Mineralogy and geochemistry of ammonian illite in intra-seam partings in Permo-Carboniferous coal of the Qinshui Coalfield, North China

International Journal of Coal Geology 153 (2016) 1–11

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Mineralogy and geochemistry of ammonian illite in intra-seam partings in Permo-Carboniferous coal of the Qinshui Coalfield, North China Qiming Zheng a, Qinfu Liu b,⁎, Songlin Shi a a b

School of Resources and Environment Engineering, Henan Institute of Engineering, Zhengzhou, Henan 451191, China School of Geological Science and Survey Engineering, China University of Mining and Technology, Beijing 100083, China

a r t i c l e

i n f o

Article history: Received 14 April 2015 Received in revised form 18 November 2015 Accepted 18 November 2015 Available online 19 November 2015 Keywords: Qinshui Coalfield Ammonian illite Geochemical and mineralogical characteristics Paleo-salinity Diagenesis

a b s t r a c t Ammonian illite is observed to be presented in the intra-seam coal partings of the Permo-Carboniferous coal seams from the Qinshui Coalfield, North China. This paper provides new insights into its geochemical and mineralogical characteristics, as well as the factors influencing its formation and its nitrogen isotope ratios. The interlayer cations of ammonian illite in the samples presented in this study were dominated by NH+ 4 with a certain + + amount of K+ and distributed homogeneously in each illite layer. Ammonian illite has an NH+ 4 /(NH4 + K ) ratio of 0.87 on average, with a basal spacing (d001, 10.267 Å on average) greater than those of potassian illite and muscovite, but lower than that of tobelite. The average stoichiometric formula of the ammonian illite was inferred as (NH40.67,K0.11)(Al1.90,Fe0.06,Mg0.04)(Al0.68,Si3.32)O10(OH)2, and the average Si/AlIV ratio (4.88) was found to be higher than that of tobelite. This indicates that the conversion of ammonian illite to tobelite includes the expulsion of Si from the tetrahedral sheets and the incorporation of Al into the octahedral and tetrahedral sheets. + + IV The d005 is influenced not only by the NH+ 4 /(NH4 + K ) ratio but also, to some extent, by the entrance of Al into tetrahedral sites. It is inferred that the ammonian illite was formed by the incorporation of Si into pre-existing kaolinite during diagenesis, and that its formation was influenced not only by the diagenetic temperature but also by the depositional environment. Brackish water favors the formation of ammonian illite during deposition. The NH+ 4 of the ammonian illite was mainly derived from pyrrolic (N-5) and pyridinic (N-6) nitrogen groups at the margins of the coal's carbon matrix during diagenesis. On average, it has a δ15N value of +8.0‰, much higher than that of total coal nitrogen (+3.9‰ on average). This is related mainly to the higher δ15N values of N-5 and N-6 than the quaternary nitrogen group (N-Q) within the carbon matrix. © 2015 Elsevier B.V. All rights reserved.

1. Introduction Geochemical and mineralogical studies have shown that isomor+ phous substitution of NH+ 4 for K can occur during various geological + processes because of their similar ionic radii (NH+ 4 , 1.48 Å; K , 1.33 Å) and that NH4-bearing minerals form as a result (Itihara and Suwa, 1985; Juster et al., 1987). Common NH4-bearing aluminosilicate minerals include buddingtonite, ammonian illite, and NH4-bearing biotite (Erd et al., 1964; Gulbrandsen, 1974; Higashi, 1978, 1982; Itihara and Suwa, 1985; Juster et al., 1987; Daniels and Altaner, 1993; Daniels et al., 1994; Ward and Christie, 1994; Nieto, 2002; Dai et al., 2012a; Permana et al., 2013). The term “ammonian illite” is used in the current study in reference to NH4-bearing illite (i.e., illite in which NH+ 4 is the dominant interlayer cation), in accordance with the requirements of the International Mineralogical Association (Nickel and Mandarino, 1987). The interlayer + cations of ammonian illite are dominated by NH+ 4 , followed by K ⁎ Corresponding author. E-mail addresses: [email protected] (Q. Zheng), [email protected] (Q. Liu).

http://dx.doi.org/10.1016/j.coal.2015.11.008 0166-5162/© 2015 Elsevier B.V. All rights reserved.

(Higashi, 1978, 1982; Juster et al., 1987; Liu and Zhang, 1997). Some geochemical and mineralogical studies of ammonian illite have been undertaken, and X-ray diffraction analysis has indicated that ammonian illite has higher basal spacing (N10.15 Å) than potassian illite (b10.05 Å) (Juster et al., 1987; Liu and Zhang, 1997). This is mainly attributed to the + larger ionic radius of NH+ 4 relative to K and to the tetrahedral characteristics of NH+ (Juster et al., 1987; Liu and Zhang, 1997). 4 The ammonian illite associated with high-rank coal has been studied by Juster et al. (1987). They reported that the ammonian illite, which is an authigenic mineral, forms largely because of the reaction of potassian + illite with NH+ 4 , and that the NH4 is derived mainly from ambient coal nitrogen during coalification. Liu et al. (1996), Liang et al. (2005), and Dai et al. (2012a) identified ammonian illite in the PermoCarboniferous coals of North China and they reported that the precursor minerals of the ammonian illite are dominated by kaolinite. The proportion of ammonian illite within clay minerals increases with coalification (Liu and Zhang, 1997). Juster et al. (1987) reported that the starting temperature for the formation of ammonian illite associated with coal is 250 °C, but Liu et al. (1996) and Liang et al. (2005) have suggested that the temperature is actually much lower (i.e., 105 °C). Williams

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and Ferrell (1991), Williams et al. (1995) and Drits et al. (2002) also found ammonian illite in sedimentary rocks associated with oil. They suggested that the NH+ 4 was derived mainly from the crude oil during maturation and migration. They further concluded that the formation of the ammonian illite (tobelitization reaction) probably occurred during the processes of oil generation and oil expulsion (oil window, T = 80–140 °C), which is consistent with the findings of Liu et al. (1996) and Liang et al. (2005). However, some geological information including stoichiometric formula and nitrogen isotope of ammonian illite associated with coal and the geological factors influencing its formation (i.e., coalification and depositional environment) needs to be further discussed. Although Higashi (1982) has proposed the ideal stoichiometric formula: NH4Al2(Si3Al)O10(OH)2, the accurate chemical composition of the ammonian illite (tobelite) associated with coal also remains unclear. The divergence in the values quoted for the temperature necessary for the formation of ammonian illite indicates that in addition to the diagenetic temperature, other geological factors that influence its formation (such as depositional environment and precursor minerals) remain unclear. In the present study, intra-seam mudstone parting samples in Permo-Carboniferous coal were collected from the Qinshui Coalfield, North China, and the ammonian illite contained within those samples was studied mineralogically and geochemically. This paper reports new information regarding the structural features and chemical composition of the ammonian illite associated with coal. Furthermore, its formation process and the influence of other geological factors on its formation are discussed. 2. Geological setting The Qinshui Coalfield is located in the eastern margin of Shanxi Province, North China (Fig. 1). Coal-bearing sequences in the Qinshui Coalfield include the Taiyuan Formation (Upper Carboniferous) and

