Mineralogical and geochemical characteristics of pyrometamorphic rocks induced by coal fires in Junggar Basin, Xinjiang, China

Journal Pre-proof Mineralogical and geochemical characteristics of pyrometamorphic rocks induced by coal fires in Junggar Basin, Xinjiang, China

Yu Zhang, Xueqing Zhang, James C. Hower, Sherong Hu PII:

S0375-6742(19)30279-1

DOI:

https://doi.org/10.1016/j.gexplo.2020.106511

Reference:

GEXPLO 106511

To appear in:

Journal of Geochemical Exploration

Received date:

16 May 2019

Revised date:

4 November 2019

Accepted date:

24 February 2020

Please cite this article as: Y. Zhang, X. Zhang, J.C. Hower, et al., Mineralogical and geochemical characteristics of pyrometamorphic rocks induced by coal fires in Junggar Basin, Xinjiang, China, Journal of Geochemical Exploration (2018), https://doi.org/ 10.1016/j.gexplo.2020.106511

This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

© 2018 Published by Elsevier.

Journal Pre-proof

1

Mineralogical and geochemical characteristics of pyrometamorphic rocks induced by coal fires in Junggar Basin, Xinjiang, China *

*

Yu Zhang 1,2 , Xueqing Zhang 1, James C. Hower 3, Sherong Hu1 1.

College of Geoscience and Surveying, China University of Mining & Technology, Beijing 100083,China. 2.

The EMS Energy Institute and Leone Family Department of Energy and Mineral Engineering, The

3.

University of Kentucky Center for Applied Energy Research, 2540 Research Park Drive, Lexington,

Pennsylvania State University, University Park, PA 16802,United States

Kentucky 40511, United States

Jo ur

na

lP

re

-p

ro

of

Abstract: The mineralogical and geochemical characterizations of the pyrometamorphic rocks caused by coal fires are discussed. The minerals in the combustion metamorphic rocks, as analyzed by X-ray diffraction (XRD), are anorthite, hematite, tridymite and cristobalite, quartz in clinkers; and tridymite, sekaniaite, sanidine, mullite, cristobalite, and quartz in paralavas. Tridymite and sekaniaite account for the largest mineral proportion in paralava. The major elements and rare earth elements (REEs) were determined by X-ray Fluorescence (XRF). Combustion metamorphic rocks are characterized by the major elements and REEs. Three methods to evaluate obtained normalized REE distribution patterns were used to evaluate characteristics of combustion metamorphic rocks. Chondrite-normalized distribution characteristics exhibit intense negative anomalies Eu and lack a Ce anomaly. North American Shale Composite (NASC) normalized patterns show a slight negative anomaly in δEu and lack anomalies in δCe for clinkers, but the paralavas show a positive δEu anomaly. Compared with Upper Continental Crust (UCC) normalized patterns, there is a slight difference (LREE-depleted, but HREE-enriched), and it is similar to NASC-normalized patterns. Primitive mantle-normalized trace elements show significant differences in combustion metamorphic rocks, which the clinkers show larger variations in Pb than paralavas, and paralavas have significant negative Dy anomalies. High content of Fe element may result in enrichment in Ni, Co, and Cu. Keywords: Coal fires; Mineralogy; Geochemistry; Clinkers; Paralavas

1. Introduction

Coal fires are a global catastrophe, particularly in coal mining countries such as China, Russia, the United States, Indonesia, Australia, and South Africa 1-5. Large amounts of greenhouse gases (GHGs) and toxic gases (CO, Polycyclic aromatic hydrocarbons/PAHs, Hg, etc.) are emitted to the air

2, 5-7

. In addition to environmental pollution, coal fires reshape geomorphology through

small-scale surface fracturing (fissures, cracks, funnels, and vents), and large-scale surface subsidence (sinkholes, trenches, depressions, and slides) 7. Coal fires are triggered by natural phenomenon and by human activities

7-9

. With the increased scope of human activities and of the

Journal Pre-proof

2

mining scale, the percentage of coal fires caused by the natural environment (lightning, forest fires, or strong solar heating) has decreased. Coal fire formation is an exothermic oxidation reaction process that occurs when coal is exposed to oxygen on the extraction face, coal storage piles, and coal waste piles and goafs 6; when temperatures reach 80-130 °C, the coal begins to burn 8. Considerable amount of related literature has been published in many fields, including geological, geophysical, geochemical, environmental, numerical simulation, remote sensing, and

ro

of

fire-fighting 1, 2, 7, 8, 10-12, but only few papers about the changes of rock by coal fires are available. Combustion metamorphism is a common phenomenon in coal fires due to high temperatures 12, 14, 15

.

reported that coal burning temperatures reached up to 1300 °C. High

-p

Several authors

13

re

temperatures induce changes on the mineral, textural, physical, and chemical characteristics of

lP

rocks in sediments with coal, gas, oil, or bitumen 13, 16. New reactions (such as inversion of quartz

na

to tridymite/cristobalite, and reaction rims with the non-crystalline matrix of silica grains) and rocks (clinker and paralava) are generated

17, 18

, which are an indicator of coal fires. Foit et al.

19

Jo ur

reported the pyrometamorphic assemblages (sandstone, siltstone, and shale producing a multi-colored vesicular rock resembling slag) in near-surface combustion of the Healy coal seam near Buffalo, Wyoming. Querol et al.

20, 21

studied subbituminous coal, the inorganic matter, and

its transformation by combustion experiments, and found some mineralogical and morphological characteristics of the atmospheric particulate matter may be used as tracers. Stracher

22

investigated minerals (sulfates millosevichite, alunogen, coquimbite, voltaite, godovikovite, and an unidentified phase) and mineralization processes (condensation, hydrothermal alteration, crystallization from solution, fluctuating vent temperatures, boiling, and dehydration reaction) in coal-fire gas vents. Engle et al.

23

detected the common minerals in soils and clastic sediments,

Journal Pre-proof

3

including osumilite, cristobalite, hematite, quartz, calcite, and plagioclase. Gürdal et al.

24

investigated the properties of coals contributing to spontaneous combustion and to the combustion by-products. They reported that the coal contained the pyrite, quartz, cristobalite, tridymite, kaolinite, and gypsum, but some geochemical properties of pyrometamorphic rocks were not addressed. Pone et al.25 focused on the coal-fire-gas minerals generated by the spontaneous

of

combustion, noting that gas phases condensed to new minerals with decreasing temperature. The mineral paragenesis of the fired coal gangue (cristobalite, mullite, hematite, trydimite, cordierite) 26

ro

indicated a combustion temperature of 1200 °C 12. Gatel et al.

investigated the mineralogy and

-p

petrology of oil-shale slags in Lapanouse, France. They illustrated the mineralogical diversity in

re

the slags, but the classification scheme is incomplete (e.g. without minerogenetic stages and

lP

complete combustion).

na

Various names have been applied to the heated rocks. Cosca et al.

27

called unmelted rocks

"burnt rocks" or "clinker", in which rock color changes from its initial color to orange or red, and 28

Jo ur

the solidified melt resembles igneous rocks called "paralava". Liu

claimed that protolith rocks

should be considered in naming combustion metamorphic rocks, such as burnt-mudstone. Huang and Liu 29 noted that pyrometamorphic rocks should be named as paralavas and clinkers. Zhang et al.30 classified them into baked rocks, baked–melted rocks, and melted rocks. Grapes

31-34

redefined the burnt, unmelted, reddish rocks as clinker, and the melted glassy rocks as paralava. In terms of detailed research of pyrometamorphic rocks, Clark et al.

