Analysis of Jet Structure and Physical Properties in the Coalfields of Northern China

Abstract

Archeological discoveries have identified China as one of the first countries in the world to use jet. However, many differences are evident between the jet currently found in existing mines and the archaeological discoveries of cultural relics in terms of texture and quality according to the definition of organic gem jet in gemology. This paper reports the results of microscopic analysis and coal quality analysis of the coal and jet samples from coal seams in Fushun Open-pit Mine and Datong Coalfield. The findings reveal that the physical and chemical composition of coal in different mining areas differs markedly. However, the differences between jet and coal in both mining areas are similar; that is, jet has lighter density and greater hardness (2–4) compared to coal, as well as elasticity (engravability), and both jet and coal occur in the (rock slurry) hydrothermal environment. Lastly, the analysis shows that the formation of jet depends on rubber-like hydrocarbon coal with a high degree of corruption in a sedimentary environment under the vulcanization of a hydrothermal, high-sulfur environment.

1. Introduction

Jet, a mineraloid characterized by a high degree of hardness, low density, and being easy to engrave, has been used in decorations and handicrafts for millennia, as is evident in relics from the European Stone Age and North American Indian tribes [1,2,3]. According to Systematic Gemology, jet, which is also known as coal jade and black carbon stone, is a black organic rock with strong luster, light density, toughness, and wear resistance [4]. Jet is a variant of lignite resulting from the transformation of organic matter. After death occurs, some low-grade plants are buried underground. The plants rot into humus and are gradually covered in the fine-grained silt, then transformed through long-term geological action under the influence of pressure and temperature into hard shale, called jade rock. From a petrological point of view, jet is saprolite-humus coal [5,6,7].

As early as the ancient Roman era, jet was among the most popular “black gems”. Jet’s popularity has not diminished over time. In more recent times, jet was often used for funeral jewelry by 19th century British Victorians (Figure 1a) because of its dark, dignified color [8,9].

Jet has also been a traditional Chinese carving craft materials used in Chinese carving [10]. The earliest Chinese jet products, dating from more than 7000 years ago, were found at the Xinle Cultural Site in Shenyang, where “ear pond decorations” and round beads were unearthed [11]. Analysis by the scientific research institute of the Liaoning Coalfield Geological Exploration Company of the slices of jet discovered at the site and confirmed that the raw materials came from the Fushun Coalfield near Shenyang [12]. As another example, a large amount of jet jewelry dating from 3000 to 4000 years ago was discovered at the Chifeng Xiajiadian Cultural Site. The articles unearthed include ornaments made of pipe beads, piece beads, bamboo joints, abacus plates, double holes, etc., as well as carved ornaments such as animals, plants, totems, and random shapes. Individual pieces from these archeological finds can range from simple to incredibly complex. The “Dugu Letter Seal” owned by Duguxin of the Southern and Northern Dynasties and Western Wei Dynasty, unearthed in Xunyang County, Shaanxi in 1981, is a sphere with eight edges and 26 sides, 4.5 cm high and 4.35 cm wide, featuring 18 square seals, eight triangular seals, and 14 square seals with engraved text (Figure 1b). This seal, made of jet, is currently in the Shaanxi History Museum collection. In 1955, in the fields of the “Silk Road” in the ancient city of Gaochang, Turpan, Xinjiang, a square box carved from jet was found, containing 10 Persian Sassanian silver coins, including 4 from Shabr II (310-379 AD), 5 Aldashi II (379-383 AD), and 1 Shabr III (A.D. 383-388). Archaeologists have also collected many jet products from the Western Zhou Dynasty, Spring and Autumn Period, Warring States Period, Qin, Han, Weijin, Southern and Northern Dynasties, Liao Dynasty, and other period during their explorations of more than 20 ancient tombs and sites in seven provinces, including Shaanxi, Sichuan, Henan, Xinjiang, Gansu, Guangdong, and Liaoning.

