the strata of the Tokuzidaban Group, such as coral, brachiopods, bivalves, gastropods, stromatoporoidea, bryozoans, crinoids and microfossils. The monzonitic granite, which is exposed in the northern part of the deposit, intrudes into the Lower Carboniferous Tokuzidaban Group. Additionally, a grey-green vein-like wall can occasionally be observed in close proximity to the nephrite bodies throughout the mining area. During the field investigation, samples of nephrites, wall rocks, marbles and monzonitic granites were collected.

3. Samples and Methods

Among all the samples available to us, we selected four representative nephrites for this study (Figure 4). A small part of the Mida white nephrite exhibits a porcelain-like luster (Figure 4a), while the majority show a typical ivory white color, fine texture, semitranslucent-to-opaque transparency and greasy luster (Figure 4b–d). The color, structure, texture and morphological characteristics of the samples were investigated. Refraction index (RI), specific gravity (SG), Mohs hardness, Fourier transform infrared spectroscopy (FTIR) and powder X-ray diffraction (XRD) were performed at the laboratory of the School of Gemology, China University of Geosciences, Beijing (CUGB). The hardness was examined using a TH763 digital microhardness tester and then converted to Mohs hardness through calculation. We also chose two carbonate rocks for this study (Figure 4e,f). The slices of nephrites and carbonate rocks were observed using an OLYMPUS BX5 polarizing microscope to determine the mineral composition, structural characteristics and contact relationships.

FTIR was performed in reflection mode with a resolution of 4 cm⁻¹ over the range of 400–2000 cm⁻¹. A test grating of 6 mm was used, and 32 scans were performed with a Bruker Tensor 27 Fourier transform infrared spectrometer to investigate the main minerals and compare them with Raman spectroscopy results. All samples were crushed and ground into powders with a size of less than 200 mesh using an agate mortar. The KBr pellets method was employed for analysis. XRD studies were conducted using a Smartlab Rigaku X-ray diffractometer with Cu target, 45 kV–100 mA and scanning speed of 8°/min.

The chemical compositions and backscattered electron (BSE) images of nephrites were acquired using a Shimadzu EPMA–1720 Electron Microprobe Analyzer (EPMA) at the Institute of Earth Sciences, CUGB. The instrument was operated with a 15 kV accelerating voltage, 10 nA beam current and 3 μm spot size. All counting times of element peaks and backgrounds were set to 10 s, with relative sensitivity levels ranging from 0.01 to 0.05 (wt%), an absolute sensitivity of 10⁻¹⁴ g, and relative error ranges between 1% and 3%. The instrument was equipped with an X-ray energy spectrometer for qualitative analysis of element contents in the samples to enhance observation efficiency and selection accuracy. Major elements in carbonate rocks were determined through energy-dispersive X-ray fluorescence (EDXRF) using Shimadzu EDX-7000 at the laboratory of the School of Gemology, CUGB.

The surface characteristics and mineral structure of the nephrite were examined using a ZEISS SUPRA 55 scanning electron microscopy (SEM) at the Institute of Earth Sciences, CUGB. The instrument was operated under the following conditions: room temperature of 18 °C, humidity of 30% and acceleration voltage of 15 kV. Each sample was sliced to a size of 1 × 1 cm and coated with carbon powder as a conducting medium.

4. Results

4.1. Gemological Properties

The appearance of the four nephrite jade samples investigated in this paper is similar, and the conventional gemological properties are listed in

Table 1. Gemological properties of Mida nephrite.

Scheme	Refractive Index	Specific Gravity	Mohs Hardness
MD–1	1.61	2.86	6.17
MD–2	1.61	2.91	6.42
MD–3	1.62	2.95	6.42
MD–4	1.62	2.90	6.20

.

All samples are white to grayish white in color. The refraction index ranges from 1.61 to 1.62, the Mohs hardness ranges from 5.97 to 6.21, and the specific gravity ranges from 2.86 to 2.95. The relatively low values of specific gravity, compared to the theoretical range (2.9 to 3.1 [6]), suggest that there may be some minor minerals with lower specific gravity values within the samples.

4.2. Petrography

4.2.1. Nephrite

The Mida nephrite consists of tremolite, with minor accessory minerals of quartz, calcite and diopside.