17

described the changes of

pyrometamorphism and partial melting of shales during combustion metamorphism in terms of mineralogy and texture. Pyrometamorphic rocks were discussed terms of petrography and mineralogy with respected to the oxidation of a pyritic lignite seam in the Erin Formation, SW

Journal Pre-proof

Trinidad

4

35

. Ciesielczuk et al.36 conducted three types of heating experiments to reveal

crystallization processes of the particular pyrometamorphic minerals for shales and carbonate rocks. Changes in mineralogy, textures, and glass composition were discussed in paralava and clinker in Shanxi Province, China, and lithologies and geochemistry also were used to characterize the paralava and clinker

31

. Surface thermal anomalies, pyrometamorphic rocks, and barren

patches of land or soil with locally different emissivities are common in coal fire areas 6, 22. New

ro

of

techniques are being used to capture structural features such as rock and coal 37-42. Sokol et al. 43, 44 studied the mineralogy of annealed and fused waste rocks via optical microscopy, X-ray

-p

diffraction (XRD), X-ray Fluorescence (XRF) and microprobe analyses, and some rare minerals

re

were found (tridymite, crystobalite, mullite, K-bearing cordierite, K-Mg-osumilite, and Fe3+- and

lP

Al-rich Caclinopyroxene, as well as hexagonal and orthorhombic analogues of anorthite).

na

Major elements and rare earth elements (REEs) also are used to evaluate clinkers and paralavas by quantitative chemical analyses of the synthesized mixtures. Ciesielczuk et al.36

Jo ur

proved that mineralogies are similar to the rocks found in the burning coal-mine dumps. Concentrations of major, trace, and rare elements in the coal gangue and in the different lithologies were used to illustrate environmental characteristics of coal gangue dumps

14

. The

distribution and concentration of REEs are significant in geochemistry, which provide information about material sources, diagenetic environment, sedimentary environment, origin, and evolution 45-47

.

Chondrites are considered the original chemical mixture from which the Earth was formed

48

. Taking such a mixture as a standard for construction can clearly reflect the sample change from

the original composition of the earth 49. The smoothness of a normalized distribution pattern (to Upper Continental Crust, UCC; and North American Shale Composite, NASC)

49

provides a

Journal Pre-proof

5

simple, but reliable, basis for testing the quality of REE chemical analyses of sedimentary rocks 50. In this study, samples from eight clinkers and paralavas produced by coal combustion in Xinjiang, China, were selected for X-ray diffraction (XRD), optical microscopy, and X-ray Fluorescence (XRF). The study provides detailed information about pyrometamorphic rocks caused by coal fires, and contributes to the knowledge of coal fire by determining location and

of

burning intensity.

2. Geological setting and methods Geological setting

ro

2.1

-p

Xinjiang is the largest provincial autonomous region in north-west China with an area of 1.66

re

million square kilometers, accounting for one sixth in land area of China, located at 34°25′- 48°10′

lP

north and 73°40′- 96°18′ east (Fig. 1). There are many coal fire areas in China, especially in 31

.

na

northern China, extending ∼5000 km in an E-W direction and ∼750 km in an N-S direction

The coal accumulation process in Xinjiang occurred from the Paleozoic Carboniferous to the

Jo ur

Mesozoic Jurassic, among them, the coal-bearing strata and coal seams are mainly distributed in the early and middle Jurassic strata with large exposed area, wide distribution, large coal seam layers, large single-layer thickness and large resource potential

51, 52

. Tianshan Mountains of

Xinjiang (Fig. 1) has some of the most serious coal fires in China. Eight samples were collected from Junggar basin: Wucaiwan coal mine (WCW) in eastern Junggar basin, Sikeshu coal mine (SKS) in northern Junggar basin, Baiyingou (BYG), Dahuangshan (DHS), Shuixigou (SXG), No.105 (105), Fukang (FK), and Hoxtolgay (HT) mines in southern Junggar.

Journal Pre-proof

6

2.2 Methods 2.2.1 Petrologic analysis The fresh rock samples were selected for preparation. A thin sliver (0.03 mm) of rock was cut from the sample and ground optically flat. Those samples were mounted on glass slides and then ground smooth using progressively finer abrasive grits. The size of rock thin sections was 22×22×0.03 mm, and the thickness was determined using quartz as the optical gauge

of

(Michel-Lévy interference colour chart). The Leica DM4500 P, a high-end polarization

ro

microscope with intelligent light and contrast management, was used to determine the rock fabric

Jo ur

na

lP

re

-p

and extent and characteristics of the pyrometamorphic rocks.

7

Jo ur

na

lP

re

-p

ro

of

Journal Pre-proof

Fig. 1 pyrometamorphic rocks sampling location and coal-fire distribution map of Xinjiang Region, China 52, 53. 2.2.2 Mineral analysis Eight rock samples (paralavas and clinkers) were collected from the coal fires area in Xinjiang, China. The samples were pulverized to a maximum size of 200 mesh, then, dried at 105°C for 2 hours. The mineral composition of the sample were analyzed by X-ray diffraction using a Rigaku D/max-2500/PC XRD diffractometer with Ni-filtered Cu-Kα radiation and a scintillation detector at the China University of Mining & Technology, Beijing. The X-ray

Journal Pre-proof

8

intensities are measured in the range of 2θ=2.5-70°with a scanning rate of 4°/min operating at 40 kV and 150 mA. The XRD patterns of the samples were used to quantify the mineral composition utilizing Siroquant, commercial interpretation software developed by Taylor

54

based on the

Rietveld refinement approach 55. 2.2.3 Element distributions Analysis of major and trace element compositions were conducted in Analytical Laboratory

of

Beijing Research Institute of Uranium Geology (ALBRIUG). Rock samples were first pulverized

ro

to powder to pass through 200 mesh (75 μm) prior to mineral analysis and element distributions.

-p

Then, the powdered samples were dried at 105°C for 2 hours. 0.7 g of sample powder was

re

weighed and mixed with 5.2 g lithium tetraborate (Li2B4O7), 0.4 g lithium fluoride, and 0.3 g

lP

ammonium nitrate at 1150–1250 °C for 10–15 min.

Major elements were analyzed by XRF (Philips PW2404) on fused glass beads with an

na

excitation condition of 50 kV/50 mA and 30-mm diameter of viewed light beam, and analytical

Jo ur

precision is generally better than 2 ‰. Gravimetry was used to measure the loss on ignition when the samples of temperature were heated to 1100 °C. The major elements analysis was carried out according to Chinese National Standard (GB/T14506.30-2010). As for trace element compositions (La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, Rb, Ba, Th, U, K, Pb, Sr, P), 50 mg of sample powder was dissolved in distilled HNO3 + HF (1 mL + 0.5 mL), and then ultrasonically stirred. Thereafter, the solutions were evaporated to dryness and the residue was dissolved with HNO3 + HF (1.5 mL + 0.5 mL). Subsequently, the solutions were heated at 130 °C for 3h, and the solutions were diluted to 50 ml using ultrapure H2O. Those solutions were analyzed by Element XR Inductively-coupled plasma-mass spectrometry (Element XR/ICP-MS). The analytical uncertainties are less than 7%, estimated from analyses of two

Journal Pre-proof

9

Chinese National standards (GBW 07106 and GBW07312).

3. Results 3.1 Macroscopic characteristics of combustion metamorphic rocks Combustion metamorphic rocks are evidently different from regional and magmatic thermal metamorphic rocks. Types of mineralization were divided into high-temperature (>800 °C), mid-temperature (~500-800 °C), and low-temperature (<500 °C)

56

. For the high-temperature

of

stage, the new rocks (clinkers, buchites, parabasalts, and slags) are generated; for mid-temperature

4, 56, 57

. Therefore, several textures are defined in combination

-p

supergene alterations are generated

ro

stage, gas condensation and gas-waste interaction are generated; and for the low-temperature,

re

with the textures of sedimentary, metamorphic, and magmatic rocks.

lP

1) Residual structures preserve the textures of protoliths found in clinkers as presented in Fig.

low-temperature process.

na

3a. Protolith (mudstone) can be clearly observed by the naked eye, corresponding to a

Jo ur

2) Pore structures occur because combustible or volatile materials are burned or volatilized, and the rock surface produces pores, corresponding to a mid-temperature process. Pore structures are shown in almost all combustion metamorphic rocks, as depicted in Figs. 2b-e. 3) Breccia structures of particles with a size of 2–64 mm produced by the collapse of combustion sinter and deposit are named microbreccia. Macrobreccia differ in particle size (>64 mm) (Figs. 2c-d). Breccia structures correspond to high-temperature processes. 4) Melted structures form from rocks melted at high temperatures and generate new rocks (slag/paralava), as shown in Fig. 2e.