According to Chen Mugeng’s “Seal Cutting Needle Degree” from the Qing Dynasty, jet is black in color, tough in texture, light in weight, similar to black rhino horns, produced in Qinzhong, and usable for seals. During the late Qing Dynasty, there were workshops specializing in jet carving were located in Fushun, Sichuan, Shaanxi, and other places where jet was abundant. In May 2009, a Liu Desix Tibetan jet stone pen holder from the late Qing Dynasty was auctioned at Beijing Poly’s “Antique Treasure Day Fair” for 16,800 Yuan. Modern artists, such as Zhao Kunsheng and his disciples in Fushun, have continued the traditional of coal carvings; his works are also displayed in the Great Hall of the People. The terms “coal carvings” and “jet” refer to the same mineraloid, as its varying origins and characteristics have generated multiple names for this material, including jet, coal jade, coal root stone, charcoal extract, coal, and black jade [9,10,11,12,13].

Coal formations result from biological mineralization in nature, and their occurrence and distribution are primarily controlled by the structural pattern of the coal they feature, which is dictated by the types of ancient plants, paleoclimate, and paleogeography involved in their creation [14,15,16]. Different tectonic backgrounds and sedimentary environments have led to the formation of different coal-bearing structures as the products of crustal tectonic evolution during different periods.

Coal-bearing construction was mainly accomplished by the tectonic stretching movement during different geohistorical periods that had a direct impact on coal accumulation. The structures of different scales affected the shape of coal-accumulating basins, as well as the distribution and migration of coal-accumulating canters and coal-rich belts. Such structural differences are especially the case for large structures, which determined the distribution of coal-accumulating areas and controlled the advance and retreat of seawater and the migration of biota [17].

The Paleozoic Hercynian movement, Mesozoic Yanshan movement, and Cenozoic Himalayan movement of the North China and Yangtze plates of China featured differences in intensity, movement stages, gyrations, and directional migration, all of which directly affected the regular changes in plant populations and coal accumulation [18,19,20]. The formation of a series of large-scale structural basins provided a good place for coal accumulation. At the same time, the tectonic pattern and sedimentary environment also affected the occurrence and reproduction of plants throughout earth’s history, from lower to higher plants, from aquatic to semi-aquatic to terrestrial plants, from simple to complex, forming a wide variety of plant ecosystems and providing the material basis for coal accumulation. For example, aquatic single-celled algae (micro-paleophytes) were deposited as saprolite. Meanwhile, humus coal was formed by the deposition of coastal and terrestrial higher plants, while humus saprolite formed locally in the deep-water reducing environment of the structural core [21].

Different tectonic periods and plant communities have suggested inherited and staged evolution and development. The coal accumulation pattern in Chinese geography can be described as having four stages, with each stage characterized by certain types of coal and rock properties. The first stage is mainly a neritic coal accumulation mode dominated by colonies, forming Early Paleozoic coal, and the coal is sapropelic coal [22,23]. The second stage is mainly the sea-land transitional coastal and terrestrial coal collection mode dominated by ferns, forming late Paleozoic coal, the coal is humic coal, a small amount of sapropel coal, and humic sapropel coal [24,25]. The third stage is mainly for the large and medium-sized continental basins dominated by gymnosperms, in which the inland lakes and rivers gather coal, forming the Mesozoic coal [26]. The fourth stage is mainly the mode of coal accumulation in inland lakes and rivers dominated by medium and small continental basins dominated by angiosperms, forming Cenozoic coal [27,28].

(1) Early Paleozoic coal mainly originated from shallow sea coal accumulation dominated by bacteria and algae, forming sapropel coal [22,23];

(2) The genesis of Late Paleozoic coal was mainly based on the fern-based sea and land crossing sea and land coal-gathering patterns. As a result, humus coal formed, with a small amount of saprolite and humus saprolite [24,25];

(3) Mesozoic coal was formed mainly in gymnosperm-based large and medium-sized continental basins, inland lakes, and rivers facies, featuring coal accumulation in sedimentary facies [26]. This process yielded humus coal, along with a small amount of saprolite and humus saprolite;

(4) Cenozoic coal mainly originated in angiosperm-based medium and small continental basins dominated by inland lakes and rivers following the facies coal accumulation model [27,28]. This coal is comprised of humus coal, a small amount of saprolite, and humus saprolite.