the Shanxi Formation (Lower Permian) and have a total thickness of 144–212 m (Fig. 2). The Taiyuan Formation, with an average thickness of 99.5 m, conformably overlies the Benxi Formation, and is composed mainly of sandstone, siltstone, mudstone, limestone, and coal seams. The Nos. 6, 8, 12, and 15 coals are the major minable coal seams in the Taiyuan Formation. The No. 15 Coal, which is located in the lowermost part of the Taiyuan Formation, has a thickness of 4.97–8.69 m (average 6.78 m) in the northern part of the Qinshui Coalfield and a thickness of 0.30–6.17 m (average 3.21 m) in the southern part. One to three continuous intra-seam partings, composed mainly of clay minerals, are contained within the No. 15 Coal. This coal was formed in a coastal peat swamp, which was developed from a lagoon before a large-scale transgression, and this coal was influenced mainly by brackish water in the stage of peat accumulation (Ge et al., 1985; Liang et al., 2002; Shao et al., 2008). The Shanxi Formation, with an average thickness of 69.5 m, conformably overlies the Taiyuan Formation and is composed mainly of sandstone, siltstone, mudstone, and coal seams. The No. 3 Coal is the only minable coal within this formation and has a thickness of 5.04–7.16 m (average 6.11 m) in the southern Qinshui Coalfield, thicker than that in the northern part of the coalfield (0–3.80 m; average 1.92 m). One to two continuous intra-seam partings are contained within the No. 3 Coal. This coal was formed in a freshwater peat swamp, which had a lagoon–lacustrine environment during a local regression. The No. 3 Coal was influenced mainly by freshwater in the stage of peat accumulation but also, to a lesser extent, by brackish water (Ge et al., 1985; Liang et al., 2002; Shao et al., 2008). The strata overlying the Shanxi Formation are the non-coal-bearing lower Shihezi Formation. The Permo-Carboniferous coal in the Qinshui Coalfield has a high rank, dominated by low-volatile-matter bituminous coal and anthracite (with vitrinite maximum reflectances of 1.5–4.5%; Zhang et al., 2002).

Fig. 1. Location of the Qinshui Coalfield and sampling sites in the coalfield.

Q. Zheng et al. / International Journal of Coal Geology 153 (2016) 1–11

3

cut over an area 10-cm wide and 10-cm deep following the method outlined by Dai et al. (2012a), and then was stored immediately in plastic bags to minimize contamination and oxidation. Prior to analysis in the lab, all samples were crushed and ground to b200 μm. 3.2. X-ray diffraction (XRD) analysis The mineralogy of the parting samples was determined using X-ray diffraction (XRD) analysis, which was performed by a D/max-2500/PC powder diffractometer. Operating conditions were as follows: power, 6 kW (40 kV, 150 mA); scanning speed, 4°/min; step, 0.02°; div slit, 1°; and rec slit, 0.3 mm. The XRD pattern was recorded over a 2θ interval of 4–45° for non-oriented powder samples (b200 μm). The b 2-μm fraction (dominated by clay minerals) was obtained by preparing and extracting a suspension of powder samples, which was then centrifuged according to the method outlined by Lin (1990). The oriented pattern (air-dried, glycol-saturated, and thermally treated) for the b 2-μm fraction was recorded over a 2θ interval of 2–70°. X-ray diffractograms of the parting samples, including the oriented and non-oriented patterns, were subjected to quantitative mineralogical analysis using Quan and Clay software developed by Lin (1990), who followed the method outlined by Chung (1974a, 1974b, 1975) and Chinese Petroleum and Gas Industry Standard SY/T 5163-2010. XRD analysis was performed by the State Key Laboratory of Coal Resources and Safe Mining, China University of Mining and Technology, Beijing. 3.3. X-ray fluorescence (XRF) analysis Following the methods outlined by Dai et al. (2012a, 2012b), the X-ray fluorescence (XRF; ARL ADVANT'XP+) spectrometry was used to determine the oxides of the major elements of high-temperature ash (815 °C) of each parting sample, including SiO2, Al2O3, K2O, Na2O, CaO, MgO, Fe2O3, and TiO2. For the XRF analysis, each powder sample (b200 μm) was treated thermally at 815 °C for 2 h to prepare the high-temperature ash sample. Then, the ash sample was covered uniformly by borate and pressed into a flat disk (diameter: 35 mm) using a hydraulic press (15 MPa) equipped with molds. The XRF analysis was performed by the State Key Laboratory of Coal Resources and Safe Mining, China University of Mining and Technology, Beijing. 3.4. Calculation for stoichiometric formula of ammonian illite

Fig. 2. Sedimentary sequences and depositional environment of the Qinshui Coalfield. Modified from Ge et al. (1985), Liang et al. (2002), Shao et al. (2008)).

3. Samples and methods 3.1. Sample collection In total, 20 samples of Permo-Carboniferous coal partings were collected from the exposed faces of 11 coal mines in the Qinshui Coalfield. Ten of the samples were taken from the No. 3 Coal of the Shanxi Formation, and the remaining samples were taken from the No. 15 Coal of the Taiyuan Formation (Table 1). Each parting sample (about 1–2 kg) was

The stoichiometric formula of natural ammonian illite remains unclear. A chemical composition: (NH4x1, Kx2)(Alx3, Fex4, Mg2 − x3 − x4)(Six5, Al4 − x5)O10(OH)2, was set for ammonian illite according to that outlined by Zhao and Zhang (1990) for illite, and the stoichiometric compositions of the other minerals found in the samples (discussed in Section 4.1.1) are as follows: quartz, SiO2; pyrite, FeS2; plagioclase, (Na,Ca)(Al3,Si5)O16; boehmite, AlO(OH); kaolinite, Al4Si4O10(OH)8; and anatase, TiO2. These stoichiometric formulae were used in the following calculation. The percentage of the major element oxides containing xi(i = 1, 2, 3, 4, 5) was calculated from the mineral assemblage indicated by the Quan and Clay software using the method described by Ward et al. (1999) and Dai et al. (2012a, 2012b). This process includes a calculation to allow for the loss of volatile matter, such as hydroxyl water and NH+ 4 from the clay minerals. The abundances of the major element oxides, determined by the XRF analysis, were also recalculated to provide normalized percentages of the major element oxides. Simultaneous equations were established according to these two sets of data, as shown below:

0:5
I


MK 2 O
x2 ¼ P K
P D MI

ð1Þ

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Q. Zheng et al. / International Journal of Coal Geology 153 (2016) 1–11

Table 1 Mineral compositions of parting samples determined applying X-ray diffraction and Quan and Clay software (wt.%). Age

Coal seam

Sampling site

Sample

Permian

No. 3 Coal

Sihe Mine

SH-3-g1 SH-3-g2 CP-3-g1 CP-3-g2 CP-3-g3 CP-3-g4 DJZ-3-g1 DJZ-3-g2 YW-3-g ZC-3-g1 YQ1-15-g1 YQ1-15-g2 YQ1-15-g3 YQ2-15-g1 FHS-15-g GSY-15-g1 GSY-15-g2 WTP-15-g3 CZ-15-g3 CZ-15-g4 Av

