Selected Papers on Coal Petrology and Coal Geochemistry in Ren Deyi
Minerals in coal are an important part of coal. From the genetic point of view, the composition and characteristics of minerals in coal not only reflect the geological background of coal-accumulating environment, but also sometimes reflect various geological processes experienced after the formation of coal seams, which is helpful to clarify the basic theoretical issues such as the genesis of coal seams, coalification and regional geological historical evolution (Ward, 2002). From the perspective of coal utilization, the mineral content in coal directly affects the calorific value level and processing and utilization characteristics of coal (Han Dexin, 1996), and is also the main source of wear, corrosion and pollution in coking metallurgy. In addition, the content, existing forms and environmental pollution of most trace harmful elements in coal are also related to minerals in coal (Vassilev et al., 1994), and minerals are the main carriers of trace elements in coal (Tang et al., 2004). Gupta et al. (1999) think that most problems in coal utilization are caused by minerals in coal, not organic macerals in coal. On the other hand, rare elements and radioactive elements that meet the requirements of industrial grade are associated useful minerals, and some minerals can play a catalytic role in the process of coal utilization and processing, which improves the economic and technical value of coal. Therefore, it is of great theoretical and practical significance to study the composition, content, source and occurrence of minerals in coal.
I. Minerals found in coal
The minerals in coal mainly include chronological, clay minerals (mainly kaolinite, illite and illite/montmorillonite mixed layer minerals), carbonate minerals (siderite, calcite and dolomite) and sulfide minerals (such as pyrite) (Ward,1978,2002; Harvey et al.,1986; Palmer et al., 1996). Scholars at home and abroad have made extensive research on the occurrence characteristics and geological causes of minerals in coal, especially these four types of minerals (Martinez-Tarazona et al.,1992; Patterson et al.,1994; Huang Wenhui et al.,1999; Hower et al, 2001; Ward, 2002; Dai et al., 2003), and a variety of trace minerals in coal, such as monazite, zircon, xonotlite, bischofite, collophanite, chromite and so on. (Querol et al.,1997; Rao et al.,1997; Ward,1989; Deere et al.,1999; Vasiliwete et al.,1998; Li et al; Ding Zhenhua et al., 2002). According to the data of Finkelman( 198 1), more than 125 minerals have been identified in coal. Bou？ Ka et al. (2000) think that coal may contain 145 minerals; Tang et al. (2004) summarized the literature reports at home and abroad and listed 20 1 species of identifiable crystal minerals in coal.
According to previous research data, the hydroxide minerals found in coal include limonite, bauxite, goethite, lepidolite, diaspore, gibbsite, boehmite, wolframite, brucite and calcium hydroxide. Limonite, bauxite and goethite are common in coal, and there are many studies on their causes (Dill et al., 1999). Glass fiber is rare in coal and mainly exists in peat (Bou? Ka et al.,1997); The content of diaspore in coal is low, which mainly exists in the coal seam with gangue in volcanic ash layer (Burger et al.,1971). Gibbsite is rare in coal (Bou? Ka et al., 2000); There are few or few minerals such as boehmite, wolframite, brucite and calcium hydroxide in coal (Ward,1978; Bou？ Ka and so on. ,2000; Don et al., 2004).
It is worth noting that although boehmite can exist in clay dikes in some coal measures strata, some research work has been done on it (Mao Yuan et al.,1994; Beam, etc. Liu et al., 1997), but there is no publicly reported information about the occurrence and genesis of boehmite in coal at home and abroad, mainly because it is rare in coal. Bou？ Ka et al. (2000) think that boehmite is very rare in coal; Ward( 1977, 1984, 2002) thinks that there may be a small amount of boehmite in individual coals, but the high content of boehmite in coals is unusual. Goodarzi et al. (1985), Harvey et al. (1986), Patterson et al. (1994) and Vassilev( 1994) studied the minerals in coal from Canada, Australia, the United States and Bulgaria respectively, but no Bohm was found. Tatsuo et al. (1993, 1996) and Tatsuo( 1998) found a small amount of boehmite in the low-temperature ashing products of Paleogene coal in Ishikariwan coalfield, Hokkaido, Japan (only 8 of the 85 coal samples collected contained boehmite, with the highest content. In addition, diaspore in coal has not been publicly reported at home and abroad.