At present, literature exploring the formation of jet remains limited, especially in terms of gem identification. The large number of producing areas in China has led to a wide variation in the quality of jet produced. For example, Fushun and Datong jet do not meet the current national standard (GB/T 16553-2017 jewelry and gem identification) test parameters, precluding issuance of the jet identification certificate. Moreover, the quality of ancient jet differs from the current standards. For example, Guo Jingwen of Northwest University and others have identified 16 works made from jet that were discovered in numerous ancient tombs in Xinjiang and Shaanxi. The jet precision form these pieces was determined to fall between 1.29 and 1.53, which is inconsistent with the existing standards. Based on the analysis of the genesis of jet and the determination and analysis of jet parameters in different eras and regions, this paper shows that the coal fines produced in China have the characteristics of having occurred in multiple geological periods (Carboniferous, Jurassic, Cretaceous-Tertiary), numerous producing areas, and large changes in jet parameters. Therefore, when evaluating the characteristics of jet, it is necessary to pay careful attention to the engraving and quality of the ornamental aspects while ensuring the parameters of high-quality jet.

2. Materials and Methods

2.1. Materials

We collected coal and jet samples from Datong Coalfields and Fushun Coal Basin of Northern China. In Northern China, with the exception of the early Paleozoic era, jet has been found in the coal-measure strata of the Late Paleozoic (Datong Coalfield), Mesozoic (Datong Coalfield), and Cenozoic (Fushun Coal Basin). Fushun jet occurs in lignite, while Datong Coalfield jet occurs in weakly caking coal, gas coal, and long-flame coal. To date, jet has been discovered in many coal fields in China, especially in Fushun, but it has not been systematically analyzed and tested [29]. Thus, in this study, which is the first time to examine Chinese jet, coal and jet samples from the main mineable coal seams of the Carboniferous and Jurassic periods in the Datong Coalfield, and the main mineable coal seams of the Paleogene in the Fushun Open-pit Mine were collected and sliced. The samples were analyzed, and their microscopic characteristics were observed.

The identification of coal essence (or jet) has different evaluation standards in international, national and regional. According to the local standards of Fushun City, the Mohs hardness of coal essence is 2~4 and the density is 1.17~1.39 g/cm³. In comparison with the current national standards of the People’s Republic of China, the Mohs hardness of coal essence is 2~4, the density is 1.32 (±0.02) g/cm³, the optical characteristic is a homogeneous body, and the refractive index of the spot measurement method is usually 1.66 (±0.02). However, according to the jewelry identification standard of coal essence in China, the Mohs hardness of coal essence is 2.5 to 4, the specific gravity is 1.30 to 1.34, and the refractive index is 1.66.

2.2. Methods

In order to analyze the characteristics of coal and jet in each coal seam, this paper uses laboratory chemical tests to determine the physical and chemical parameters of coal and jet, including water content, ash content, volatile matter, density, fixed carbon content, ignition point, spontaneous ignition and hardness.

In addition to this, coal and jet were sampled in sections, and their microscopic characteristics were visually identified under an Olympus BX53 microscope. The Olympus BX53 microscope uses UIS2 optical system, the observation head has a wide field of view, the number of fields of view is 22, the eyepiece magnification is 10, the vertical movement range of the stage is 25 mm, and it is equipped with a coarse adjustment limiter, which can be adjusted by the coarse adjustment knob. It contains four types of condensers, namely Abbe condenser, swing achromatic condenser, achromatic/aspheric condenser and universal condenser.