Changpping Mine

Dongjiazhuang

Carboniferous

No. 15 Coal

Tunliu Mine Zhangcun Mine First Guoyang Mine

Second Guoyang Mine Fenghuangshan Mine Gushuyuan Mine Wangtaipu Mine Chengzhuang Mine

Quartz

Plagioclase

Pyrite

Anatase

Boehmite

2.5 0.8 1.9 4.6 2.0

1.3 1.7 1.2 4.5 2.5 1.4

1.4 0.7

2.6

29.7 8.9 12.4 2.2

2.9

2.1 0.8 0.6 1.6 2 1.2 1.9 0.5

0.7

1.2 7.2 1.1

0.5

1.0 6.2

1 1.5

4.4 2.3 0.8 3.9 1.6

5.2 0.7

Ammonian illite

Kaolinite

8.8 30.0 91.7 90.6 87.6 86.0 94.2 68.5 97.0 60.0 92.5 98.2 97.9 8.2 59.3 38.0 18.6 6.7 94.0 10.6 61.9

88.7 69.0 5.8 6.6 7.5 1.9 29.4 3.0 40.0 2.8

60.5 29.8 42.8 70.1 89.4 85.5 31.6

Av., average. SH-3-g1 and SH-3-g2 were collected from the No. 3 Coal of the Sihe Mine; CP-3-g1, CP-3-g2, CP-3-g3 and CP-3-g4 were collected from the No. 3 Coal of the Changping Mine; DJZ-3-g1 and DJZ-3-g2 were collected from the No. 3 Coal of the Dongjiazhuang Mine; YW-3-g was collected from No. 3 Coal of the Tunliu Mine; ZC-3-g1 was collected from No. 3 Coal of Zhangcun Mine; YQ1-15-g1, YQ1-15-g2 and YQ1-15-g3 were collected from No. 15 Coal of the First Guoyang Mine; YQ2-15-g1 was collected from the No. 15 Coal of the Second Guoyang Mine; FHS-15-g was collected from the No. 15 Coal of the Fenghuangshan Mine; GSY-15-g1 and GSY-15-g2 were collected from the No. 15 Coal of the Gushuyuan Mine; WTP-15-g3 was collected from the No. 15 Coal of the Wangtaipu Mine; CZ-15-g3 and CZ-15-g4 were collected from the No. 15 Coal of Chengzhuang Mine.

1:5
L



MAl2 O3
ð4 þ x3 −x5 Þ ¼ P Al
P D MI

0:5
P


I


3.5. Boron content analysis

M Al2 O3 MAl2 O3 M Al2 O3 þ2
K
þ 0:5
B
þ 0:5
I ML MK MB

M Fe2 O3 M Fe2 O3 þ 0:5
I

x4 ¼ P Fe
P D MP MI

MMgO
ð2−x3 −x4 Þ ¼ P Mg
P D MI

ð2Þ

ð3Þ

In order to estimate the paleosalinity, the boron content of the parting samples was determined using inductively coupled plasma mass spectrometry (ICP-MS) following the method outlined by Dai et al. (2014). They used samples treated with 3 mL HNO3 (65%) + 1 mL HF (40%) + 0.5 mL H3PO4 (85%) in a closed PTFE vessel to prevent boron volatilization before its determination. The ICP-MS analysis was performed by the CNNC Beijing Research Institute of Uranium Geology.

ð4Þ 3.6. Nitrogen isotopic analysis

Q þ5
L


MSiO2 MSiO2 M SiO2 þ4
K
þI

x5 ¼ P Si
P D ML MK MI

ð5Þ

where Q , L, K, I, B, P, and A represent the proportions of quartz, plagioclase, kaolinite, ammonian illite, boehmite, pyrite, and anatase, respectively, as indicated by the XRD analysis; PSi, PAl, PK, PMg, and PFe represent the normalized percentages of SiO2, Al2O3, K2O, MgO, and Fe2O3, respectively, as calculated from the XRF data; MSiO2, MAl2O3, MK2O, MMgO, and MFe2O3 represent the molar masses of SiO2, Al2O3, K2O, MgO, and Fe2O3, respectively; and MP, MK, MB, MI, and MLrepresent the molar masses of pyrite, kaolinite, boehmite, ammonian illite, and plagioclase, respectively; PD represents the proportion of the volatilematter-free fractions calculated from the XRD data. P D ¼ Q þ L þ 0:5
P



MAl2 O3 M Fe2 O3 MK −72 þ 0:5
B
þAþK
þI MK MP MB

MI −18−26x1 : MI

The formula based on the charge balance is as follows: x1 þ x2 þ x3 þ x5 ¼ 6:

There are two conventional methods for determining nitrogen isotopes in rocks and minerals: one based on the decomposition of samples in acid solutions (Haendel et al., 1986; Brӓuer and Hahne, 2005) and the other based on pyrolysis techniques under vacuum (Boyd et al., 1993; Busigny et al., 2005). In the current study, the nitrogen isotope ratio for NH+ 4 fixed in the mineral lattices of the parting samples was determined using the acid-dissolution method. All the parting samples were first treated with H2O2 to remove dispersed organic matter. Then, they were prepared by treatment with HCl (5 mol/L) + HF (≥ 22.6 mol/L) and NaOH + Na2S (370 g NaOH + 30 g Na2S/L). This resulted in the transformation of the NH+ 4 fixed in the mineral lattices to free NH3, which was absorbed by boric acid. The boric acid containing NH3 was then treated by NaOBr under vacuum, resulting in the oxidization of NH3 to N2, which was injected directly into a Finnigan MAT-252 mass spectrometer to determine the nitrogen isotope ratios. The repeatability limit and precision were 0.5% and ±0.1‰, respectively. The nitrogen isotope analysis was performed by the Lanzhou Center for Oil and Gas Resources, Institute of Geology and Geophysics, CAS. 4. Results

ð6Þ

Each xi(i = 1, 2, 3, 4, 5) was calculated using the method of generalized least squares, and the stoichiometric formula of ammonian illite for each parting sample was obtained.

4.1. Mineralogy 4.1.1. Minerals found in intra-seam partings The proportion of each crystalline phase identified by the XRD was calculated by Quan and Clay software and the results are presented in

Q. Zheng et al. / International Journal of Coal Geology 153 (2016) 1–11

Table 1. The minerals in the parting samples are dominated by ammonian illite (average 61.9%) and kaolinite (average 31.6%), with abundant quartz (b 30%) in some cases. Trace amounts of plagioclase, anatase, pyrite, and boehmite were also identified in some samples (Fig. 3). 4.1.2. XRD characteristics of ammonian illite The basal crystallographic spacing (d001) of the ammonian illite varied from 10.191 to 10.334 Å (average 10.267 Å), higher than those of potassian illite and muscovite but lower than that of tobelite (Table 2). This suggests that interlayer cations were dominated by + NH+ 4 with some percentage of K . Other d00l values were also higher than those of potassian illite and muscovite and lower than that of tobelite, and largely had an integer-multiple relationship with d001. The 001 peaks of ammonian illite in all the samples were sharp under both air-dried and glycol-saturated conditions. After glycolation, the 001 peaks did not shift to a higher 2θ value or have a decreased intensity, as occurs with mixed-layer ammonian illite–smectite (Bobos, 2012; Bobos and Eberl, 2013). This indicates that the ammonian illite examined in the current study was pure illite and that no expandable layers (smectite) were interstratified (Fig. 4). No difference was observed for the position, FWHM, and profiles of the 001 peaks after glycolation, and this is also attributed to the absence of smectite layers in the ammonian illite structure. Drits et al. (1997, 2005) proposed two models for the occurrence for ammonian illite: Model I, in which the il+ + lite layers contain either K+ or NH+ 4 and Model II, in which K and NH4 are distributed homogeneously in each illite layer. The main XRD difference between these two models is that basal reflections with the same value of l have different FWHM00l values. The average values of FWHM005/FWHM001, FWHM003/FWHM001, and FWHM002/FWHM001 of the ammonian illite in the current study were 1.01, 1.00, and 0.97, respectively (i.e., all close to 1). This indicates that the FWHMl does not increase with l and thus, K+ and NH+ 4 are distributed homogeneously in each illite layer, corresponding to Model II of Drits et al. (1997, 2005).