10

ro

of

Journal Pre-proof

re

-p

Fig. 2 Structure of combustion metamorphic rocks a) clinker with residual structures; b) buchite with pore structures; c) Microbreccia with clinker; d) Parabasalt with breccia; and e) slag with melted structures Note: the coin and the watch with 24mm; the pen is 140mm in length and 20mm in width.

lP

Optical microscopy was conducted to acquire information about texture, rims, and paragenesis. It is difficult to distinguish the rims. Cracks may be caused by volume-change and a

Jo ur

seen in Fig. 3.

na

likely temperature-related event in coal fires, as shown in Fig. 3 b) and h). SiO2 polymorphs are

Journal Pre-proof

re

-p

ro

of

11

lP

Fig. 3 The optical microscopy for clinkers and paralavas, Brown: decomposed organic matter and iron oxide products, White: SiO2 polymorph (most minerals are hard to distinguish)

na

3.2 Minerals in rocks

Eight XRD patterns of metamorphic rocks were used to identify the mineral species and

Jo ur

quantify the mineralogical compositions by XRD and Siroquant. The minerals in the eight metamorphic rocks are mainly anorthite, hematite, tridymite, cristobalite, sekaninaite, sanidine, quartz, mullite, illite, cordierite, and anatase (their relative concentrations are listed in Table 1). Tridymite and sekaninaite are common minerals in the paralava, shown in Fig. 4. Table 1. Mineralogy of pyrometamorphic rocks using Siroquant (wt.%) Clinker

Paralava

Mineral 1-WCW

2-BYG

3-SKS

4-DHS

5-105

6-SXG

7-FK

8-HT

Anorthite

9.0

7.0

–

13.6

–

–

–

–

Hematite

72.3

–

–

11.8

–

–

7.1

–

Tridymite

7.4

31.2

–

–

40.9

39.4

35.1

27.7

Cristobalite

11.3

8.1

–

–

–

13.4

6.0

8.3

Sekaninaite

–

–

–

–

34.7

9.4

18.3

24.6

Cordierite

–

–

–

–

13.1

–

17.1

15.2

Sanidine

–

–

–

–

–

15.0

6.2

-

Journal Pre-proof

12

–

44.8

71.8

–

2.1

14.9

4.6

18.4

–

8.9

–

–

6.2

7.8

5.2

5.8

Illite

–

–

26.2

–

3.0

–

–

–

Anatase

–

–

2

–

–

–

–

Calcite

–

–

–

68.5

–

–

–

– –

Augite

–

–

–

6.1

–

–

–

–

Qtz 1500 Trd Trd

Aug Hem Cal Cal An Cal Hem Cal Aug Cal Cal

1000 0

30

2

40

50

Ill Crd Ill

Crd

Mul Crd

10

Crd

Ab

20

30

600 400 200 0 0

Mul Sek Qtz Crd Sek Ank S Mul Qtz Trd MulQtz Trd

40

2θ/°

Sek Sek TrdQtz Hem MulTrd Crd Hem Sek Trd Ab Mul Trd Hem Hem Mul Hem Qtz Trd Mul Sek AbCrd

800

10

20

30

40

2

0

70

10

50

60

70

20

50

60

30

40

Qtz QtzQtzQtzQtz 50

70

Qtz

Trd Crs Trd Mul

1000

Trd

500 Sek

Mul

Ab SekTrd

Sek

Mul

Mul

Sek

Qtz

0

70

60

2

(f)

1500

Mul

Sek

Sek

Trd

1000

60

2500

1000

Trd

1200

50

Qtz Qtz Ill

Ill Ant

0

2000

0

70

40

2

Qtz

Qtz

Trd An Crs An Trd

Trd

(g)

1400

Intensity (counts)

60

Qtz

4000 2000

Mul

re

20

30

lP

10

20

(e)

1800 1600 1400 1200 1000 800 600 400 200 0

na

0

10

Intensity (counts)

2000

0

Mul

6000

ro

3000

Mul

Qtz 8000

of

500

Crs Mul Qtz Qtz

Intensity(Counts)

Intensity(counts)

Intensity (counts)

Cal

Crs An

-p

(d)

4000

Mul Trd

1000

0

(c)

Qtz

2000

2θ/°

5000

10000

(b)

Intensity(Counts)

2500

1800 (a) Hem 1600 1400 Hem 1200 1000 Hem 800 Hem Crs 600 Hem Hem Hem An 400 Crs Crs Trd Trd Hem 200 T T 0 0 10 20 30 40 50 60 70

Intensity (counts)

Intensity (counts)

Quartz Mullite

0

10

20

30

40

2θ/°

50

Qtz Sek Mul Qtz 60

70

(h)

Trd Qtz Crd

800 600

Sek

Sek

400

Qtz Trd

200

Crd Mul

Sek

Trd Trd

0 0

10

20

Mul Sek Trd Sek

30

Mul

Crs

Qtz Qtz Qtz TrdSek Sek Crd Crd Crd

40

50

60

70

2/

Jo ur

Fig. 4. XRD diagrams of a-c) clinkers and d-h) paralavas. Anorthite-An, Hematite-Hem, Tridymite-Trd, Cristobalite-Crs, Sekaninaite-Sek, Cordierite-Crd, Sanidine-Sa, Quartz-Qtz, Mullite-Mul, Illite-Ill, Anatase-Ant, Calcite-Cal, Augite-Aug

3.3 Major elements

The percentages of major elements are shown in Table 2. Major element oxides in the samples are dominated by SiO2, Al2O3, Fe2O3, and CaO. Two distributions were expressed in all rock samples data, i.e., high-SiO2 and low-SiO2. The proportion of SiO2 ranges from 23.61 to 80.07%. Beside the sample of No.1 and No.4, the element percentages of SiO2 in the other samples are higher than those world clays58, 59. The Fe2O3 concentration ranges from 1.11 to 35.48%, and the highest value being in the low-SiO2 rocks. The CaO concentration ranges from 0.19 to 23.27%, the high being in the low-SiO2 samples. The element percentages of CaO in No.1

Journal Pre-proof

13

and 4 are higher than those for world clay58, 59, but other samples are lower than those for world clay58, 59. The Al2O3 concentrations have small variation, with average values of 15.51%, which is close to the world clay58, 59. Table 2． Major elements of pyrometamorphic rocks (clinkers and paralavas) Xinjiang and Shanxi (whole-rock basis, %). SiO2

Al2O3

Fe2O3

MgO

CaO

Na2O

K2O

MnO

TiO2

P2O5

LOI

SiO2/ Al2O3

1

24.25

8.09

35.48

0.53

10.24

1.43

0.49

0.42

0.42

0.79

7.24

3.00

2

72.10

17.81

1.54

1.13

0.24

0.46

2.97

0.03

0.78

0.07

1.04

4.05

3

75.01

18.16

1.37

0.72

0.19

0.57

1.82

0.01

0.84

0.06

0.76

4.13

4

23.61

11.11

21.56

2.72

23.27

1.09

0.34

0.33

0.41

0.10

14.34

2.13

5

62.28

17.89

2.28

1.50

0.79

0.33

1.25

0.13

0.85

0.07

4.77

3.48

6

80.07

11.51

1.12

0.60

0.95

0.96

2.21

0.02

0.40

0.04

0.73

6.96

7

69.57

14.91

5.56

1.16

1.11

1.88

2.98

0.07

0.83

0.03

1.37

4.67

8

69.44

15.16

1.11

1.83

0.771

0.92

2.71

0.11

0.69

0.13

2.47

4.58

9*

65.63

21.37

2.02

1.44

1.80

0.03

2.26

0.13

0.07

0.30

0.64

3.07

10*

59.31

19.08

10.76

2.35

5.38

0.01

1.60

0.29

0.69

0.42

0.51

3.11

re

-p

ro

of

Sample

3.4 Trace elements

lP

9*. Siltstone Fused siltstone (clinker); 10*. Paralava * Data from Rodney Grapes 31.

na

The concentrations of trace elements are listed in Table 3 from 8 analyses

60

. The

Jo ur

distributions of the total amount of REE (∑REE) in pyrometamorphic rocks range from 98.0 to 219.16 μg/g. The mean values are 159.16μg/g for clinkers and 157.43 μg/g for paralavas, which are lower than in common Chinese whole coals (162.51 μg/g)

61

and the NASC (173.41 μg/g) 62.