Second, the geological background and experimental methods
Zhungeer Coalfield is located in the northeastern margin of Ordos Basin, with a length of 65 kilometers from north to south and a width of 26 kilometers from east to west, covering an area of 1700 square kilometers, with proven geological reserves of 26.8 billion tons of coal. It is the most abundant coal seam in Ordos basin and the most obvious zone of sedimentary phase transition. Limestone all pinches out in the coalfield and gradually becomes terrigenous clastic rock. The coal-bearing strata in Zhungeer Coalfield include upper Carboniferous benxi formation, Taiyuan Formation and lower Permian Shanxi Formation, with a total thickness of 1 10 ~ 160m. The floor of coal-bearing strata is limestone of Middle Ordovician, and the overlying strata are non-coal-bearing strata such as Lower Shihezi Formation, Upper Shihezi Formation, Shiqianfeng Formation and Liujiagou Formation. No.6 coal seam in this area is located at the top of Taiyuan Formation, and its thickness is generally 2. 7 and 35 m, with an average thickness of 30m and a maximum thickness of 50 m, are extremely thick coal seams formed under the background of delta sedimentary system (Liu et al., 1997).
According to the sampling specifications of GB 482- 1995 and MT 262-9 1, combined with the actual situation of coal seam mining in Zhungeer mining area, the coal samples of No.6 coal seam in Heidaigou Coal Mine were collected in layers. The number, thickness and characteristics of samples are shown in figure 1. The number of coal seams from top to bottom is ZG6- 1, ZG6-2, ZG6-3, ZG6-4, ZG6-5, ZG6-6 and ZG6-7. The mineral composition of coal seam was studied by X-ray diffraction analysis (XRD), and the morphological characteristics of minerals were observed by scanning electron microscope (SEM-EDX) with energy spectrometer and MPV-ⅲ microphotometer. According to GB 8899-88, the microscopic components and minerals of coal were quantitatively counted. The unit of test results is volume percentage (vol.%), and the allowable difference between the two test results is less than 4. 5%.
Figure 1 Mineral composition of columnar layered No.6 coal seam in the study area
3. Discovery and occurrence characteristics of boehmite and its special mineral assemblage
From the point of mineral composition, the D section of Zhungeer No.6 coal seam is obviously divided into four sections from top to bottom, the 1 section is composed of ZG6- 1, the second section is composed of ZG6-2, ZG6-3 and ZG6-4, the third section is composed of ZG6-5, and the fourth section is composed of ZG6-6 and ZG6-. The mineral compositions of these four profiles are quite different (Figure 1). Top-down functions are as follows:
(1) X-ray diffraction analysis (Figure 2a) and optical microscope determination show that the mineral composition of ZG6- 1 is mainly timely, with the content as high as 16. 4% (table 1), which is in a dispersed form (plate I- 1). It will lead to serious coal mineralization in time. According to the morphological characteristics of the season, it is angular and uniform in particle size, mostly 5 ~ 10μ m (plate I-3). Mainly distributed in matrix vitrinite, syngenetic clay minerals and homogeneous vitrinite. The content of clay minerals (mainly kaolinite) is 5. 5% (table 1). The SEM-EDX test results of layered weathering and clay minerals are shown in Table 2.
Table 1 Coal and Rock Composition of No.6 Coal Seam in Zhungeer Coalfield
Note: bdl is below the detection limit.
Fig. 2 XRD pattern of stratified samples of No.6 coal seam in the study area
(2) The components of ZG6-2, ZG6-3 and ZG6-4 are mainly abnormally enriched boehmite with the content of 1 1. 9%, 13. 1% and 1 1% respectively. Table 1), there is such a high content of boehmite in coal, which has not been reported at home and abroad. In addition, the content of kaolinite in these three layers is 4. 3%, 3.6% and 4. 4% respectively. Boehmite exists in the form of aphanitic in this coal seam, and its occurrence state is varied, but it is mainly distributed in matrix vitrinite in blocks, some in single blocks or irregular blocks (plates I-4 ~ 6), some in continuous blocks or beads, and some in caves of coal-forming plants (plates I-7). The particle size of massive boehmite varies greatly, ranging from 1 to 300μ m. Under the polarizing microscope, the main differences between boehmite and clay minerals are: boehmite is dense, while clay minerals are loose (Figure I-8), the reflection color of boehmite is lighter than clay minerals, and the protrusion of boehmite is higher (Figure I-6), while clay minerals are not protruding (Figure I-8). In these coal seams rich in boehmite, the mineral combination associated with boehmite is also very special. These minerals include rutile, forsterite, zircon, siderite, galena, chalcopyrite and selenium galena. The rutile content in ZG6-2 is high (1. 6%). Rutile appears in the form of single crystal or knee twin, and there is a banded structure phenomenon (plate II- 1, 2). There is dawsonite in ZG6-2 and ZG6-3. Dawsonite is mainly filled in the cell cavities of filamentous bodies, and is in the form of round particles with a particle size of 1 ~ 2 microns (plates I-7 and II-3). There are galena, chalcopyrite and selenium galena in ZG6-3. These three minerals are round (plate II-4), with special internal structure and many holes, which seem to have obvious signs of mineralization of bacteria, algae and other lower organisms (plate II-5). Zircon is found in ZG6-2 and ZG6-3, and fracture traces indicate that it comes from provenance (plates II-6 and 7). In addition, there is a small amount of siderite in the rich boehmite horizon (Plate Ⅱ-8). Because the contents of rutile, dawsonite, zircon and siderite are not high, they can't be detected by X-ray diffraction analysis. The crystal morphology and material composition were identified by polarizing microscope and scanning electron microscope (SEM-EDX) with energy spectrometer.