Tremolite

Under the polarizing microscope, at least two types of tremolite grains can be observed (Figure 5). The first type of grains, which are rare, appear as flakes and bundles approximately 200 μm long and 30 μm wide. Most of the edges and the interiors of these grains exhibit a metasomatic corrosion texture. The second type of tremolite grains display randomly oriented micro to cryptocrystalline fibrous–felted textures with intertwined boundaries. This structure is probably the main factor determining the fine and smooth macroscopic appearance of nephrite.

SEM images reveal that the jade domains contain tremolite–actinolite crystals in the form of long, slender fibers measuring approximately 100 µm in length and ranging from 0.1 to 2.0 µm in diameter. Most of these fibers are straight, compacted and oriented parallel or occasionally cross-cutting, while a few exhibit curvature. Additionally, granular and flaky tremolite with interstitial pores can be observed (Figure 6).

Quartz

Based on the shape and stage of formation, quartz grains can be divided into two types (Figure 5). The first type exhibits a metasomatic relict texture, and wave-like extinction is visible under a polarized microscope. The size of quartz grains varies from 1.0 to 400 μm, and they have xenomorphic crystalline shapes. Furthermore, we observe that the granules are dispersed in the tremolite matrix, with their edges being replaced by fibrous tremolite. The second type of quartz occurs as veins that cross-cut through the tremolite matrix.

Calcite

The calcite grains exhibit a granular, crystalloblastic texture with a grain size ranging from 0.2 to 2 mm, and their edges are often replaced by tremolite (Figure 5g,h). Sometimes, irregular calcite veins disseminating in the nephrite can be observed.

Diopside

Granular and flaky diopside grains with a diameter of approximately 50 to 100 μm are observed, occurring alongside fine granular tremolite (Figure 5i).

4.2.2. Carbonate Rocks

The carbonate rocks are mainly composed of marbleized limestone and marble. Among them, the polarizing microscope reveals the presence of biogenic detritus and a micrite matrix (Figure 7). The biogenic detritus primarily consists of Echinodermata, such as fossil crinoid stems and forams.

4.3. Mineral Chemistry

4.3.1. Amphibole

According to the classification diagram based on the chemical composition (

Table 2. Chemical composition of tremolite in Mida nephrite (wt%).

Sample	MD–1	MD–2	MD–3	MD–4–1	MD–4–2
SiO2	59.25	59.04	59.40	59.55	59.13
TiO2	0.02	0.01	0.01	0.00	0.03
Al2O3	0.13	0.29	0.15	0.09	0.11
FeO	0.09	0.17	0.24	0.16	0.09
MnO	0.02	0.38	0.32	0.09	0.08
NiO	0.00	0.03	0.09	0.00	0.06
MgO	24.83	24.10	25.19	24.73	24.77
CaO	13.84	13.18	12.30	13.05	12.71
Na2O	0.04	0.62	0.09	0.05	0.07
K2O	0.01	0.21	0.04	0.04	0.08
Total	98.23	98.00	97.84	97.75	97.04
Si	7.98	8.00	8.00	8.00	8.00
Al	0.02	0.00	0.00	0.00	0.00
SumT	8.00	8.00	8.00	8.00	8.00
Al VI	0.00	0.04	0.02	0.01	0.02
Fe3+	0.00	0.00	0.03	0.02	0.00
Mg	4.99	4.87	4.95	4.95	4.98
Fe2+	0.01	0.02	0.00	0.00	0.00
Mn	0.00	0.04	0.00	0.01	0.00
SumC	5.00	4.97	5.00	4.99	5.00
Mg	0.00	0.00	0.11	0.00	0.02
Ni	0.00	0.00	0.01	0.00	0.01
Mn	0.00	0.00	0.04	0.00	0.01
Ca	2.00	1.91	1.78	1.88	1.84
Na	0.00	0.09	0.02	0.01	0.02
SumB	2.00	2.00	1.95	1.89	1.89
Na	0.01	0.07	0.00	0.00	0.00
K	0.00	0.04	0.01	0.01	0.01
SumA	0.01	0.11	0.01	0.01	0.01
Mg/(Mg+Fe2+)	1.00	1.00	1.00	1.00	1.00
Minerals	Tr	Tr	Tr	Tr	Tr

Notes: The amphibole formulae were calculated on the basis of 23 oxygens. Tr—tremolite. Fe³⁺ and Fe₂O₃ contents were estimated based on the charge balance [42].

and Figure 8), the amphiboles belong to the calcic subgroup, which is close to the tremolite end number [42].