Sample No.1 (low-SiO2) shows the lowest concentration of the ∑REE for clinkers, however, No. 4 (low-SiO2) shows the highest concentration of the ∑REE for paralavas. Compared with other high-SiO2 samples, clinkers show a higher ∑REE for clinkers than those of paralavas. The ratios of LREE/HREE range from 3.82 to 7.54, which were lower than in the Shaanxi’ clinker (No. 9), but the paralavas were higher than Shaanxi’ paralava (No. 9). The concentration of the light rare earth elements (LREEs) is higher than those of heavy rare earth elements (HREEs), which are in accordance with the clastic rocks in West Junggar, Xinjiang 63, 64.

Journal Pre-proof

14

Table 3. The REE of pyrometamorphic rocks (clinkers and paralavas) Xinjiang and Shaanxi (whole-rock basis, μg/g). Sample

1

2

3

4

5

6

7

8

9*

10*

La

20.00

35.90

33.20

49.30

19.50

31.50

23.70

33.20

20

12.12

Ce

37.80

73.80

64.30

84.00

38.00

70.50

46.30

67.50

43.3

25

Pr

4.72

8.36

7.81

9.70

4.12

8.13

5.72

8.18

5.02

2.7

Nd

23.60

35.20

34.00

40.70

17.50

35.00

24.40

33.70

20.4

10.72

Sm

5.08

6.85

7.07

8.02

2.99

6.60

4.47

6.03

3.94

2.18

Eu

1.46

1.28

1.41

1.78

0.74

1.52

1.12

1.41

0.85

0.47

Gd

7.00

7.63

7.14

7.61

3.10

6.38

4.42

6.63

3.79

2.21

Tb

1.41

1.35

1.24

1.37

0.74

1.13

0.82

1.25

0.54

0.36

Dy

7.30

6.96

6.37

7.19

4.41

5.68

4.31

6.30

2.91

2.13

Ho

1.29

1.30

1.34

1.29

0.78

1.03

0.77

1.16

0.59

0.46

3.53

4.14

3.80

3.85

2.61

3.02

2.47

3.50

1.56

1.21

0.49

0.61

0.52

0.55

0.40

0.45

0.39

0.49

0.24

0.19

Yb

2.82

4.29

3.61

3.28

2.70

3.03

2.81

3.62

1.7

1.3

Lu

0.40

0.60

0.51

0.52

0.41

0.43

0.39

0.53

0.27

0.21

ro

of

Er Tm

116.90

188.27

172.32

219.16

98.00

174.40

122.09

173.5

105.11

61.26

92.66

161.39

147.79

193.50

82.85

153.25

105.71

150.02

93.51

53.19

HREE

24.24

26.88

24.53

25.66

15.15

21.15

16.38

23.48

11.60

8.07

LREE/HREE

3.82

6.00

6.02

7.54

5.47

7.25

6.45

6.39

8.06

6.59

re

Chondrite-normalized data

-p

ΣREE LREE

5.09

6.00

6.60

5.18

7.46

6.05

6.58

7.95

6.30

δEu

0.75

0.54

0.61

0.70

0.74

0.72

0.77

0.68

0.66

0.65

δCe

0.95

1.04

0.98

0.94

1.04

1.08

0.97

1.00

0.99

0.99

LaN/YbN

0.67

0.79

δEu

1.15

0.83

δCe

0.92

1.01

LaN/YbN δEu δCe

1.42

0.68

0.98

0.80

0.86

1.11

0.88

0.93

1.07

1.14

1.10

1.18

1.05

1.03

1.01

0.95

0.91

1.01

1.05

0.94

0.97

1.03

1.04

Jo ur

UCC-normalized REE

0.87

na

NASC-normalized REE

10.78

lP

LaN/YbN

6.68

7.89

8.67

14.16

6.81

9.80

7.95

8.64

11.09

8.79

0.81

0.58

0.65

0.75

0.80

0.77

0.83

0.74

0.73

0.71

0.81

0.76

0.73

0.80

0.83

0.75

0.77

0.82

0.83

0.74

29

* Data from Huang Lei .

Journal Pre-proof 1000

1 2 3 4 5 6 7 8

100

Concentration coefficients

15

10

1

0.1

0.01

of

Li Be Sc V Cr Co Ni Cu Zn Ga Rb Sr Y Mo Cd In Sb Cs Ba La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu W Tl Pb Bi Th U

Fig.5. Concentration coefficients (CC) of trace elements in the pyrometamorphic rocks,

ro

normalized by average trace element concentrations in the world clay59

-p

The pyrometamorphic rock in the No.2 shows significantly enriched in Cd, In, Sb, Tl, Pb,

re

and Bi (more than 10), especially in Bi, the concentration coefficients amount to 487, which is

lP

pretty high than the world clay59. The most elements (Li, Be, Sc, Co, Ni, Zn, Rb, Cd, In, Sb, Cs, Tl,

na

Pb, Bi) are depleted in pyrometamorphic rocks (relative to the average for world clays). The chondrite-normalized REE patterns of pyrometamorphic rocks values show a negative

Jo ur

anomaly in δEu and no anomalies in δCe (Fig. 6a). The low-SiO2 clinker shows a different characteristic with other clinkers (high δEu), however, the high-SiO2 clinker shows a lower values of δEu than that of paralavas. The (La/Yb)N values range from 5.09 to 10.78. Besides the low-SiO2 clinkers, the NASC REE patterns of clinkers show a negative anomaly in δEu and no anomalies in δCe (Fig. 6b), but the paralavas show a positive anomaly in δEu and the (La/Yb)NASC values are <1, except for the sample No.4 (Fig. 6c). The Upper Continental Crust (UCC) patterns are a slightly depleted in LREE, but are enriched in HREE. The UCC-normalized REE patterns present a slightly negative anomaly in δEu for clinkers and a positive anomaly in δEu or clinkers for paralavas, which is similar to NASC. The primitive mantle-normalized trace element plot (Fig. 6d)

Journal Pre-proof

16

shows enrichment in Rb, Ba, U, and Nd, and depletion of Th, Sr, P, and Ti. The clinkers and paralavas show similar trace element patterns, but the clinkers show larger variations in Pb than paralavas. Paralavas have significantly negative Dy anomalies.

Clinker Paralavs 1 4 2 5 3 6 9 7 8 10

2.5 2.0 1.5

of

100

1.0 0.5

La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu

ro

10

-p

La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu

lP

re

Rock/Primitive Mantle

Clinkers Paralavas 1 4 10000 2 5 3 6 1000 9 7 8 10 100

100 c)

10

na

REE/UCC

Clinkers Paralavas 4 1 5 2 6 3 7 9 8 10

(b)

REE/NASC

REE/Chondrite

(a)

Clinkers Paralavas 1 4 2 5 3 6 7 8

10

1 RbBaTh U K La Ce Pb Pr Sr P NdSmEu Ti Dy Y HoYbLu

Jo ur

La Ce Pr NdSmEu Gd Tb DyHo Er TmYb Lu

(d)

Fig. 6 a) NASC-normalized of REE distribution patterns, b) UCC-normalized of REE distribution patterns, c) primitive mantle-normalized trace element patterns, d) clinkers and paralavas. Chondrite-normalized (Sun and McDonough (1989), and NASC-normalized and UCC-normalized (Taylor and McLennan, 1985) Note: 1. the values of Clinker (No. 9) and paralava (No. 10) are from Huang Lei 29.

4. Discussion 4.1Minerals 4.1.1 SiO2 polymorphs Tridymite, cristobalite, and quartz are SiO2 polymorphs. SiO2 polymorphs have a low content in clinkers shown in Fig. 7a), however, high-SiO2 polymorphs are shown in paralava Fig. 7b). Higher contents of quartz and lower amounts of tridymite are present in clinkers than paralava.