In the microscopic observation of coal and jet slices, first turn on the air switch and the host key, start the computer software, then press the AIR button to release the vacuum, gently pull out the sample chamber and put in the sample, close the sample chamber and press the EVAG button to vacuumize. Then select the vacuum mode and observation probe, then adjust the height of the sample stage, select the voltage, adjust the appropriate magnification to observe the sample, focus to eliminate scattering, adjust the brightness and contrast. Select the photo mode, capture the photo, save the name and date of the final captured image record in the selected folder. Finally, lower the sample stage, press the AIR key to release the vacuum, gently pull out the sample compartment to take out the sample, close the sample compartment and press the EVAG key to evacuate, launch the software, shut down, and turn off the air switch.

3. Results

3.1. Analysis of the Main Components of Coal and Jet

The water content of coal and jet is very low, which is quite lower than the coal’s water content. The maximum moisture content of coal shall not exceed 5%, while the maximum moisture content of jet shall not exceed 3%. Specifically, the moisture content of Qianjing jet 1 is 0.7%, Qianjing jet 2 is 0.6%, Wangping jet is 0.58%, and Fushun Yaokeng jet is 2.44%. In comparison, the moisture content of Qianjing Coal 1 is 2.3%, Qianjing Coal 2 is 3.74%, Wangping Coal is 1.19%, and Fushun Yaokeng coal is 4.17%.

Except for the ash content of Qianjing No. 11 coal 2 and Qianjing jet 2, and the ash content of Fushun Yaokeng coal and Fushun Yaokeng jet are similar, the ash content of jet in other areas is about half of that of coal. The ash content of Qianjing Coal 1 is 12%, Qianjing Coal 2 is 11.4%, Wangping Coal is 10.28%, Fushun Yaokeng Coal is 3.43%, and Qianjing No. 11 Coal 1 has an ash content of 28%. Qianjing No. 11 coal 2 is 12.38%, Wangping No. 8 coal is 10.28%, and Fushun Yaokeng coal is 3.71%.

The volatile content of coal and jet is obviously different, and the volatile content of jet is 10–40% higher than that of coal. The volatile matter content of Qianjing Coal 1 is 61.5%, Qianjing Coal 2 is 74.61%, Wangping Coal is 70.4%, Fushun Yaokeng Coal is 58.38%, and Qianjing No. 11 Coal 1 has a volatile content of 38.6%, Qianjing No. 11 coal 2 is 32%, Wangping No. 8 coal is 29.93%, and Fushun Yaokeng coal is 45%.

The apparent density of jet is smaller than that of coal, the apparent density of jet is about 1.2, while the apparent density of coal is about 1.3~1.6. The apparent density of Qianjing Coal 1 is 1.23, Qianjing Coal 2 is 1.2, Wangping Coal is 1.2, Fushun Yaokeng Coal is 1.24, and Qianjing No. 11 Coal has an apparent density of 1.42 and Qianjing No. 11 Coal 2 is 1.5, Wangping No. 8 coal is 1.65, and Fushun Yaokeng coal is 1.32.

The fixed carbon content of jet is at least 10% lower than that of coal, and the maximum fixed carbon content is 37%, while the fixed carbon content of coal is more than 30%. The fixed carbon content of Qianjing Coal 1 is 25.6%, Qianjing Coal 2 is 13%, Wangping Coal is 19%, Fushun Yaokeng Coal is 37%, and Qianjing No. 11 Coal 1 has a fixed carbon content of 40%. Qianjing No. 11 coal 2 is 49%, Wangping No. 8 coal is 30%, and Fushun Yaokeng coal is 43%.

There is little difference in oxygen absorption between jet and coal. The oxygen absorption capacity of Qianjing Coal 1 is 0.56, Qianjing Coal 2 is 0.76, Wangping Coal is 0.7, Fushun Yaokeng Coal is 0.82, and Qianjing No. 11 Coal 1 has an oxygen absorption capacity of 0.61 and Qianjing No. 11 Coal 2 is 0.73, Wangping No. 8 coal is 0.61, and Fushun Yaokeng coal is 0.8.