4.3.2. Diopside

Diopside in Mida nephrite exhibits a nearly constant chemical composition (

Table 3. Chemical composition of diopside in Mida nephrite (wt%).

Sample	MD–2	MD–3–1	MD–3–2
SiO2	54.48	54.2	54.7
TiO2	0.00	0.00	0.05
Al2O3	0.01	0.51	0.07
Cr2O3	0.03	0.02	0.02
FeO	0.55	0.79	0.96
MnO	0.04	0.02	0.12
MgO	18.48	17.69	18.19
CaO	26.34	26.12	26.46
Na2O	0.07	0.02	0.01
K2O	0.00	0.00	0.02
Total	100.01	99.49	100.6
Si	1.96	1.97	1.97
Al	0.00	0.02	0.00
Fe3+	0.02	0.02	0.03
Mg	0.99	0.96	0.98
Ca	1.02	1.02	1.02
Na	0.01	0.00	0.00
Sum	4	3.99	4
Minerals	Di	Di	Di

Notes: The diopside formulae were calculated on the basis of 6 oxygens. Di—diopside. Fe³⁺ and Fe₂O₃ contents were estimated according to the charge balance [42].

). SiO₂ content ranges from 54.20 to 54.70 wt%, CaO content ranges from 26.12 to 26.46 wt%, and MgO content ranges from 17.69 to 18.48 wt%. FeO content is low, ranging from 0.55 to 0.96 wt%. Based on the general mineral formula (XY[T₂O₆]) and sample calculations, the chemical characteristics of diopside are as follows: Si occupies 1.96 to 1.97 atoms per formula unit (a.p.f.u.) on the T site, Mg (M₁) occupies 0.96 to 0.99 a.p.f.u on the Y site, and Ca (M₂) occupies approximately 1.02 a.p.f.u on the X site.

4.3.3. Quartz and Calcite

The chemical compositions of quartz and calcite (

Table 4. Chemical composition of quartz and calcite in Mida nephrite (wt%).

Sample	MD–1	MD–2–1	MD–2–2	MD–4	MD–2	MD–3	MD–4
SiO2	99.73	99.36	99.80	99.48	0.02	0.06	0.01
TiO2	0.00	0.04	0.04	0.00	0.00	0.00	0.00
Al2O3	0.05	0.02	0.01	0.02	0.01	0.00	0.01
Cr2O3	0.01	0.01	0.00	0.00	0.04	0.00	0.00
FeO	0.02	0.00	0.00	0.03	0.04	0.00	0.00
MnO	0.04	0.04	0.01	0.02	0.00	0.00	0.00
NiO	0.00	0.03	0.00	0.00	0.00	0.04	0.00
ZnO	0.00	0.03	0.06	0.03	0.10	0.05	0.00
MgO	0.02	0.29	0.00	0.28	0.00	0.02	0.01
CaO	0.04	0.15	0.03	0.13	56.66	58.13	57.10
Na2O	0.05	0.02	0.04	0.01	0.03	0.05	0.00
K2O	0.03	0.01	0.01	0.01	0.00	0.00	0.00
Total	100.00	100.00	100.00	100.00	57.03	58.35	57.19
Minerals	Qtz	Qtz	Qtz	Qtz	Cal	Cal	Cal

Notes: Qtz—quartz. Cal—calcite.

) in Mida nephrite indicate that quartz is primarily composed of SiO₂ (98.04–99.44%), while calcite is mainly composed of CaO (56.66–58.13%).

4.4. Fourier Transform Infrared Spectroscopy

The infrared ranges of 1200 to 900 cm⁻¹ and 600 to 400 cm⁻¹ are indicative of the mineral components in Mida nephrite (Figure 9).