Journal Pre-proof

17

The quartz transforms to the cristobalite and tridymite in high-temperature processes 65. Cosca et al. 27 indicated that even minor (0.1-0.3 wt.%) additional components (Al, Fe, and Ti) in the SiO2 phases may lead to significant the transformation of quartz to cristobalite or tridymite, as with tridymite in silica bricks. Therefore, rapid changes in temperature may result in solid solutions 43, 44

presenting metastable formation of any of the polymorphs. Sokol et al.

point out that

of

tridymite-bearing assemblages in pyrometamorphic rocks were formed at a higher temperature

ro

than quartz-bearing assemblages. High proportions of cristobalite were also considered to be the result of the thermal metamorphism of quartz, starting at about 900 °C

35

. The silica such as

-p

cristobalite and tridymite were also found in clinker and paralava, which may indicate a high

re

temperature (767-1125 °C) 66.

80

100

60

na

Mineral content (%)

80

40

0 Trd

Jo ur

20

Crs

Hem Minerals

Cal

An

Mineral content (%)

lP

Clinker Paralava

Clinkers Paralavas 5 2 6 3 7 8

b)

70 60 50 40 30 20 10 0

Aug

Qtz Trd Crs Mul Ill

Sek Crd Hem An

Ab Ant

Minerals

Fig. 7. Mineral percentages a) low-SiO2, and b) high-SiO2. Qtz, quartz; Trd, tridymite; Crs, cristobalite; Hem, hematite; Sek, sekaninaite; Crd, cordierite; Ill, illite; Mul, mullite; Cal, calcite, Aug, augite; Sa, Sanidine; An, anatase. 4.1.2 Fe-bearing precursor minerals The results of numerous analysis approaches 67-70 showed that decomposition products varied depending on environment

70

. Pyrite transforms to the hematite when temperature reach

300-530 °C 67, 69, 71. Hematite is one of the dominant mineral phases in coal ash deposits at a probe temperature of 500°C 72. Therefore, hematite also may be considered as an indicator that is used to evaluate the temperature of coal fires. When the presence of hematite is found in

Journal Pre-proof

18

pyrometamorphic rocks, it can be speculated that the combustion temperature at that time was 500°C. 4.1.3 Sekaninaite-cordierite Fig. 6b shows that sekaninaite and cordierite are found in paralava, which agree with other research

31, 33

. Sekaninaite is considered to be a typical product of combustion metamorphism 33,

and it typically occurs in paralava when temperature is higher than 1000 °C

73

. Sekaninaite,

of

cordierite, and tridymite together constitute the main components of the SiO2-rich paralava.

ro

Other Si-bearing or Al-bearing, Kaolinite metamorphosed to mullite under high temperature 17

-p

, which is also represented in paralava as an indicator of high-temperature.

re

4.1.4 Ca-bearing phases

The presence of anorthite in clinkers may imply that the high-temperature transformation

lP

products of fluxing minerals (calcite) included in the coal reacted with the high-temperature

na

transformation products 74. 4.2 Major element concentrations

Jo ur

For the high-SiO2 metamorphic rocks, the average Fe2O3 and FeO contents amount to 2.56 % and 0.73 % for clinkers, however, the paralava samples show a higher content than that of clinker samples, showing 6.08 % of Fe2O3 and 3.24 % of FeO. The difference may be attributed to strong iron ore enrichment, especially in the formation process of paralava when temperatures reach a certain range (∼580−600 °C)

75

. For instance, in the Heshituoluogai coal fire area in Xinjiang,

magnetic limonite is transformed into hematite during the action of coal fires

8, 76

. This

characteristics is also shown in Shanxi’s pyrometamorphic clinkers and paralavas 31, as presented in Table 2. The SiO2/Al2O3 ratios for pyrometamorphic rocks are much higher than the theoretical

Journal Pre-proof

19

SiO2/Al2O3 ratio of kaolinite (1.18), indicating the existence of free SiO2 in the rocks and consistent with the relatively high proportions of quartz in the mineral matter77. From the Fig. 8, CaO contents show a good linear relationship with the LOI (R2=0.93). Some Samples with high CaO and LOI may due to some containing Ca material (CaCO3 or clay) produces CO2 or water vapour. 16 Y=0.0554X+1.49

ro

10 8 6

-p

LOI (%)

12

of

14

4

0 0

5

re

2 10

15

20

25

lP

CaO (%)

na

Fig. 8. The relationship between CaO and LOI 4.3 REE and other trace elements

Jo ur

Fig. 6 shows that the distribution pattern of rare earth elements is consistent, indicating that each sample has a common source and a similar genetic mechanism 78. The clinker data of δEu are higher than the paralavas, but the rocks in the northeastern Ordos Basin exhibit different characteristics with those in Xinjiang. The distribution patterns of Ce and Eu are mainly controlled by the temperature, pressures, and speciation of REEs, especially at higher temperatures 75, 76, 78-81. The δEu has two valence states: Eu2+ and Eu3+; Eu3+ always exists in normal circumstances, however Eu3+ can change into Eu2+ at high temperature

82-84

. The negative anomalies in δEu and

lack of anomalies in δCe may be caused by the high temperature. The oxygen fugacity required for the redox of Ce4+/Ce3+ increases with increasing temperature, and the Ce anomaly is almost

Journal Pre-proof

impossible at high temperature

85

20

. The geochemical characteristics of rare earth elements can be

used to infer the formation environment of pyrometamorphic rocks. The NASC-normalized REE patterns shows a flat trend without an obvious anomaly (Fig. 6), which is similar with shale-normalized REEs patterns49, indicating that rock samples were similar in sedimentary environment and epigenetic evolution 63. Unlike the Chondrite-normalized results, the NASC-normalized REE patterns for clinkers present a slight negative anomaly in δEu and

of

have no δCe anomalies, but the paralavas show a positive anomaly in δEu. Temperature is

ro

considered to be a dominant factor controlling the species of REE 80. Positive anomalies in δEu for

-p

paralavas indicate that more soluble Eu2+ was formed during high-temperature combustion

re

(>250 °C) 80, 86.

lP

For the low-SiO2 samples (No.1 and 4), rich elements (Ni, Co, Cu) are considered as trace

na

elements, which is related to iron deposit (Fig.9a). The places in samples of No.1 and 4 may be potential iron deposit. Compared with low-SiO2 samples, the Ni, Co, and Cu elements were

Jo ur

depleted (Fig.9b) in high-SiO2 samples, which may be due to low Fe-containing. Cd, In, Sb, Bi, Pb, and Tl enrich in No. 2, which may be due to the dark and especially the black shales have the highest values and a large dispersion of Cd, Bi, Tl and Pb87. However， those elements (Cd, In, Sb, Bi, Pb, and Tl) are more closely related to sulfur than to organic carbon, which are presumably related to sulfide minerals 87. The place collected from No.2 may contain commercially valuable sulphide deposit.

Journal Pre-proof

21

Concentration coefficients

1000

1 4

100

10

1

0.1

0.01

ro

b)

100

-p

10

re

1

2 3 5 6 7 8

0.1

lP

Concentration coefficients

1000

of

Li Be Sc V Cr Co Ni Cu Zn Ga Rb Sr Y Mo Cd In Sb Cs Ba La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er TmYb Lu W Tl Pb Bi Th U

0.01

na

Li Be Sc V Cr Co Ni Cu Zn Ga Rb Sr Y MoCd In Sb Cs Ba La Ce Pr NdSmEu Gd TbDyHo Er TmYb Lu W Tl Pb Bi Th U

Fig. 9. Concentration coefficients (CC) of trace elements in a) the low-SiO2 samples, b) the

Jo ur

high-SiO2 samples normalized by average trace element concentrations in the world clay

5. Conclusions

In this paper, the products of coal fires (pyrometamorphic rocks) are discussed and classified, the main conclusions are as follows: (1) Tridymite, cristobalite, and quartz (SiO2 polymorphs) were also determined in clinker and paralava, indicating that clinker formed in the temperature ranging from 767 to 1125 °C. Sekaninaite and cordierite are rich in paralava, which revealed that the pyrometamorphic rocks experienced a high temperature (more than 1000 °C). At temperatures >300 °C, pyrite can

Journal Pre-proof

22

transform into hematite. Mullite is formed by recrystallization of kaolinite at high temperature. (2) The rocks show negative anomalies in δEu and without anomalies in δCe for chondrite-normalized REE. The clinkers show larger variations in Pb than paralavas. Paralavas have significantly negative Dy anomalies, however clinkers have positive Dy anomalies. High content of Fe element may result in enrichment in Ni, Co, Cu. The Cd, In, Sb, Bi, Pb, and Tl are

of

more closely related to sulfur than to organic carbon, which are presumably related to sulfide minerals. The place collected from No.2 may contain commercially valuable sulphide deposit.

ro

Acknowledgements

-p

This work is supported by the National Natural Science Foundation of China (Nos. 41672153

re

and 41430640) and Strategic Priority Research Program-Climate Change: Carbon Budget and

lP

Related Issues” of the Chinese Academy of Sciences, Grant No. XDA05030201. Thanks are also due to candidate Dr. Xiaoyun Yan (China University of Mining & Technology, Beijing) and

Author Information

na

Congjun Huang (Chengdu University of Technology) their technical assistance.