5

reason, only weak correlation was observed between K2O and ammonian illite (r = 0.22) because of the dominance of NH+ 4 rather than K+ in the interlayer spaces of the illite component. Magnesium is thought to occur mainly as octahedral cations in the ammonian illite because of the absence of common Mg-bearing minerals, such as dolomite and magnesite, in the studied samples. In addition to the Fe in the sulfide minerals (pyrite), some Fe also appears to occur as octahedral cations in the ammonian illite (see discussion in Section 4.3). Sodium and Ca are considered to occur in plagioclase (Table 1), although plagioclase was not detected in some samples, possibly because its concentration was lower than the detection limit of the XRD system. Some Ca may occur in phases such as calcite in coal seams, (e.g., Bouška et al., 2000; Dai and Chou, 2007), but calcite was not detected by the XRD analysis of the samples from the intra-seam partings of the present study. 4.2.2. Boron The B content of the Permo-Carboniferous intra-seam coal partings varies from 45 to 698 μg/g with an average of 228 μg/g (Table 4). Boron in coal is thought to occur mainly in organically bound form, although some is fixed into clay minerals and in the tourmaline (Ward, 1980; Querol et al., 1999; Dai et al., 2013a, 2013b; Oliveira et al., 2013). Partings in coal seams, however, are dominated by clay minerals with a certain amount of dispersed organic matter; thus, substitution for Si in the tetrahedral layers of the clay minerals is considered to be the dominant mode of occurrence for B in coal partings (Bohor and Gluskoter, 1973; Boyd, 2002; Zhao and Zhang, 1990). Therefore, the boron content in the partings observed in this study might largely represent the boron absorbed on to and fixed in the clay minerals during deposition. The paleosalinity of clay-bearing sediments and sedimentary rocks can be calculated using the method proposed by Couch (1971), which is based on the correlation between the salinity and boron content of clay minerals. ð lgB−0:11Þ 1:28

4.2. Geochemistry

Sp ¼ 10

:

4.2.1. Major elements The abundances of major element oxides in the parting samples are listed in Table 3. The major element oxides are dominated by Al2O3 (average 28.43%) and SiO2 (average 41.18%), as might be expected from the mineralogical compositions, which are dominated by kaolinite and ammonian illite. Because of the absence of orthoclase, potassium is thought to occur mainly as interlayer cations in ammonian illite. For this

Sp and B represent paleosalinity of depositional environment and boron content of clay minerals, respectively. However, different types of clay minerals have different abilities to absorb boron (Couch, 1971; Walker and Price, 1963). Xu et al. (2003) reported that the ability ratio of illite, smectite, kaolinite, chlorite, and pyrophyllite was 4:2:1:1:1, and in order to reflect paleosalinity accurately, the boron content in different types of clay minerals must be modified. According to

Fig. 3. X-ray powder diffraction patterns of non-oriented parting samples.

ð7Þ

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Q. Zheng et al. / International Journal of Coal Geology 153 (2016) 1–11

Table 2 d00l and FWHM00l (l = 1, 2, 3, 5) and ratios of FWHM005/FWHM001 and FWHM002/FWHM001 for the ammonian illite in parting samples (d00l in Å). Sample

d001

FWHM001

d002

FWHM002

d003

FWHM003

d005

FWHM005

FWHM002 FWHM001

FWHM003 FWHM001

FWHM005 FWHM001

SH-3-g1 SH-3-g2 CP-3-g1 CP-3-g2 CP-3-g3 CP-3-g4 DJZ-3-g1 DJZ-3-g2 YW-3-g ZC-3-g1 YQ1-15-g1 YQ1-15-g2 YQ1-15-g3 YQ2-15-g1 FHS-15-g GSY-15-g1 GSY-15-g2 WTP-15-g3 CZ-15-g3 CZ-15-g4 Av. Tobelitea Muscovitea Illitea

10.264 10.310 10.297 10.310 10.288 10.309 10.287 10.288 10.334 10.312 10.264 10.262 10.314 10.241 10.237 10.216 10.225 10.191 10.309 10.264 10.276 10.334 9.985 10.022

n.d. 0.255 0.383 0.371 0.324 0.196 0.581 0.508 0.391 0.252 0.307 0.311 0.240 n.d. 0.453 0.449 n.d. n.d. 0.225 n.d. 0.350 n.d. n.d. n.d.

n.d. 5.148 5.151 5.154 5.149 5.154 5.073 5.142 5.149 5.149 5.143 5.143 5.156 n.d. 5.119 5.102 n.d. n.d. 5.143 n.d. 5.138 5.167 4.991 5.011

n.d. 0.256 0.376 0.396 0.323 0.193 0.560 0.513 0.343 0.243 0.275 0.288 0.209 n.d. 0.464 0.447 n.d. n.d. 0.223 n.d. 0.341 n.d. n.d. n.d.

n.d. 3.428 3.424 3.428 3.426 3.431 3.467 3.421 3.426 3.431 3.423 3.428 3.439 n.d. 3.415 3.403 n.d. n.d. 3.426 n.d. 3.428 n.d. n.d. n.d.

n.d. 0.252 0.400 0.376 0.331 0.208 0.581 0.504 0.356 0.256 0.305 0.277 0.232 n.d. 0.459 0.436 n.d. n.d. 0.242 n.d. 0.348 n.d. n.d. n.d.

n.d. 2.060 2.057 2.057 2.057 2.060 2.051 2.057 2.065 2.062 2.048 2.059 2.061 n.d. 2.043 2.033 n.d. n.d. 2.056 n.d. 2.052 2.067 1.997 n.d.

n.d. 0.246 0.384 0.362 0.344 0.213 0.610 0.485 0.372 0.265 0.306 0.272 0.247 n.d. 0.445 0.452 n.d. n.d. 0.246 n.d. 0.350 n.d. n.d. n.d.

n.d. 1.00 0.98 1.07 1.00 0.98 0.96 1.01 0.88 0.96 0.90 0.93 0.87 n.d. 1.02 1.00 n.d. n.d. 0.99 n.d. 0.97 n.d. n.d. n.d.

n.d. 0.99 1.04 1.01 1.02 1.06 1.00 0.99 0.91 1.02 0.99 0.89 0.97 n.d. 1.01 0.97 n.d. n.d. 1.08 n.d. 1.00 n.d. n.d. n.d.