The vibration absorption peaks observed in all samples at 540 cm⁻¹, 511 cm⁻¹, 468 cm⁻¹ and 420 cm⁻¹ are attributed to the characteristic Si-O bending vibration, M-O bending vibration and external lattice vibration. Additionally, the peaks at 761 cm⁻¹, 688 cm⁻¹, 663 cm⁻¹ and 644 cm⁻¹ can be detected in the samples and are assigned to the Si-O-Si symmetric stretching vibration. Furthermore, the peaks at 1136 cm⁻¹, 1041 cm⁻¹, 995 cm⁻¹ and 925 cm⁻¹ are due to the O-Si-O and Si-O-Si asymmetric stretching vibrations as well as O-Si-O symmetric stretching vibration. Moreover, the presence of other minerals is indicated by certain characteristic vibrational absorption peaks found in some samples, such as diopside (850 cm⁻¹ peak), calcite’s CO₃²⁻ bending vibration (875 cm⁻¹ peak), quartz’s Si-O asymmetric stretching vibration (1091 cm⁻¹ peak) and quartz’s Si-O-Si symmetric vibrations (800 cm⁻¹ and 783 cm⁻¹ peaks). The band positions and assignments can be found in

Table 5. FTIR band assignment of the nephrite samples from Mida nephrite deposit.

Wavenumbers (cm−1)	Band Assignment	Suggested Minerals
1136, 1041, 995, 92	O-Si-O and Si-O-Si asymmetric stretching vibration; O-Si-O symmetric stretching vibration	Tr
761, 688, 663, 644	Si-O-Si symmetric stretching vibration
540, 511, 468, 420	Si-O bending vibration
1091	Si-O asymmetric stretching vibration	Qtz
800, 783	Si-O-Si symmetric vibration
875	CO32− bending vibration	Cal

Notes: Tr—tremolite. Qtz—quartz. Cal—calcite.

.

4.5. Powder X-ray Diffraction

The results indicate that Mida white nephrites are relatively pure and mainly composed of tremolite, with minor minerals such as calcite and quartz (Figure 10). The diffraction peaks and their assignment to the identified mineral species are listed in

Table 6. XRD diffraction peaks and their assignment to the identified mineral species.

2θ (°)	d (Å)	Minerals	2θ (°)	d (Å)	Minerals
9.76	9.0516	Tr	30.36	2.9416	Tr
10.50	8.4183	Tr	32.38	2.7623	Tr
17.36	5.1042	Tr	33.08	2.7058	Tr
18.16	4.8729	Tr	34.50	2.5972	Tr
19.59	4.5275	Tr	35.30	2.5403	Tr
20.85	4.2563	Qtz	36.54	2.4573	Qtz
22.04	4.0218	Tr	38.53	2.3345	Tr
22.81	3.8944	Tr	39.38	2.2865	Cal
23.04	3.8568	Cal	41.71	2.1635	Tr
26.23	3.3947	Tr	43.13	2.0959	Cal
27.18	3.2784	Tr	47.49	1.9131	Cal
28.51	3.1268	Tr	48.44	1.8777	Cal
29.36	3.0391	Cal	50.16	1.8171	Qtz

Notes: Tr—tremolite. Qtz—quartz. Cal—calcite.

.

4.6. Chemical Characteristics of Carbonate Rocks

The chemical composition of carbonate rocks is listed in

Table 7. Chemical composition of carbonate rocks (wt%).

Sample	WY-1-1	WY-1-2	WY-2
SiO2	0.58	0.53	0.58
Al2O3	0.37	0.32	0.36
Fe2O3	0.18	0.12	0.19
MnO	0.07	0.04	0.06
NiO	0.04	0.03	0.03
MgO	1.14	0.74	1.22
CaO	52.28	53.44	52.04
K2O	0.11	0.08	0.11
CuO	0.03	0.03	0.03
SrO	0.03	0.03	0.03
SO3	0.01	0.01	0.01
SUM	54.83	55.36	54.66

. In all the analyzed samples, the major element contents show limited variations: CaO (52.04–53.44%) and MgO (0.74–1.22%).

5. Discussion

5.1. Cause of the Ivory White Color and Porcelain-like Appearance of Nephrite

Quartz is present in a granular or disseminated form within the tremolite matrix of a small portion of white Mida nephrite. Samples with a minor amount of quartz exhibit a porcelain-like appearance at a macroscopic level, without any apparent internal features observed under transmitted light. While quartz is mentioned as a minor mineral in many deposits, such as in Utah (USA), Tashisayi and Ruoqiang in Xinjiang (China), Luodian in Guizhou (China), and Sanchakou and Golmud in Qinghai (China) [1,11,41,43,44], the porcelain-like appearance seems to be present only in nephrites with smaller quantities of quartz from the Mida deposit.