The ignition temperature of jet is between 260 and 330 °C, which is lower than that of coal, and it is easy to occur naturally. The ignition temperature of Qianjing Coal 1 is 330 °C, Qianjing 2 is 280 °C, Wangping Coal is 300 °C, Fushun Yaokeng Coal is 260 °C, and Qianjing No. 11 Coal 1 has an ignition temperature of 400 °C. No. 11 coal 2 is 300 °C, Wangping No. 8 coal is 330 °C, and Fushun Yaokeng coal is 270 °C.

It can be seen that the result in

Table 1. Comparison of the results of the coal and jet test results for each coal seam.

	Mad 1 (%)	Ad 2 (%)	Vdaf 3 (%)	St,d (%)	Apparent Density (g/cm3)	Fixed Carbon (%)	Oxygen Uptakevd (cm3/g)	Glia Index	Ignition Temperature (°C)	Spontaneous Ignition
Qianjing No. 11 Coal 1	2.3	28	38.6	2.26	1.42	40	0.61	47/8.5 Smooth droop type	400	Spontaneous
Qianjing jet 1	0.7	12	61.5	2.67	1.23	25.6	0.56	75/0 Smooth ramp-down type	330	easy
Qianjing No. 11 Coal 2	3.74	12.38	32	1.14	1.5	49	0.73		300	Spontaneous
Qianjing jet 2	0.6	11.4	74.61	1.93	1.2	13	0.76		280	easy
Wangping No. 8 Coal	1.19	29.93	37.59	1.4	1.65	30	0.61		330	Spontaneous
Wangping No. 8 jet	0.58	10.28	70.4	0.8	1.2	19	0.7		300	easy
Fushun Yaokeng Coal	4.17	3.71	45	0.7	1.32	43	0.8		270	easy
Fushun Yaokeng jet	2.44	3.43	58.38	0.7	1.24	37	0.82		260	easy

¹ Mad refers to the percentage of moisture content. ² Ad refers to the percentage of ash content. ³ Vdaf refers to the percentage of distribution content.

demonstrates that the physical and chemical parameters of coal from each coal seam and jet are quite different. The moisture content, ash content, density and ignition point of jet are all lower than coal, and jet and coal have significant differences in volatile matter content and fixed carbon content. The volatile matter content of jet is much higher than that of coal and the fixed carbon content of coal is much lower than that of jet. Jet is easy to spontaneously combust, with greater hardness and better elasticity.

Due to the variations in the depositional environment of various coal fields, the physical and chemical parameters of coal and jet differ greatly. However, in China, the current national standard for jet identification is based on international standards and does not consider the large changes in the access and exit of China’s coal fields. As another consequence, jet from the Fushun and Datong coalfields does not meet China’s national identification standards, but is in line with international standards, which are broader than national standards.

3.2. Microscopic Observation Characteristics of Coal and Jet Slices

3.2.1. Microscopic Observation Characteristics of Coal Slices from Different Layers of the Datong Coalfield and Fushun Open-Pit Mine (

Table 2. Microscopic observation characteristics of coal slices from different layers of the Datong Coalfield and Fushun Open-pit Mine.