n.d. 0.96 1.00 0.98 1.06 1.09 1.05 0.95 0.95 1.05 1.00 0.87 1.03 n.d. 0.98 1.01 n.d. n.d. 1.09 n.d. 1.01 n.d. n.d. n.d.

n.d., not determined; Av., average. The FWHM values have been corrected for angular reflection broadening by multiplication with the cosθ factor according to Drits et al.'s (1997) method, where θ is the Bragg angle of the corresponding reflection. a From Quan and Clay software PDF.

the proportions of different clay minerals, Xu et al. (2003) has modified boron content as follows:

B ¼

B 4xi þ 2xs þ xk þ xc þ xpy

ð8Þ

where xi, xs, xk, xc, and xpy represent the contents of illite, smectite, kaolinite, chlorite, and pyrophyllite, respectively, and B and B* represent the boron and modified boron contents, respectively. In the present

study, if the ammonian illite was transformed mainly from kaolinite during diagenesis (discussed in Section 5.2), the boron contained in the ammonian illite would have been absorbed mainly by pre-existing kaolinite during peat accumulation. Therefore, the boron content of the samples was modified according to the Reaction 2 (discussed in Section 5.2), using the following equation:

B ¼

B 0:65xa þ xk

Fig. 4. X-ray diffraction patterns of air-dry (AD) and glycol-saturated (GS) oriented parting sample YQ1-15-g2.

ð9Þ

Q. Zheng et al. / International Journal of Coal Geology 153 (2016) 1–11

7

Table 3 Abundances of major element oxides in parting samples from Qinshui Coalfield (%, on whole-parting basis). Sample

SiO2

Al2O3

Na2O

K2O

CaO

MgO

Fe2O3

TiO2

P2O5

SiO2/Al2O3

LOIs

SH-3-g1 SH-3-g2 CP-3-g1 CP-3-g2 CP-3-g3 CP-3-g4 DJZ-3-g1 DJZ-3-g2 YW-3-g ZC-3-g1 Ap. YQ1-15-g1 YQ1-15-g2 YQ1-15-g3 YQ2-15-g1 FHS-15-g GSY-15-g1 GSY-15-g2 WTP-15-g3 CZ-15-g3 CZ-15-g4 Ac. At.

40.45 42.22 44.52 47.91 47.63 30.18 40.74 48.33 33.99 44.57 42.06 38.12 42.97 44.33 24.83 53.16 46.68 41.33 35.85 39.65 36.16 40.31 41.18

34.65 35.87 32.04 32.27 32.02 20.26 27.59 32.91 22.11 32.47 30.22 23.28 30.29 32.13 13.16 30.40 25.82 29.42 26.40 29.64 25.96 26.65 28.43

0.38 0.49 0.70 1.14 1.07 0.49 0.65 0.82 0.34 0.56 0.66 0.51 0.22 0.27 0.17 0.31 0.24 0.15 0.06 0.01 0.01 0.19 0.43

0.22 0.24 0.63 0.59 0.57 0.39 1.36 0.40 0.28 0.15 0.48 1.08 0.44 0.40 0.38 1.68 1.52 0.79 0.52 1.65 0.27 0.87 0.68

0.74 0.11 0.16 0.28 0.28 0.14 0.16 0.23 0.38 0.01 0.26 0.13 0.06 0.05 0.03 0.12 0.07 0.09 0.08 0.07 0.07 0.08 0.17

0.51 0.07 0.21 0.31 0.30 0.16 0.47 0.23 0.15 0.09 0.25 0.33 0.07 0.07 0.11 0.57 0.41 0.34 0.20 0.21 0.03 0.23 0.24

0.30 0.28 0.51 0.86 0.69 0.97 1.62 0.53 2.5 0.42 0.87 0.50 0.06 0.22 0.20 0.79 1.54 1.95 0.70 0.37 0.21 0.66 0.76

0.70 0.79 1.76 0.58 0.62 6.98 1.57 0.55 1.90 0.59 1.60 1.59 0.91 0.88 0.91 1.88 1.64 1.19 1.03 1.09 1.26 1.24 1.42

0.41 0.04 0.03 0.01 0.01 0.06 0.44 0.02 0.05 0.01 0.11 0.05 0.02 0.02 0.03 0.04 0.03 0.05 0.05 0.03 0.02 0.03 0.07

1.17 1.18 1.39 1.48 1.49 1.49 1.48 1.47 1.54 1.37 1.41 1.64 1.42 1.38 1.89 1.75 1.81 1.41 1.36 1.34 1.39 1.54 1.47

21.64 19.89 19.44 16.05 16.81 40.37 25.4 15.98 38.3 21.13 23.49 34.41 24.96 21.63 60.18 11.05 22.05 24.69 35.11 27.28 36.01 29.74 26.62

Ap., average of Permian partings; Ac., average of Carboniferous partings; At., average of all the partings; LOI, loss of ignition.

where xa and xk represent the contents of ammonian illite and kaolinite, respectively. The paleosalinity of the intra-seam partings of the present study was calculated according to Eqs. (7) and (9), and the results indicate that the paleosalinity varied from 21.4‰ to 200.9‰ (average 75.7‰) (Table 4), which is much higher than contemporary seawater (35‰). 4.2.3. Nitrogen isotope ratio of ammonian illite Of all the minerals in the intra-seam partings, only ammonian illite contains nitrogen; therefore, the nitrogen isotope ratio of the parting samples determined by the acid-dissolution method described in Section 3.6, should represent that of the ammonian illite. The nitrogen isotope ratios of the ammonian illite and the corresponding coal samples are listed in Table 5. The results indicate that δ15N for the ammonian illite of the current study varies from + 2.5‰ to + 11.7‰ (average + 8.0‰), lower than that of sediments and sedimentary rocks (+10‰; Holloway and Dahlgren, 2002) but higher than that of the corresponding coal samples (average + 3.9‰). The isotopic gap (Δδ15N = δ15Nammonian illite − δ15Ncoal nitrogen) between the ammonian illite and the corresponding coal for the Permian samples is higher than that of the Carboniferous samples. Williams et al. (1995) reported that no isotopic fractionation between the fixed NH4 and corresponding organic nitrogen has been identified during diagenesis. Furthermore, Schimmelmann and Lis (2010) used a 5-year heating experiment to prove that the isotopic gap between the fixed NH4 and organic nitrogen could be buffered by NH+ 4 in pore fluid. In the present study, the higher Table 4 Boron content, modified Boron content and paleosalinity of parting samples (boron content and modified boron content in μg/g, paleosalinity in ‰). Sample

B

B*

Paleo-salinity

Sample

B

B*

Paleo-salinity

SH-3-g1 SH-3-g2 CP-3-g1 CP-3-g2 CP-3-g3 CP-3-g4 DJZ-3-g1 DJZ-3-g2 YW-3-g ZC-3-g1

90 n.d. 317 220 222 144 n.d. 129 51 152

95 n.d. 532 340 349 227 n.d. 175 77 192

28.9 n.d. 110.6 78.0 79.6 56.9 n.d. 46.3 24.33 50.0

YQ1-15-g1 YQ1-15-g2 YQ1-15-g3 YQ2-15-g1 FHS-15-g GSY-15-g1 GSY-15-g2 WTP-15-g3 CZ-15-g3 CZ-15-g4