References 1.

Jo ur

Corresponding Author：Sherong Hu and Yu Zhang

Song, Z. Y.; Kuenzer, C.; Zhu, H. Q.; Zhang, Z.; Jia, Y. R.; Sun, Y. L.; Zhang, J. Z., Analysis of

coal fire dynamics in the Wuda syncline impacted by fire-fighting activities based on in-situ observations and Landsat-8 remote sensing data. International Journal of Coal Geology 2015, 141, 91-102. 2.

van Dijk, P.; Zhang, J.; Jun, W.; Kuenzer, C.; Wolf, K.-H., Assessment of the contribution of

in-situ combustion of coal to greenhouse gas emission; based on a comparison of Chinese mining information to previous remote sensing estimates. International Journal of Coal Geology 2011, 86, (1), 108-119. 3.

Stracher, G. B., Coal fires burning around the world: A global catastrophe. International Journal

of Coal Geology 2004, 59, (1-2), 1-6. 4.

Stracher, G. B., Coal fires burning around the world: Opportunity for innovative and

interdisciplinary research. GSA Today 2007, 17, (11). 5.

Kuenzer, C.; Zhang, J.; Sun, Y.; Jia, Y.; Dech, S., Coal fires revisited: The Wuda coal field in the

aftermath of extensive coal fire research and accelerating extinguishing activities. International Journal of Coal Geology 2012, 102, 75-86.

Journal Pre-proof 6.

23

Kuenzer, C.; Stracher, G. B., Geomorphology of coal seam fires. Geomorphology 2012, 138, (1),

209-222. 7.

Stracher, G. B.; Taylor, T. P., Coal fires burning out of control around the world: thermodynamic

recipe for environmental catastrophe. International Journal of Coal Geology 2004, 59, (1-2), 7-17. 8.

Song, Z.; Kuenzer, C., Coal fires in China over the last decade: A comprehensive review.

International Journal of Coal Geology 2014, 133, 72-99. 9.

Zhang, Y.; Yu, M.; Huang, P.; Song, W.; Zhang, T.; Hu, S., Impacting factors of coal seam

spontaneous combustion based on principal component analysis (PCA)—— a case study in Wuda coalfield. Coal geology of China 2015, 27, (5), 12-15. 10. Sun, Q.; Sun, B.; Sun, F.; Yang, Q.; Chen, G.; Yang, M., Accumulation and Geological Controls of Low-Rank Coalbed Methane in Southeastern Junggar Basin. Geological Journal of China Universities 2012, 18 (3), 460-464.

of

11. Wang, H.; Dlugogorski, B. Z.; Kennedy, E. M., Analysis of the mechanism of the low-temperature oxidation of coal. Combustion and Flame 2003, 134, (1-2), 107-117.

ro

12. Ribeiro, J.; da Silva, E. F.; Flores, D., Burning of coal waste piles from Douro Coalfield (Portugal): Petrological, geochemical and mineralogical characterization. International Journal of Coal

-p

Geology 2010, 81, (4), 359-372.

13. Bentor, Y. K.; Kastner, M.; Perlman, I., Combustion metamorphism of bituminous sediments and

re

the formation of melts of granitic and sedimentary composition. Geochimica et Cosmochimica Acta 1981, 45, (11), 2229-2255.

14. Querol, X.; Izquierdo, M.; Monfort, E.; Alvarez, E.; Font, O.; Moreno, T.; Alastuey, A.; Zhuang,

lP

X.; Lu, W.; Wang, Y., Environmental characterization of burnt coal gangue banks at Yangquan, Shanxi Province, China. International Journal of Coal Geology 2008, 75, (2), 93-104. 15. Heffern, E. L.; Coates, D. A., Geologic history of natural coal-bed fires, Powder River basin, USA.

na

International Journal of Coal Geology 2004, 59, (1-2), 25-47. 16. Ž´ček, V.; Skála, R.; Chlupáčová, M.; Dvořák, Z., Ca-Fe3+-rich, Si-undersaturated buchite from

Jo ur

Želénky, North-Bohemian brown coal basin, Czech Republic. European Journal of Mineralogy 2005, 17, (4), 623-634.

17. Clark, B. H.; Peacor, D. R., Pyrometamorphism and partial melting of shales during combustion metamorphism: mineralogical, textural, and chemical effects. Contributions to Mineralogy and Petrology 1992, 112, (4), 558-568.

18. Kruszewski, Ł., Ciesielczuk, J., and Misz-Kennan, M. In Mineralogy of some metacarbonate rocks from burned coal-mining dump in Przygórze (Lower Silesian Coal Basin) and its analogy to "olive" rocks from the Hatrurim Formation, 4th Central-European Mineralogical Conference - CEMC 2014 Skalský Dvůr, 2014; Gadas, P., Ed. Proceedings of the international symposium: Skalský Dvůr,

2014; p 77. 19. Foit, F. F.; Hooper, R. L.; Rosenberg, P. E., An unusual pyroxene, melilite, and iron oxide mineral assemblage in a coal-fire buchite from Buffalo, Wyoming. American Mineralogist 1987, 72, (1-2), 137-147. 20. Querol, X.; Alastuey, A.; Lopez-Soler, A.; Mantilla, E.; Plana, F., Mineral composition of atmospheric particulates around a large coal-fired power station. Atmospheric Environment 1996, 30, (21), 3557-3572. 21. Querol, X.; Turiel, J. F.; Soler, A. L., The behaviour of mineral matter during combustion of Spanish subbituminous and brown coals. Mineralogical Magazine 1994, 58, (390), 119-133.

Journal Pre-proof

24

22. Stracher, G. B., New mineral occurrences and mineralization processes: Wuda coal-fire gas vents of Inner Mongolia. American Mineralogist 2005, 90, (11-12), 1729-1739. 23. Engle, M. A.; Radke, L. F.; Heffern, E. L.; O'Keefe, J. M.; Hower, J. C.; Smeltzer, C. D.; Hower, J. M.; Olea, R. A.; Eatwell, R. J.; Blake, D. R.; Emsbo-Mattingly, S. D.; Stout, S. A.; Queen, G.; Aggen, K. L.; Kolker, A.; Prakash, A.; Henke, K. R.; Stracher, G. B.; Schroeder, P. A.; Roman-Colon, Y.; ter Schure, A., Gas emissions, minerals, and tars associated with three coal fires, Powder River Basin, USA. Science of the Total Environment 2012, 420, 146-59. 24. Gürdal, G.; Hoşgörmez, H.; Özcan, D.; Li, X.; Liu, H.; Song, W., The properties of Çan Basin coals (Çanakkale—Turkey): Spontaneous combustion and combustion by-products. International Journal of Coal Geology 2015, 138, 1-15. 25. Pone, J. D. N.; Hein, K. A. A.; Stracher, G. B.; Annegarn, H. J.; Finkleman, R. B.; Blake, D. R.; McCormack, J. K.; Schroeder, P., The spontaneous combustion of coal and its by-products in the

of

Witbank and Sasolburg coalfields of South Africa. International Journal of Coal Geology 2007, 72, (2), 124-140.