(3) The mineral composition of ZG6-5 is mainly kaolinite with the content of 1 1. Containing a small amount of boehmite (3. 3%) and trace pyrite.
(4) The minerals of ZG6-6 and ZG6-7 are mainly kaolinite, accounting for 22%, 19. It contains trace pyrite, calcite and calcite, but no boehmite (Figures 2e and f).
4. Preliminary study on the genesis of boehmite and its associated minerals.
Boehmite is the weathering product of silicate rocks, and often coexists with gibbsite, diaspore, kaolinite, chalcedony, ammonium mica and other minerals. In addition, it may be a low-temperature hydrothermal product, coexisting with zeolite (Kondakov et al.,1975; Hrinko， 1986； Beam, etc. Banerji,1998; Cheng Dong et al, 200 1). However, in the coal seam rich in boehmite, except kaolinite, no such biogenic minerals were found, and no evidence of low-temperature hydrothermal minerals or hydrothermal activities was found.
According to the research of Wang Shuangming et al. (1996), in the early stage of the formation of No.6 coal seam in Zhungeer coalfield (corresponding to the coal seams numbered ZG6-7 and ZG6-6), the terrain of Zhungeer coalfield was high in the northwest and low in the south, and the terrigenous clastic materials mainly came from Mesoproterozoic potash feldspar granite widely distributed in Yinshan ancient land in the northwest, so in ZG6-7 and ZG6, Coalfield is located in Yinshan ancient land low-lying area in the west of the north and benxi formation uplift area in the east of the north, where coal accumulation continues. According to the environmental significance represented by the oxygen and carbon isotope values of Carboniferous limestone, it is concluded that Carboniferous limestone was formed in a normal marine environment, and the average paleotemperature of Taiyuan Formation was 29 ~ 32℃, indicating that the climate in this area was hot at that time (Liu Huanjie et al.,1991; Cheng Dong et al, 200 1). According to the study of Carboniferous paleomagnetism in this area by Lin (1984) and Cheng Dong (200 1), the paleolatitude of late Carboniferous in Zhungeer coalfield is about north latitude 14. This tropical humid climate is beneficial to the formation of gibbsite in the weathering crust of benxi formation (Cheng Dong et al., 200 1). Gibbsite is the product of open environment oxidation. Gibbsite and a small amount of clay minerals are transported to Zhungeer peat swamp in a short distance in the form of colloid under the action of water flow. According to Wang Shuangming's research (1996), Zhungeer coalfield is only about 50 kilometers away from the weathering crust. With the continuous accumulation of peat, when the corresponding coal seam is ZG6- 1, the benxi formation uplift in the northeast direction descends, and the replenishment of terrigenous debris becomes the Mesoproterozoic potash feldspar granite in Yinshan ancient land in the northwest direction. ZG6- 1 is mainly clay minerals except a large amount of time. In the early stage of peat accumulation diagenesis, gibbsite colloidal solutions in ZG6-5, ZG6-4, ZG6-3 and ZG6-2 layers were dehydrated to form boehmite under the compaction of overlying sediments. Judging from the occurrence form of boehmite, most boehmite is flocculated, which also reflects the characteristics of its colloidal origin. Liu Changling et al. (1985) think that the formation of boehmite is mainly related to the weak acidity in diagenetic stage and the medium environment from weak oxidation to weak reduction, and boehmite is more likely to form in peat swamp. Benxi formation Bauxite in Hequ, Shanxi Province is rich in boehmite, and the heavy minerals of Jinyu Bauxite are zircon, rutile and galena. This is similar to the result of rich boehmite coal seam (Liu Changling et al., 1985), and it is also the evidence that boehmite in No.6 coal seam originated from benxi formation bauxite. The formation of high content boehmite in No.6 coal is different from boehmite or boehmite in coal measures kaolinite. Liu et al. (1997) pointed out that the formation of diaspore or boehmite in coal measures kaolinite is mainly due to the formation of kaolinite in medium (pH
Table 2 SEM -EDX test results of boehmite and its associated minerals
Note: Min is the minimum value; Max is the maximum value; AM is the arithmetic mean; Bdl is below the detection limit.