189 478 396 45.3 n.d. n.d. n.d. n.d. 698 60

300 749 622 69 n.d. n.d. n.d. n.d. 1142 65

70.7 144.4 125.0 22.4 n.d. n.d. n.d. n.d. 200.9 21.4

n.d., no data.

values of δ15N in the ammonian illite, in comparison with coal nitrogen, can be attributed mainly to the isotopic divergence of the different forms of organic nitrogen in the coal. Studies by Mitra-Kirtley et al. (1993), Thomas (1997), Boudou et al. (2008), and Valentim et al. (2011) showed that the organic nitrogen in coal is present mainly as the pyrrolic (N-5, 50–80%), pyridinic (N-6, 20–40%), and quaternary nitrogen (N-Q, 0–20%) functional groups. The N-5 and N-6 groups are located on the margins of the carbon matrix and they have lower chemical stability than the N-Q within the carbon matrix. Compared with N-Q, the N-5 and N-6 groups have greater amounts of ambient H available and they are liberated preferentially as NH3 during coalification, which is when they can provide NH+ 4 for ammonian illite formation, whereas N-Q is liberated preferentially as N2. Pyrrolic nitrogen and N-6 were subject to much stronger isotopic fractionation than N-Q during

Table 5 δ15N of ammonian illite and its corresponding coal samples (‰). Age

Parting

δ15N

Coal

δ15Na

Permian

SH-3-g1 SH-3-g2 CP-3-g1 CP-3-g2 CP-3-g3 CP-3-g4 DJZ-3-g1 DJZ-3-g2 YW-3-g ZC-3-g1 Ap. YQ1-15-g1 YQ1-15-g2 YQ1-15-g3 YQ2-15-g1 FHS-15-g GSY-15-g1 GSY-15-g2 WTP-15-g3 CZ-15-g3 CZ-15-g4 Ac. At.

+5.1 +11.6 +11.7 +10.2 +10.6 +9.6 n.d. +10.4 n.d. +8.6 +9.7 +4.8 +8.3 +8.1 +2.5 +11.4 +8.7 +7.8 +3.8 +7.8 +3.7 +6.7 +8.0

SH-3-c

+4.4

CP-3-c

+5.5

DJZ-3-c

+1.2

YW-3-c ZC-3-c Ap. YQ1-150-c

n.d. −0.3 +2.0 +1.2

YQ2-15-c FHS-15-c GSY-15-c

+6.8 +6.9 +10.0

WTP-15-c CZ-15-c

+8.1 +1.4

Ac. At.

+5.0 +3.9

Carboniferous

n.d., no data; Ap., average of Permian partings; Ac., average of Carboniferous partings; At., average of all the partings. a From Zheng et al. (2015).

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peat accumulation and coalification because of their lower chemical stability. Compared with N-Q and total coal nitrogen, N-5 and N-6 usually have higher values of δ15N (Boudou et al., 2008). Therefore, the value of δ15N of the ammonian illite, consistent with that of N-5 and N-6, is higher than that of the total coal nitrogen. In addition, the value of δ15N of the ammonian illite shows an increasing trend with paleosalinity (Fig. 5), possibly due to the higher δ15N value for brackish-influenced coal compared with freshwater-influenced coal (Xiao and Liu, 2011; Zheng et al., 2015).

very sensitive to the thickness of the coherent scattering domains + + (CSDs). Therefore, the NH+ 4 /(NH4 + K ) ratio for Model II can be calculated from d005, which does not depend on the CSDs; the calculation equations are shown in the following:

4.3. Crystal chemistry of ammonian illite

d001 ¼ 5d005 :

The calculated stoichiometric formulae of the ammonian illites in the study are shown in Table 6. Those parting samples with b 20% ammonian illite were not used in the calculation. The mean stoichiometric formula of the ammonian illite in this study is (NH40.67,K0.11)(Al1.90,Fe0.06,Mg0.04)(Al0.68,Si3.32)O10(OH)2, with higher Si/AlIV (ranging from 3.33 to 6.22; average 4.88) compared with tobelite (Si/AlIV = 3). A small proportion of Fe and Mg in six-fold coordination, which is absent in tobelite, occurs in the octahedral sheets. Weaver (1979) and Bayan and Hower (2012) reported that conversion of potassian illite to muscovite involves the incorporation of interlayer K+, substitution of Al for Si in the tetrahedral layers, and loss of octahedral Mg and Fe. Therefore, it can be inferred that during subsequent diagenesis and metamorphism, the conversion of ammonian illite to tobelite should include the expulsion both of Fe and Mg from the octahedral sheets and of Si from the tetrahedral sheets, and the incorporation both of Al into the octahedral and tetrahedral sheets, and of interlayer + NH+ 4 and/or K . However, only weak correlation (r = 0.16) was observed between the Si/AlIV of the ammonian illite and the volatile matter on a dry and ash-free basis (Vdaf) for the corresponding coal samples (Fig. 6). This indicates that the conversion of ammonian illite to tobelite is influenced not only by the degree of diagenesis, but also by other fac+ tors such as the concentrations of Al3+, NH+ 4 , and K in the ambient pore fluid. The interlayer cations of ammonian illite include not only NH+ 4 but + + also some K+, and the NH+ 4 /(NH4 + K ) ratio varies from 0.58 to 0.97 (average 0.87). This is the reason why the ammonian illite of the current study has lower and higher d00l spacings than tobelite and muscovite, respectively.

+ + The NH+ 4 /(NH4 + K ) of the ammonian illite calculated by the method of Drits et al. (1997, 2005) varies from 0.51 to 0.98 (average 0.83) (Table 6), somewhat lower than that calculated from the stoichiometric formulae (average 0.87). This indicates that the variation of d005, as well as that of d001⁎, is influenced not only by the incorporation of IV NH+ 4 but also by the substitution of Al for Si in the tetrahedral layers. Multiple linear regression was used to identify the relation of d005 to + IV + NH+ 4 /(NH4 + K ) and Si/Al , as well as their degree of influence on d005. The regression equation used is as follows:

NH4 þ  ¼ 2:884d001 −28:80: NH4 þ þ Kþ

ð10Þ

The d001⁎ was calculated from d005 as follows: 

ð11Þ


IV d005 ¼ 2:000−0:00045Si=Al þ 0:065NH4 þ = NH4 þ þ Kþ ðP ¼ 0:00Þ: ð12Þ + IV + The standardized coefficients of NH+ 4 /(NH4 + K ) and Si/Al are 0.91 and − 0.04, respectively, suggesting that d005 is influenced not + IV + only by NH+ 4 /(NH4 + K ) but also, to some extent, by Si/Al . The species of interlayer cations partially represent the cation composition of the pore fluid when the ammonian illite was formed. The interlayer cations of the ammonian illite of the present study are + dominated by NH+ 4 , with a lesser proportion of K . According to the isomorphous principle (Han and Ma, 2003), the ability of K+ to be incorporated into the lattice of ammonian illite is greater than that of NH+ 4 . + Hence, ammonian illite forms under NH+ 4 -enriched and K -depleted + + conditions, and the NH+ 4 /(NH4 + K ) ratio must be lower than that of the ambient pore fluid.