Serial Number	Coal Field	Coal Seam	Coal Section	Component Analysis
1	Datong Jinhua Palace	J-2		Homogeneous vitrinites and semi-filament bodies are distributed in a striatum, the chitinous group is underdeveloped; the vitrinite group accounts for about 55%, the semi-filamentous body for about 35%, and the mineral content about 10%.
2	Datong Jinhua Palace	J-3		Homogeneous vitrinite (100%) contains pores and cracks.
3	Datong Jinhua Palace	J-7		The vitrinite group accounts for 60%, the inertite group for 40%; the coal rock type is micromirror inert coal, the homogeneous vitrinite is in the shape of a block or strip, and the filamentous body is in the shape of a band.
4	Datong Yungang	J-8		Inert components (45%) are inserted between successive vitrinite components (55%) due to a local oxidation.
5	Datong Jinhua Palace	J-9		Semi-filament body (black-brown25%), transitional stage as thin cell wall, vitrinite (40%), semi-filament body (20%), and silk body (15%) at top-bottom.
6	Datong Yungang	J-11		The black is silky (20%), the black-brown is semi-silky (45%), and the brown is vitrinite (35%); The cell cavity of the semi-filament body is hollow and the orientation is not obvious.
7	Datong Jinhua Palace	J-11		Structural vitrinites (60%), the cell structure (40%) is obvious, neatly arranged, into a honeycomb shape, as cell wall swells absorbing water and cell cavity filled with matrix vitrinite.
8	Datong Jinhua Palace	J-12		From bottom to top is the transition stage from vitrinite group (60%) to inertin group (40%), which are silk body (black), semi-silk body (black-brown), and vitrinite (red-brown).
9	Datong Yungang	J-14		Homogeneous vitrinite (60%) and silken body (30%), semi-filament body (10%) are interlayered as a transition phenomenon, and cracks in the later period lead to the bending and discontinuity of the laminae.
10	Datong Jinhua Palace	J-15		Silken body (30%), cell wall as black and opaque (70%); the cell structure is clear and the cavity is hollow. An oxidizing environment and hydrodynamic force is weak, with relatively complete biological cell structure.
11	Datong Xiaoyu	C-3		The vitrinite group accounts for 70%, the chitin group is for 20%, and the inert group is for 10%. The coal rock type is micro-triad coal, and vitrinite accounts for the most important component.
12	Datong Xiaoyu	C-5		A large number of keratinocytes (30%) are developed in yellow or yellow-white thin strips, with jagged inner edges, often inlaid around vitrinites (70%).
13	Datong Wangping	C-5		Vitrinite occupies 55%, inertite occupies 45%, and the coal rock type is micromirror inert coal; structure of vitrinite is filledin inertite as massive or linear.
14	Datong Wangping	C-8		Algae body (left), yellow 30%; silken body (right), black, opaque, hollow, visible cell structure 40%, but part of cell wall is swollen 30%; red is the mass vitrinite; yellow and orange megaspores (lower) are distributed along the layer.
15	Datong Qianjing	C-11		Structural vitrinite (70%), cell cavity is still black (30%) filled with pitch in the cell cavity. The stratum corneum (yellow) underneath is clearly jagged.
16	Hunyuan	C-8		Resin body (30%), Algae body (70%)
17	Hunyuan	C-11		Algae body (70%), Sapropel (30%)
18	Ningwu Tongying	C-2		The keratinous body (30%) coexists with the vitrinite (70%) in the edge, and the matrix vitrinite and inert body are filled into the structural vitrinite.
19	Fushun Open-pit Mine	East pit		Resin body (30%), yellow lens body, cell wall expansion and rupture, cavity filled with shell debris and fine granules (70%).
20	Fushun Open-pit Mine	Waist pit		Mass vitrinite (40%), elliptical, reddish-brown, surrounded by shell debris bonded by matrix vitrinite (60%).
21	Fushun Open-pit Mine	West Pit		Vitrinite accounts for 45%, the chitin body is for 55%, the coal rock type is slightly bright coal; shell crumbs are distributed in the matrix vitrinite in a directional arrangement.

)

The study took samples from the coal seams of eight coal fields of Datong Jinhuagong, Datong Yungang, Datong Xiaoyu, Datong Wangping, Datong Qianjing, Hunyuan, Ningwu Tongying and Fushun open-pit mine (Table 2). The coal seams of Jinhuagong in Datong include J-2, J-3, J-7, J-9, J-11, J-12, J-15, and the coal seams of Yungang in Datong include J-8, J-11, J- -14, the coal seams of Datong Xiaoyu are C-3 and C-5, the coal seams of Datong Wangping are C-5 and C-8, the coal seams of Datong Qianjing is C-11, and the coal seams of Hunyuan are C-8 and C-11, the coal seam of Ningwu Tongying is C-2, and the coal seam of Fushun open-pit mine includes Dongkeng, Yaokeng and Xikeng. After the sample coal was sliced and observed under a microscope, it can be found that the coal seam is mainly composed of vitrinite, silk and inertite, and basically does not contain algae and lipids. The coal seam in the Hunyuan coal-producing area is mainly composed of algae and resin, and the coal seam in Fushun open-pit mine is mainly composed of vitrinite and resin, but all kinds of coal have no elastic characteristics, are brittle, and are not easy to carve.