The primary factors that determine the light scattering ability are particle size, volume of the second-phase particles and relative refractive index [45]. In all types of jade, the degree of compaction of the granules also influences light penetration because loose parts can be filled with a gas substance having the lowest refractive index. When the grains are uniform in size, well-oriented and tightly interlocking, light can effectively penetrate nephrite, achieving a high level of transparency and exhibiting a greasy to vitreous luster (Figure 11). However, the occurrence of an ivory white color and a porcelain-like appearance in Mida white nephrite may indicate a different internal texture compared to the aforementioned situation. Observations under a polarizing microscope and scanning electron microscope have shown that the white Mida nephrite contains tremolite and quartz grains, with the latter dispersed within the matrix of the former. The size of tremolite grains varies greatly, ranging from 0.05 to 10 μm in width. Similarly, there is significant variability in the size of quartz grains, ranging from 0.05 to 1 mm. A low degree of compaction can also be detected between these grains and their pores.

Consequently, the substantial difference in size between tremolite and quartz grains, along with a low degree of compaction and random orientation of the grains, leads to a reduction in the ability of light to pass through Mida white nephrite (Figure 11c). The significant variation in mineral grain size and random orientation also contribute to an increase in inter-particle pores, the formation of surfaces that are not very smooth and lower internal density. All these factors ultimately cause light to scatter more and consume more energy while passing through some Mida white nephrites. As a result, its ability to reflect surface light and transmit internal light decreases, resulting in a macroscopic appearance characterized by an ivory white color and porcelain texture (Figure 12a). Examples of nephrites corresponding to models a and b are shown in Figure 12b,c.

5.2. Origin of Quartz in Mida Nephrite

Based on differences in geological background, nephrite deposits can generally be divided into two types: those related to the replacement of dolomite (D-type) and those related to the replacement of serpentinite (S-type) [1,46]. The Mida nephrite deposit is more closely related to the former due to metasomatism between Si-rich hydrothermal fluids and host carbonate rocks. However, it differs from typical D-type nephrites as its occurrence stratum consists of limestone rather than dolomite or dolomitic marble, which distinguishes it from other nephrite deposits along the Kunlun–Altyn Tagh Mountains. The presence of quartz may be connected with this new type of limestone replacement nephrite deposit. However, further study is needed to understand its mechanism.

Although quartz has been mentioned in other nephrite deposits as a minor component, its origin in these deposits and in Mida nephrites has not been extensively investigated. Therefore, determining its origin could be of significant importance. By analyzing the occurrence of quartz in this context, two possible explanations for its presence can be proposed.

The first possibility is that quartz may originate from Si-rich hydrothermal fluids. During the late stage of metasomatism, when the Mg in the ore-forming fluids was nearly depleted, and pressure and temperature values decreased, abundant silica in the hydrothermal fluids began to crystallize as quartz. Crystallization might have occurred within the less compact tremolite matrix, resulting in dispersed quartz grains throughout it.

The second possibility is that quartz may have originated from the host rocks, which are primarily composed of limestone. If the host rocks contained a significant number of quartz particles, they would have participated in reactions during the thermal effect and led to the formation of tremolite. This hypothesis would rely on the evidence of a substantial quantity of quartz grains in the host rocks. However, our observations on the host rock did not confirm this hypothesis.

Therefore, the occurrence of quartz in Mida white nephrites is more likely due to the first hypothesis, suggesting that quartz grains overlapped the less compact tremolite matrix during the formation of the Mida white nephrites. This hypothesis illuminates the potential processes that lead to the formation of nephrites.

6. Conclusions

The gemological and mineralogical characteristics of a white nephrite from the newly discovered Mida deposit are presented. The nephrite exhibits an ivory white color and a porcelain-like appearance with semitranslucent-to-opaque transparency and a porcelain-to-greasy luster. It is mainly composed of tremolite, with minor amounts of quartz, diopside and calcite. Quartz was found in petrographic observation, electron probe microanalysis, Fourier transform infrared spectroscopy and powder X-ray diffraction. Quartz grains were found dispersed in the tremolite matrix, and these observations may be correlated with the appearance of an ivory white color and a porcelain-like appearance rather than the traditional greasy luster.

After investigating the grain size and inter-particle pores, we hypothesize that the difference in size between tremolite and quartz grains, along with low compaction and random grain orientation, is related to the ivory white color and porcelain-like appearance. The occurrence of quartz in nephrite is relatively rare, which could provide valuable insights into the ore-forming process, making it a focal point for further investigation of the Mida nephrite deposit.