5.2. Conversion of kaolinite to ammonian illite 5. Discussion 5.1. Structural feature of ammonian illite The structure of the ammonian illite in the intra-seam coal partings of the current study corresponds to Model II, as proposed by Drits et al. + (1997, 2005), and the NH+ are distributed homogeneously 4 and K within each ammonian illite interlayer (i.e., they have the same NH+ 4 / + (NH+ 4 + K ) ratio). Higashi (2000) and Juster et al. (1987) reported 2+ + that the NH+ + Na+ + K+) ratio in ammonian illite is 4 /(NH4 + Ca positively correlated with d001, mainly because of the higher radius of + NH+ 4 than K . However, Drits et al. (1997, 2005) proposed that d001 of + + ammonian illite is not only influenced by NH+ 4 /(NH4 + K ), but also

Studies by Liu et al. (1996) and Liang et al. (2005) showed that the ammonian illite associated with the coal-bearing strata in North China was formed mainly by the reaction of NH+ 4 with ambient kaolinite rather than smectite or mixed-layer illite–smectite, and the NH+ 4 was produced during the coalification stage. The absence of smectite layers in the ammonian illite structure accords with this hypothesis. The layer types in the crystal structures of kaolinite and ammonian illite are 1:1 and 2:1, respectively. The Al in kaolinite is in six-fold coordination and that of ammonian illite is in both six- and four-fold coordination (Zhao and Zhang, 1990; Bayan and Hower, 2012). Therefore, the conversion of kaolinite to ammonian illite should have restructuring of the octahedral and tetrahedral sheets and the introduction of some octahedral Al into the tetrahedral sites. Because of the difference in the Si/Al atomic ratios between ammonian illite (Si/Al = 1.29) and kaolinite (Si/Al = 1), two possible conversion processes (i.e., expulsion of Al and incorporation of Si) are thus proposed as follows: ðReaction 1Þ 3:32Al4 ðSi4 O10 ÞðOHÞ8 þ 2:68NH4 þ þ 0:44 Kþ þ 0:24Fe2þ þ 0:16Mg2þ 3þ ¼ 4ðNH40:67 ; K0:11Þ ðAl1:90 ; Fe0:06 ; Mg0:04 ÞðAl0:68 ; Si3:32 ÞO10 ðOHÞ2 þ 2:16Al þ 4:96OH− þ 6:80H2 O

ðReaction 2Þ 2:58Al4 ðSi4 O10 ÞðOHÞ8 þ 2:68NH4 þ þ 0:44 Kþ þ 0:24Fe2þ þ 0:16Mg2þ þ 2:96Si4þ þ 1:56H2 O ¼ 4ðNH40:67 ; K0:11Þ ðAl1:90 ; Fe0:06 ; Mg0:04 ÞðAl0:68 ; Si3:32 ÞO10 ðOHÞ2 þ 15:76 Hþ : Fig. 5. Plot of δ15N of ammonian illite proportion in parting samples vs. their paleo-salinity.

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9

Table 6 + + Stoichiometric formula, Si/AlIV, NH+ 4 /(NH4 + K ) and (NH4)2O amount of ammonian illite calculated from XRD and XRF data ((NH4)2O in %). Sample

Stoichiometric formula

Si/AlIV

CNH4 þ a

CNH4 þ b

(NH4)2O

SH-3-g1 SH-3-g2 CP-3-g1 CP-3-g2 CP-3-g3 CP-3-g4 DJZ-3-g1 DJZ-3-g2 YW-3-g ZC-3-g1 YQ1-15-g1 YQ1-15-g2 YQ1-15-g3 YQ2-15-g1 FHS-15-g GSY-15-g1 GSY-15-g2 WTP-15-g3 CZ-15-g3 CZ-15-g4 Av.

n.c. (NH4 0.93,K0.07)(Al1.92,Fe0.05,Mg0.03)(Al0.92,Si3.08)O10(OH)2 (NH4 0.55,K0.06)(Al1.95,Fe0.03,Mg0.02)(Al0.56,Si3.44)O10(OH)2 (NH4 0.71,K0.06)(Al1.92,Fe0.05,Mg0.03)(Al0.69,Si3.31)O10(OH)2 (NH4 0.71, K0.06)(Al1.92,Fe0.04,Mg0.04)(Al0.69,Si3.31)O10(OH)2 (NH4 0.75,K0.06)(Al1.88,Fe0.09,Mg0.03)(Al0.69,Si3.31)O10(OH)2 (NH4 0.80,K0.14)(Al1.84,Fe0.10,Mg0.06)(Al0.78,Si3.22)O10(OH)2 (NH4 0.62,K0.05)(Al1.93,Fe0.04,Mg0.03)(Al0.60,Si3.40)O10(OH)2 (NH4 0.88,K0.04)(Al1.79,Fe0.19,Mg0.02)(Al0.71,Si3.29)O10(OH)2 (NH4 0.68,K0.02)(Al1.95,Fe0.04,Mg0.01)(Al0.65,Si3.35)O10(OH)2 (NH4 057,K0.13)(Al1.92,Fe0.04,Mg0.04)(Al0.62,Si3.38)O10(OH)2 (NH4 0.66,K0.04)(Al1.99,Fe0.00,Mg0.01)(Al0.69,Si3.31)O10(OH)2 (NH4 0.70,K0.04)(Al1.98,Fe0.01,Mg0.01)(Al0.72,Si3.28)O10(OH)2 n.c. (NH40.54,K0.25)(Al1.83,Fe0.07,Mg0.10)(Al0.62,Si3.38)O10(OH)2 (NH4 0.49,K0.36)(Al1.77,Fe0.12,Mg0.11)(Al0.62,Si3.38)O10(OH)2 n.c. n.c. (NH4 0.52,K0.18)(Al1.95,Fe0.02,Mg0.03)(Al0.65,Si3.35)O10(OH)2 n.c. (NH4 0.67,K0.11)(Al1.90,Fe0.06,Mg0.04)(Al0.68,Si3.32)O10(OH)2

n.c. 3.33 6.22 4.82 4.78 4.77 4.10 5.67 4.66 5.19 5.48 4.78 4.55 n.c. 5.49 5.45 n.c. n.c. 5.17 n.c. 4.88

n.c. 0.93 0.90 0.93 0.93 0.92 0.85 0.93 0.96 0.97 0.81 0.94 0.95 n.c. 0.69 0.58 n.c. n.c. 0.74 n.c. 0.87

n.c. 0.91 0.86 0.86 0.86 0.91 0.78 0.86 0.98 0.93 0.73 0.89 0.92 n.c. 0.66 0.52 n.c. n.c. 0.85 n.c. 0.83

n.c. 6.36 3.84 4.91 4.92 5.16 5.45 4.32 5.99 4.73 3.94 4.60 4.87 n.c. 3.69 3.31 n.c. n.c. 3.59 n.c. 4.65

+ + n.c., not calculated; CNH4 þ , ratio value of NH+ 4 /(NH4 + K ). Parting samples with ammonian illite proportion b20% were not used to calculate a Calculated from the stoichiometric formulae of ammonian illite in parting samples. b Calculated by Drits et al.'s (1997, 2005) method.