The occurrence of high-content boehmite in late Paleozoic coal in the study area is not a simple and isolated geological event, and its unique occurrence, genesis and associated mineral combination are inextricably linked with its surrounding geological bodies, the formation and evolution of coal seams, paleogeography and paleoclimate when coal seams were formed.
Acknowledgement: Thanks to Professor Zeng from Institute of Geology and Geophysics of China Academy of Sciences and Professor Zhong Ningning from China University for their careful guidance and great help.
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Chart description
Plate Ⅰ
SEM is in 1. ZG6- 1。
2.ZG6- 1 The mineralization is serious (oil immersion, reflection single polarization, 320 ×).
3. In ZG6-1,the timely edges and corners are obvious and the particle size is uniform (SEM).
4. Regular massive boehmite (SEM) in ZG6-2.
5. Irregular massive boehmite (SEM) in ZG6-2.
6. Irregular massive boehmite in ZG6-3 with high protrusion (oil immersion, single reflection polarization, 320 ×).
7. The boehmite and aluminum phosphate (SEM) in ZG6-3 filled in the cavity of the filament.
8. The clay minerals in ZG6-5 have no protrusion (oil immersion, reflective single polarization, 320 ×).
Plate Ⅱ
Rutile crystal 1. ZG6-2 (oil immersion, reflective single polarization, 320 ×).
2. Knee twins of rutile in ZG6-2.
3. Scanning electron microscope observation of ZG6-3 filled cell cavity.
4. Selenogalena (SEM) produced in ZG6-3 is round.
5. Internal structure (SEM) of ZG6-3 chalcopyrite.
6. Zircon (SEM) in ZG6-2.
7. Zircon (SEM) in ZG6-3.
8. The siderite in ZG6-3 (SEM).
Dai Shifeng et al.: Discovery of abnormal enrichment of boehmite in Zhungeer coalfield in northeastern margin of Ordos Basin.
Plate Ⅰ
Selected Papers on Coal Petrology and Coal Geochemistry in Ren Deyi
Dai Shifeng et al.: Discovery of abnormal enrichment of boehmite in Zhungeer coalfield in northeastern margin of Ordos Basin.
Plate Ⅱ
Selected Papers on Coal Petrology and Coal Geochemistry in Ren Deyi
Discovery of extremely rich boehmite in Zhungeer coalfield, northeastern Ordos Basin.
Dai Shifeng 1 2, Ren Deyi 1 2, Li Shengsheng 2, Chen Linqiu 3.
(1.CUMT Key Laboratory of Coal Resources,100083; 2. Department of Resources and Earth Sciences, China University of Mining and Technology, Beijing,100083; 3. Illinois Geological Survey, IL6 1820, USA)
Abstract: By using X-ray diffraction analysis, scanning electron microscope with energy spectrometer and optical microscope, an extremely rich boehmite and its associated minerals were discovered for the first time in No.6 extra-thick coal seam in Qiongge coalfield in the northeast of Ordos Basin. The content of boehmite is as high as 13. 1%, and the associated minerals are diaspore, zircon, rutile, goethite, galena, chalcopyrite and seleno galena. The heavy mineral assemblage is similar to bauxite in benxi formation, North China. Gaoboehmite in coal mainly comes from thin-shell bauxite in benxi formation, a coal-accumulating basin in Northeast China. The gibbsite colloidal solution migrated from bauxite to peat swamp, and boehmite was formed by compaction and dehydration of gibbsite colloidal solution during peat accumulation and early diagenesis.
Keywords: coal; Bohm stone; Late Paleozoic; Zhungeer coalfield
(This paper was co-authored by Dai Shifeng, Ren Deyi and Li Shengsheng, originally published in Journal of Geology, vol. 80, No.2, 2006).