ro

26. Gatel, P.; Žáček, V.; Kruszewski, Ł.; Devouard, B.; Thiéry, V.; Eytier, C.; Eytier, J.; Favreau, G.; Vigier, G.; Stracher, G., Combustion mineralogy and petrology of oil-shale slags in Lapanouse,

-p

Sévérac-le-Château, Aveyron, France: analogies with hydrocarbon fires. In Coal and Peat Fires: A Global Perspective ed.; Elsevier Science: 2015; Vol. 28, pp 681-742.

re

27. Cosca, M. A.; Essene, E. J.; Geissman, J. W.; Simmons, W. B.; Coates, D. A., Pyrometamorphic rocks associated with naturally burned coal beds, Powder River Basin, Wyoming. American Mineralogist 1989, 74, (1-2), 85-100. Review 1959, 19, (5), 209-211.

lP

28. Liu, Z., Characteristics, causes on burnt rocks and underground fire burning regularity. Geological 29. Huang, L.; Liu, C., Products of combustion of the Yan’an Formation coal seam and their

na

characteristics in the Northeastern Ordos Basin. Acta Geologica Sinica 2014, 118, (15), 4132-4139. 30. Zhang, Y.; Sherong, H.; Jichao, P.; Tongtong, Z.; Jin, L., Metamorphic products of coal 1789-1805.

Jo ur

combustion and its macroscopic models in North China. Journal of China Coal Society 2016, 41, (7), 31. Grapes, R.; Zhang, K.; Peng, Z., Paralava and clinker products of coal combustion, Yellow River, Shanxi Province, China. Lithos 2009, 113, (3-4), 831-843. 32. Grapes, R., Pyro-metamorphism. Springer Science & Business Media: 2010. 33. Grapes, R.; Korzhova, S.; Sokol, E.; Seryotkin, Y., Paragenesis of unusual Fe-cordierite (sekaninaite)-bearing paralava and clinker from the Kuznetsk coal basin, Siberia, Russia. Contributions to Mineralogy and Petrology 2010, 162, (2), 253-273. 34. Grapes, R., Pyrometamorphism. Springer: Berlin, 2006. 35. Baboolal, A. A.; Knight, J.; Wilson, B., Petrography and mineralogy of pyrometamorphic combustion metamorphic rocks associated with spontaneous oxidation of lignite seams of the Erin Formation, Trinidad. Journal of South American Earth Sciences 2018, 82, 181-192. 36. Ciesielczuk, J.; Kruszewski, Ł.; Majka, J., Comparative mineralogical study of thermally-altered coal-dump waste, natural rocks and the products of laboratory heating experiments. International Journal of Coal Geology 2015, 139, 114-141. 37. Takagi, H.; Maruyama, K.; Yoshizawa, N.; Yamada, Y.; Sato, Y., XRD analysis of carbon stacking structure in coal during heat treatment. Fuel 2004, 83, (17-18), 2427-2433. 38. Okolo, G. N.; Neomagus, H. W. J. P.; Everson, R. C.; Roberts, M. J.; Bunt, J. R.; Sakurovs, R.;

Journal Pre-proof

25

Mathews, J. P., Chemical–structural properties of South African bituminous coals: Insights from wide angle XRD–carbon fraction analysis, ATR–FTIR, solid state 13C NMR, and HRTEM techniques. Fuel 2015, 158, 779-792. 39. Roberts, M. J.; Everson, R. C.; Neomagus, H. W. J. P.; Van Niekerk, D.; Mathews, J. P.; Branken, D. J., Influence of maceral composition on the structure, properties and behaviour of chars derived from South African coals. Fuel 2015, 142, 9-20. 40. Everson, R.; Neomagus, H.; Kaitano, R.; Falcon, R.; Alphen, C.; Ducann, V., Properties of high ash char particles derived from inertinite-rich coal: 1. Chemical, structural and petrographic characteristics. Fuel 2008, 87, (13-14), 3082-3090. 41. Song, Y.; Jiang, B.; Mathews, J. P.; Yan, G.; Li, F., Structural transformations and hydrocarbon generation of low-rank coal (vitrinite) during slow heating pyrolysis. Fuel Processing Technology 2017, 167, 535-544.

of

42. Yu, S.; Bo, J.; Hewu, L.; Fengli, L.; Pei, S.; Gaoyuan, Y., Variations in stress-sensitive minerals and elements in the tectonic-deformation Early to Middle Permian coals from the Zhuxianzhuang mine,

ro

Anhui Province. Journal of Geochemical Exploration 2018, 188, 11-23.

43. Sokol, E.; Volkova, N.; Lepezin, G., Mineralogy of pyrometamorphic rocks associated with Mineralogy 1998, 10, (5), 1003-1014.

-p

naturally burned coal-bearing spoil heaps of the Chelyabinsk coal basin, Russia. European Journal of

re

44. Sokol, E. V.; Volkova, N. I.; Stracher, G., Combustion metamorphic events resulting from natural coal fires. Reviews in Engineering Geology 2007, 18, 97-115. 45. Zhu, B.; Jiang, S.; Yang, J.; Pi, D.; Ling, H.; Chen, Y., Rare earth element and Sr-Nd isotope

lP

geochemistry of phosphate nodules from the lower Cambrian Niutitang Formation, NW Hunan Province, South China. Palaeogeography, Palaeoclimatology, Palaeoecology 2014, 398, 132-143. 46. Chen, D. F.; Dong, W. Q.; Qi, L.; Chen, G. Q.; Chen, X. P., Possible REE constraints on the

na

depositional and diagenetic environment of Doushantuo Formation phosphorites containing the earliest metazoan fauna. Chemical Geology 2003, 201, (1-2), 103-118.

Jo ur

47. Yang, J.; Torres, M.; McManus, J.; Algeo, T. J.; Hakala, J. A.; Verba, C., Controls on rare earth element distributions in ancient organic-rich sedimentary sequences: Role of post-depositional diagenesis of phosphorus phases. Chemical Geology 2017, 466, 533-544. 48. Lunine, J. I., Earth: evolution of a habitable world. Cambridge University Press: 2013. 49. Taylor, S.; McLennan, S., The continental crust: Its evolution and composition. Blackwell: London, 1985.

50. Dai, S.; Graham, I. T.; Ward, C. R., A review of anomalous rare earth elements and yttrium in coal. International Journal of Coal Geology 2016, 159, 82-95. 51. He, S.; Li, S.; Wang, J.; Roza, A.; Tian, J., Coal Resources Hosting Pattern and Prediction in Xinjiang. Coal Geology of China 2011, 23, (8), 82-84. 52. Dai, S.; Finkelman, R. B., Coal geology in China: an overview. International Geology Review 2018, 60, (5-6), 531-534. 53. Zhang, J., Underground Coal Fires in China: Origin, Detection, Fire-fighting, and Prevention. China Coal Industry Publishing House: Beijing, 2008. 54. Taylor, J., Computer programs for standardless quantitative analysis of minerals using the full powder diffraction profile. Powder Diffraction 1991, 6, (1), 2-9. 55. Rietveld, H., A profile refinement method for nuclear and magnetic structures. Journal of Applied Crystallography 1969, 2, (2), 65-71.