If the ammonian illite is formed under Reaction 1 (Al-expulsion process), some Al-hydroxide (e.g., boehmite, diaspore) should be identifiable by the XRD analysis because Al has low capacity for transfer, especially under acidic conditions (Han and Ma, 2003). However, only four of the partings sampled in the present study contained boehmite (at concentrations b10%); thus, it is inferred that the ammonian illite formed mainly under Reaction 2 (Si-incorporation process). 5.3. Geological factors influencing ammonian illite formation Ammonian illite, as an authigenic mineral, is produced by the transformation of other clay minerals (e.g., kaolinite, illite, and pyrophyllite) during diagenesis (Juster et al., 1987; Daniels et al., 1994). Studies (e.g., Juster et al., 1987; Liu et al., 1996; Liang et al., 2005) showed that the transformation of other clay minerals to ammonian illite is influenced considerably by the diagenetic temperature. However, the proportion of ammonian illite within the total clay minerals of the samples in the present study shows a decreasing trend with diagenetic temperature (coal rank), and it is also accompanied by a decreasing volatile matter (dry and ash-free basis) yield of the associated coal (Fig. 7). This indicates that ammonian illite formation should have been subjected to some geological factors other than diagenetic temperature, such as depositional conditions. The relation between the paleosalinity of the intra-seam partings and the proportion of ammonian illite within the total clay minerals (Fig. 8) shows the latter increases with the inferred paleosalinity increasing (r = 0.73) when the paleosalinity is b77.94‰ and it reaches 100% when the paleosalinity is N77.94‰. Ammonian illite

Fig. 6. Plot of Si/AlIV of ammonian illite in parting samples vs. volatile matter on dry and ash-free basis (Vdaf) of the corresponding coals. The Vdaf data are from Zheng et al. (2015).

is an authigenic mineral formed during diagenesis and therefore, its formation could not have been influenced directly by the depositional environment. The NH+ 4 in ammonian illite has been considered derived from organic nitrogen (N-5 and N-6) in the coal during coalification (Juster et al., 1987; Liu et al., 1996). As the predecessors of the NH+ 4 fixed in ammonian illite, N-5 and N-6, are oxidized preferentially in freshwater because of their relatively high oxidizability, and thus it is inferred that brackish water favors the preservation of N-5 and N-6 during peat accumulation. Therefore, the presence of brackish-influenced coal, which is in turn influenced by the depositional environment, contributed indirectly to ammonian illite formation in the intra-seam partings of the present study. 6. Conclusions The minerals in the intra-seam partings of the Permo-Carboniferous coals in the Qinshui Coalfield are dominated by kaolinite and ammonian illite. The basal spacing (d001; average 10.267 Å) of ammonian illite is greater than that of potassian illite and muscovite but lower than that of tobelite, indicating that the interlayer cations are dominated by + NH+ 4 with some minor K . The ammonian illite of the present study is a pure illite and that no expandable (smectite) layers are interstratified, and the K+ and NH+ 4 are distributed homogeneously within each illite layer. The average stoichiometric formula of the ammonian illite of the present study was inferred as

Fig. 7. Plot of ammonian illite proportion within the total clay minerals for parting samples vs. Vdaf of the corresponding coals. The Vdaf data are from Zheng et al. (2015).

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Fig. 8. Plot of ammonian illite proportion within the total clay minerals for parting samples vs. their paleo-salinity.

(NH40.67,K0.11)(Al1.90,Fe0.06,Mg0.04)(Al0.68,Si3.32)O10(OH)2, and the NH+ 4 / + IV (NH+ 4 + K ) varies from 0.58 to 0.97 (average 0.87). The Si/Al ratio of the ammonian illite (average 4.88) higher than that of tobelite indicates that the conversion of ammonian illite to tobelite includes the expulsion of Si from the tetrahedral sheets and the incorporation of Al into the oc+ tahedral and tetrahedral sheets. The substitution of NH+ 4 for K has a dominant effect on d005 of ammonian illite, followed by the introduction of AlIV into the tetrahedral sites. It is inferred that the ammonian illite of the present study formed by the incorporation of Si into pre-existing kaolinite during diagenesis. Brackish water favored the preservation of the N-5 and N-6 groups in the carbon matrix of the coal during deposition, which in turn provided the NH+ 4 for ammonian illite formation during subsequent diagenesis. The δ15N values of the ammonian illite are higher than those of the corresponding coals, mainly attributed to the δ15N values of N-5 and N-6 higher than that of N-Q. Acknowledgments This research was supported by the National Natural Science Foundation of China (Nos. 41502154 and 41072119), the Coal Seam Gas Joint Research Fund of Shanxi Province (No. 2013012005), the Technological Key Research Program of Education Department Henan Province (No. 13A170011), and Doctor Foundation of Henan Institute of Engineering (No. D2103016). Editor Shifeng Dai and the two anonymous reviews are highly appreciated for their careful and constructive comments for the manuscript. References Bayan, M.R., Hower, J.C., 2012. Illite crystallinity and coal metamorphism for selected central Appalachian coals and shales. Int. J. Coal Geol. 94, 167–172. Bobos, I., 2012. Characterization of smectite to NH4-illite conversion series in the fossil hydrothermal system of Harghita Bãi, East Carpathians, Romania. Am. Mineral. 97, 962–982. Bobos, I., Eberl, D.D., 2013. Thickness distributions and evolution of growth mechanisms of NH4-illite from the fossil hydrothermal system of Harghita Bãi, eastern Carpathians, Romania. Clay Clay Miner. 61, 375–391. Bohor, B.F., Gluskoter, H.J., 1973. Boron in illite as an indicator of paleosalinity of Illinois coals. J. Sediment. Petrol. 43, 945–956. Boudou, J.P., Schimmelmann, A., Ader, M., Mastalerz, M., Sebilo, M., Gengembre, L., 2008. Organic nitrogen chemistry during low-grade metamorphism. Geochim. Cosmochim. Acta 72, 1199–1221. Bouška, V., Pešek, J., Sýkorová, I., 2000. Probable modes of occurrence of chemical elements in coal. Acta Montana Ser. B Fuel Carbon Min. Proc. Praha 10 (117), 53–90. Boyd, R.J., 2002. The partitioning behaviour of boron from tourmaline during ashing of coal. Int. J. Coal Geol. 2002 (53), 43–54. Boyd, S.R., Hall, A., Pillinger, C.T., 1993. The measurement of δ15N in crustal rocks by static vacuum mass spectrometry: application to the origin of the ammonium in the Cornubian batholith, southwest England. Geochim. Cosmochim. Acta 57, 1339–1347. Brӓuer, K., Hahne, K., 2005. Methodical aspects of the 15N-analysis of Precambrian and Paleozoic sediments rich in organic matter. Chem. Geol. 218, 361–368. Busigny, V., Ader, M., Cartigny, P., 2005. Quantification and isotopic analysis of nitrogen in rocks at the ppm level using sealed tube combustion technique: a prelude to the study of altered oceanic crust. Chem. Geol. 223, 249–258. Chung, F.H., 1974a. Quantitative interpretation of X-ray diffraction patterns of mixtures. I. Matrix-flushing method for quantitative multicomponent analysis. J. Appl. Crystallogr. 7, 519–525.

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