3.2.2. Microscopic Observation Characteristics of Coal Jet from Different Layers in the Datong Coalfield and Fushun Open-Pit Mine

As shown in

Table 3. Microscopic observation characteristics of jet slices from different layers in the Datong Coalfield and Fushun Open-pit Mine.

Serial Number	Coal Field	Coal Seam	Coal Section	Component Analysis
1	Datong Qianjing	Floor 11 jet		Mainly algae (75%), arranged in a directional arrangement, distributed in the black matrix (25%), which contains a small amount of matrix vitrinite and mass vitrinite.
2	Datong Qianjing	Floor 11 jet		Crustaceous group accounts for 80%, the coal rock type is dark coal, and other microscopic components (20%) are distributed in fragments in a strong hydrodynamic force and reducing coal-forming environment.
3	Datong Wangping	Floor 8 jet		The size of the algae (70%) is similar, the arrangement is directional, with high content of microspores (30%). The chitinous content of this sample is outstanding, reflecting a good oil-generating potential.
4	Datong Wangping	Floor 8 jet		Mainly algae (85%), with a small amount of inert and vitrinite fragments (15%).
5	Fushun Open-pit Mine East pit	jet		Gelatinized unstructured vitrinite (30%) and resin body mainly (70%).
6	Fushun Open-pit Mine West Pit	jet		Mainly humus (70%), a small part of resin body (30%), mostly in directional arrangement.
7	Fushun Open-pit Mine Waist pit	jet		Humus (85%) and a small amount of resin (15%) are arranged in a directional arrangement, increased inorganic content.

, the study carried out jet sampling from the No. 11 coal seam of Datong Qianjing, the No. 8 coal seam of Datong Wangping (Figure 2), and the Dongkeng, Xikeng and Yaokeng of Fushun Open-pit Mine. The jet after sampling was sliced and observed under a microscope. It is found that the jet is distributed in layers, generally less than 1 m thick, and most of them are lens bodies, with a thickness of 0.5 m or less (Figure 2), and structures are developed in the parts with the largest thickness. The Carboniferous coal seam of Datong Coalfield is mainly composed of algae and lipids, and the coal essence of Fushun open-pit mine is mainly composed of vitrinite and resin. Jet has high hardness, low density, and is easy to carve.

4. Discussion

In this paper, coal and jet were sampled, sliced and analyzed under microscope, and the physical characteristics of coal and jet, such as moisture content, ash content, volatile matter content, apparent density, fixed carbon content, oxygen absorption and carbon content were obtained. Compared with coal, jet has higher volatile content, and its density and fixed carbon content are lower than coal. These differences in physical properties make jet show good elasticity and sculptability. Since organic elastic substances are related to rubber hydrocarbons, natural rubber contains 92% to 95% of rubber hydrocarbons. The linear molecules of rubber hydrocarbons are connected to each other through the bridging of some atoms or atomic groups to form a three-dimensional network structure called cross-linked structure. Therefore, hydrocarbon source material is a necessary condition for the formation of jet. Microscopic sections of jet mainly contain algae and lipids, which provide hydrocarbon source material for the formation of jet. Coal concentrate from Fushun open-pit mine mainly contains vitrinite and a small amount of lipids. The main component of coal concentrate from Datong coalfield is algae. Compared with coal concentrate from Datian coalfield, Fushun open-pit coal concentrate has a lower content of hydrocarbon source substances.