Journal Pre-proof

26

56. Kruszewski, L.; Fabianska, M. J.; Ciesielczuk, J.; Segit, T.; Orlowski, R.; Motylinski, R.; Kusy, D.; Moszumanska, I., First multi-tool exploration of a gas-condensate-pyrolysate system from the environment of burning coal mine heaps: An in situ FTIR and laboratory GC and PXRD study based on Upper Silesian materials. Sci Total Environ 2018, 640-641, 1044-1071. 57. Kruszewski, Ł., Supergene sulphate minerals from the burning coal mining dumps in the Upper Silesian Coal Basin, South Poland. International Journal of Coal Geology 2013, 105, 91-109. 58. Xie, P.; Guo, W.; Yan, X.; Zheng, X., Fluorine in Lopingian superhigh-organic-sulfur coals from the Lalang Coal Mine, Guangxi, southern China. Fuel 2017, 208, 483-490. 59. Grigoriev, N. A., Chemical Element Distribution in the Upper Continental Crust. 2009; p 382. 60. Lu, Y., GeoKit—A geochemical toolkit for Microsoft Excel. Geochimica 2004, 33, 459-464. 61. Dai, S.; Li, D.; Chou, C.-L.; Zhao, L.; Zhang, Y.; Ren, D.; Ma, Y.; Sun, Y., Mineralogy and geochemistry of boehmite-rich coals: new insights from the Haerwusu Surface Mine, Jungar Coalfield,

of

Inner Mongolia, China. International Journal of Coal Geology 2008, 74, (3-4), 185-202. 62. Haskin, L. A.; Haskin, M. A.; Frey, F. A.; Wildeman, T. R., Relative and absolute terrestrial

ro

abundances of the rare earths. In Origin and Distribution of the Elements, Elsevier: 1968; pp 889-912. 63. Tao, S.; Shan, Y.; Tang, D.; Xu, H.; Li, S.; Cui, Y., Mineralogy, major and trace element

-p

geochemistry of Shichanggou oil shales, Jimusaer, Southern Junggar Basin, China: Implications for provenance, palaeoenvironment and tectonic setting. Journal of Petroleum Science and Engineering

re

2016, 146, 432-445.

64. Tao, H.; Wang, Q.; Yang, X.; Jiang, L., Provenance and tectonic setting of Late Carboniferous Earth Sciences 2013, 64, 210-222.

lP

clastic rocks in West Junggar, Xinjiang, China: A case from the Hala-alat Mountains. Journal of Asian 65. Matjie, R.; Ward, C.; Li, Z., Mineralogical Transformations in Coal Feedstocks during Carbon Conversion, Based on Packed-Bed Combustor Tests: Part 2. Behavior of Individual Particles. Coal

na

Combustion and Gasification Products 2012, 4, (1), 55-67. 66. Thy, P.; Jenkins, B.; Grundvig, S.; Shiraki, R.; Lesher, C., High temperature elemental losses and

Jo ur

mineralogical changes in common biomass ashes. Fuel 2006, 85, (5-6), 783-795. 67. Jorgensen, F.; Moyle, F., Phases formed during the thermal analysis of pyrite in air. Journal of Thermal Analysis and Calorimetry 1982, 25, (2), 473-485. 68. Koulialias, D.; Kind, J.; Charilaou, M.; Weidler, P.; Löffler, J. F.; Gehring, A. U., Variable defect structures cause the magnetic low-temperature transition in natural monoclinic pyrrhotite. Geophysical Journal International 2015, 204, (2), 961-967. 69. Dunn, J.; De, G.; O'Connor, B., The effect of experimental variables on the mechanism of the oxidation of pyrite: part 1. Oxidation of particles less than 45 μm in size. Thermochimica Acta 1989, 145, 115-130. 70. Bhargava, S.; Garg, A.; Subasinghe, N., In situ high-temperature phase transformation studies on pyrite. Fuel 2009, 88, (6), 988-993. 71. Xu, J.; Zhao, F.; Guo, Q.; Yu, G.; Liu, X.; Wang, F., Characterization of the melting behavior of high-temperature and low-temperature ashes. Fuel Processing Technology 2015, 134, 441-448. 72. Li, J.; Zhu, M.; Zhang, Z.; Zhang, K.; Shen, G.; Zhang, D., The mineralogy, morphology and sintering characteristics of ash deposits on a probe at different temperatures during combustion of blends of Zhundong lignite and a bituminous coal in a drop tube furnace. Fuel Processing Technology 2016, 149, 176-186. 73. Li, H. X.; Zhang, Z. L.; Tang, Y. X., Effect of High-Efficiency Flux on the Melting Characteristics

Journal Pre-proof

27

of Coal Ash. Applied Mechanics and Materials 2013, 295-298, 3094-3097. 74. Matjie, R. H.; Li, Z. S.; Ward, C. R.; French, D., Chemical composition of glass and crystalline phases in coarse coal gasification ash. Fuel 2008, 87, (6), 857-869. 75. de Boer, C. B.; Dekkers, M. J.; van Hoof, T. A., Rock-magnetic properties of TRM carrying baked and molten rocks straddling burnt coal seams. Physics of the Earth and Planetary Interiors 2001, 126, (1-2), 93-108. 76. Shao, Z.; Wang, D.; Wang, Y.; Zhong, X., Theory and application of magnetic and self-potential methods in the detection of the Heshituoluogai coal fire, China. Journal of Applied Geophysics 2014, 104, 64-74. 77. Dai, S.; Zhang, W.; Seredin, V. V.; Ward, C. R.; Hower, J. C.; Song, W.; Wang, X.; Li, X.; Zhao, L.; Kang, H.; Zheng, L.; Wang, P.; Zhou, D., Factors controlling geochemical and mineralogical compositions of coals preserved within marine carbonate successions: A case study from the Heshan

of

Coalfield, southern China. International Journal of Coal Geology 2013, 109-110, 77-100. 78. Frietsch, R.; Perdahl, J.-A., Rare earth elements in apatite and magnetite in Kiruna-type iron ores

ro

and some other iron ore types. Ore Geology Reviews 1995, 9, (6), 489-510.

79. Neumann, G.; Langen, J.; Zahel, H.; Plümacher, D.; Kletowski, Z.; Schlabitz, W.; Wohlleben, D.,

-p

Temperature and pressure dependence of lattice parameters of the XCu 2 Si 2 compounds (X= Ce, Eu, Yb, La, Gd, Lu, Ca, Th). Zeitschrift für Physik B Condensed Matter 1985, 59, (2), 133-141.

re

80. Sverjensky, D. A., Europium redox equilibria in aqueous solution. Earth and Planetary Science Letters 1984, 67, (1), 70-78.

81. Wood, S. A., The aqueous geochemistry of the rare-earth elements and yttrium: 1. Review of

lP

available low-temperature data for inorganic complexes and the inorganic REE speciation of natural waters. Chemical Geology 1990, 82, 159-186.

82. Seredin, V. V.; Dai, S., Coal deposits as potential alternative sources for lanthanides and yttrium.

na

International Journal of Coal Geology 2012, 94, 67-93. 83. Biswas, K.; Sontakke, A. D.; Sen, R.; Annapurna, K., Luminescence properties of dual valence Eu

Jo ur

doped nano-crystalline BaF2 embedded glass-ceramics and observation of Eu2+ --> Eu3+ energy transfer. Journal of Fluorescence 2012, 22, (2), 745-52. 84. Pei, Z.; Zeng, Q.; Su, Q., A study on the mechanism of the abnormal reduction of Eu3+→ Eu2+ in Sr2B5O9Cl prepared in air at high temperature. Journal of Solid State Chemistry 1999, 145, (1), 212-215.

85. Huang, X.; Qi, L.; Meng, Y., Trace element and REE geochemistry of minerals from Heifengshan, Shuangfengshan and Shaquanzi (Cu–) Fe deposit, eastern Tianshan Mountains. Mineral Deposits 2013, 32, (6), 1188-1210. 86. Jiang, S.; Yu, J.; Lu, J., Trace and rare-earth element geochemistry in tourmaline and cassiterite from the Yunlong tin deposit, Yunnan, China: implication for migmatitic–hydrothermal fluid evolution and ore genesis. Chemical Geology 2004, 209, (3-4), 193-213. 87. Heinrichs, H.; Schulz-Dobrick, B.; Wedepohl, K. H., Terrestrial geochemistry of Cd, Bi, Tl, Pb, Zn and Rb. Geochimica et Cosmochimica Acta 1980, 44, (10), 1519-1533.

Journal Pre-proof

28

Jo ur

na

lP

re

-p

ro

of

Conflict of interest The authors declared that they have no conflicts of interest to this work. We declare that we do not have any commercial or associative interest that represents a conflict of interest in connection with the work submitted。

Journal Pre-proof

29

Jo ur

na

lP

re

-p

ro

of

Highlight 1. We summarized systematically the products in the whole process from ignition to extinguishing. 2. We examined the main minerals in the combustion metamorphic rocks. 3. The combustion metamorphic rocks characteristics of major elements and REE distribution are shown in this paper.