In addition to hydrocarbon source substances, the elastic characteristics of jet are also related to sulfur. Natural rubber changes from raw rubber to mature rubber through vulcanization, which greatly improved its elastic properties. The progression of the vulcanization process continuously strengthens the cross-linked structure, causing the free movement ability of the chain segment to decreases; moreover, the plasticity and elongation rate decrease, the strength, elasticity and hardness increase, and the compression set and swelling degree decrease. The formation of jet is similar to the vulcanization process of rubber, which requires sufficient rubber hydrocarbon substances and sulfur, and can be formed by vulcanization under the action of algae, resins, higher plant crude fibers and heat energy.

During the coal formation process, the high-sulfur area was primarily found at the entrance of the basement structure in the coalfield and the coalfield transgression [29]. The basement structure in the coalfield was located in a deep-water reducing environment, where provided a heat source and was more conducive to the growth of algae. In addition, during the later geological tectonic movement, the magma hydrothermal activity intensified the sulfidation so that the jet was mainly associated with the basement structure area, the structure was centered in a lens and thin layer, and the quality of the structure center was higher.

For example, the jet in the Datong Coalfield (Figure 3) is mainly produced in the structural intersection area where more deadly types of coal mine disasters tend to occur. Disasters based on water, fire, gas, and roof collapse are relatively concentrated. Critically, the jet in the coal seam is flammable and has a short ignition period, increasing the risk of fire in the coal mine goaf. In comparison, Fushun Coalfield jet was mainly produced close to outcrops or in areas where open-pit mine boundary structures were developed, stomatal almond structures are common, and metamorphism in oil shale caused resin bodies to form amber (Figure 4).

The craftsmanship involved in manual coal carving is divided into various procedures: cutting, shoveling, grabbing, grinding, throwing, rolling, chopping, planning, drilling, rubbing, and other technological processes, all of which are unique to Fushun jet carving. Many masters of Fushun coal fine carving art have offered comparison of jet from the Datong Coalfield and Fushun Open-pit Mine. Specifically, they found Fushun jet superior to Datong refined coal in terms of cutting, shoveling, grabbing, grinding, throwing, and chopping. Datong jet also exhibited better fluorescence characteristics than Fushun jet. (Fluorescence intensity is related to the amount of fluorescent substances that can be produced. Stronger fluorescence intensity means that the sample contains more conjugated substances such as benzene rings). Meanwhile, Fushun jet was reported to exhibit inferior performance in rolling, rolling, planning, drilling and rubbing is inferior to Datong jet. In practical terms, Datong jet is more suitable for modern dental carvings and drills to process more delicate figures and other shapes. These characteristics may be related to the high content of fine oil and low algae in Fushun jet in contrast to the high content of saprolite in the jet from the Datong Coalfield, the extended time involved in coal formation, and the high degree of hydrothermal metamorphism.

5. Conclusions

Jet has a long history of decoration and craftsmanship, but the standards for evaluating jet are different, and there are few studies on the cause of jet. In this paper, the samples of coal and jet from Fushun open-pit mine and Datong coalfields were sampled, sliced and observed under microscope, and their physical and chemical properties and coal quality properties were analyzed. Compared with coal, it was found that the moisture content of jet is lower, generally not more than 1%, the volatile matter content and fixed carbon content of jet are significantly higher than that of coal, the volatile matter content of jet is at least close to 60%, and the fixed carbon content is 13–37%. The apparent density of jet is about 1.2, and the ignition point is 260–330 °C, which is more natural. Identifying jet according to these physical properties does not conform to Chinese national standards, but conforms to international standards.

According to the high elastic characteristics of jet and the composition of microscopic slices, the jet slices mainly contain algae and lipids, which provide hydrocarbon source substances for the formation of jet. In addition to hydrocarbon source materials, the formation of jet also requires a high sulfur content or flowing high sulfur liquid environment and a basement structure hydrothermal development area. After these conditions are met, after vulcanization, algae or resinous rubber hydrocarbon coal with high volatile content and low density, jet can form.