Identifying sources of respirable quartz and silica dust in underground coal mines in

southern West Virginia, western Virginia, and eastern Kentucky

Steven J. Schatzel

National Institute for Occupational Safety and Health, Pittsburgh Research Laboratory, 626 Cochrans Mill Road, PO Box 18070, Pittsburgh, PA 15236, USA

E-mail address: zia6@cdc.gov.

abstract

Prior research has suggested that the source of respirable silica dust in underground coal mines is typically

the immediate top or bottom lithology adjacent to the mined seam, not mineral matter bound within the

mined coal bed. Geochemical analyses were applied in an effort to identify the specific source rock of

respirable quartz dust in coal mines. The analyses also demonstrate the compositional changes that take

place in the generation of the respirable dust fraction from parent rock material. All six mine sites were

mining coal with relatively low mineral matter content, although two mines were operating in the Fire Clay

coal bed which contains a persistent tonstein. Interpretations of Ca, Mg, Mn, Na, and K concentrations

strongly suggest that the top strata above the mined seam is the primary source of mineral dust produced

during mining. One site indicates a mixed or bottom source, possibly due to site specific conditions.

Respirable dust compositional analyses suggest a direct relationship between the quantity of mineral Si and

the quantity of quartz Si. A similar relationship was not found in either the top or bottom rocks adjacent to

the mined seam. An apparent loss of elemental Al was noted in the respirable dust fraction when compared

to potential parent rock sources. Elemental Al is present in top and bottom rock strata within illite, kaolinite,

feldspar, and chlorite. A possible explanation for loss of Al in the respirable dust samples is the removal of

clays and possibly chlorite minerals. It is expected that removal of this portion of the Al bearing mineral

matter occurs during rock abrasion and dust transport prior to dust capture on the samplers.

1. Introduction

Overexposure to respirable quartz dust can lead to the development of silicosis, a debilitating and potentially fatal lung disease. The

current federal permissible coal mining exposure limit for respirable

dust in coal mines is 2 mg/m3 (Title 30, Code of Federal Regulations,

2007). If the respirable dust sample contains more than 5% quartz,

federal regulations reduce the allowable respirable dust standard the

coal mine operator must meet. The reduced standard is defined as (10/

silica %) mg/m3 (Title 30, Code of Federal Regulations, 2007). In the

discussion of mining-related respirable silicate mineral matter, the

term “silica” has been used to include all crystalline, crypto crystalline,

and amorphous forms of silica dioxide in respirable dust from all types

of mines.

Compliance data show that a significant portion of roof bolter and

continuous miner samples collected exceeded the federal dust

standard (National Institute of Occupational Safety and Health,

2001). In samples from 2000 to 2004, 20% exceeded the MSHA

reduced standard silica dust concentration of 100 μg/m3 (Goodman,

2006). The control of quartz dust in coal mines is limited by

uncertainty regarding the origins of quartz yielded in the respirable

dust fraction. Results from several research studies indicate that the

mined seam is not the principle source of respirable silica dust (Hicks

and Nagelschmidt, 1943; Dodgson et al., 1971; Ramani et al., 1987).

Clay coatings have been noted on respirable, quartz dust grains

(Wallace et al., 1994; Harrison et al., 1997). The frequency of silicosis

appears to increase with the presence of more toxic, clean SiO2 quartz

tetrahedra (Dodgson et al., 1971; Robock and Bauer, 1990; Chen et al.,

2005; Harrison et al., 2005). A key project task for this research is to

develop methodology capable of distinguishing potential top rock and

bottom rock sources of the respirable silica dust fraction produced

during continuous mining activities to identify the parent rock source.

2. Methodology

2.1. Study area

The locations of the six cooperating mine sites are shown in Fig. 1.

The study area encompasses parts of western Virginia, southern West

Virginia, and eastern Kentucky. The coal beds included a range in rank

from high volatile A bituminous in Kentucky and West Virginia to

medium volatile bituminous in Virginia. All mines are drift, room and

pillar operations and are using on the order of 14 m3

/s of ventilation

air at the last open cross cut. The positional data and the coal bed

names for the cooperating mines are given in Table 1. More detailed

information on the study sites is contained in Appendix A.
Fig

.1. The locationof mine sites includedin this study.

2.2. Field sampling

The protocol developed in this study called for sampling of rock

and coal from known sites of at current production sections and in

previously mined locations where high quartz concentrations were

determined in respirable dust. Samples of overburden rock immediately overlying the coal bed and samples of underlying bottom rock

were retrieved. These rock strata are referred to as top and bottom

rock respectively and make up what the coal mining industry

commonly refer to as“roof” and“floor” rock. Lump samples of coal

were retrieved from rib locations after removal of the top 3 cm of

surface coal. Lump samples of coal, top and bottom were also retrieved

from operating sections. Rock binders or apparent mineralized zones

occurring within the mined coal bed were also sampled.

Dust sampling was also conducted on the selected continuous

miner production sections. Sampling was conducted at one intake and

one return location at each mine. An exception was Mine E where two

return locations were used for comparison of the dust compositional

changes with distance from the miner. Conventional personal dust

samplers, 10-mm nylon cyclones and 37-mmfilter cassettes, were

used for all sampling. The samplers were operated at 2.0 L/min. Two

dust samplers were utilized at each sampling location.

2.3. Analytical methods

Massesof al

l dust samples were measuredin the Respiratory

Hazards Control Branch (RHCB) weighing laboratoryat NIOSH.A setof

dustfilter cassettes was used for analysisby X-rayfluorescence (XRF),

a non-destructive, semi-quantitative method giving elemental composition of major cations on the filters. Following XRF analysis, the

Table 1 a Not determined. A Kentucky nd

a nda Fire Clay

B West Virginia N37° 43.456′ W081° 59.747′ Williamson

C Virginia N37° 11.488′ W081° 43.007′ Lower Horsepen

D West Virginia N37° 54.389′ W081° 34.071′ Powellton

E Kentucky N37° 10.3821′ W083° 11.737′ Fire Clay

F West Virginia N37° 44.632′ W082° 12.240′ Lower Cedar Grove

Mine site State Latitude Longitude Coal bed

Geographic positioning system (GPS) coordinates for locations near the portals of the

six coal mine study sites

Table 2X ray diffraction resultsSample type Top Top Bottom Top Top Bottom Top Bottom Bottom Top Coal binder Bottom Top Top BottomMine A A A C C C D D D E E E F F FMineral species arock4top arock11top arock10bot crock3top crock6top crock1bot drock5top drock3bot drock6bot erock6top erock7coal erock5bot frock2top frock6top frock1bot

%

Quartz 27 30 44 23 38 16 50 59 26 37 1 28 37 31 33Kaolinite 4 6 8 0 0 0 7 2 15 4 95 57 4 4 63Illite 43 36 17 49 19 62 34 29 48 40 bda 33 43

Albite 9 12 19 17 5 14 6 5 11 9 bda 13 10 10 bda

Chlorite 18 16 11 10 1 4 4 bda bda 10 bda 1 16 12 bdaHematite bda bda bda bda bda bda bda bda bda bda bda bda bda bda bdaPyrite bda bda bda bda 19 bda bda bda bda bda bda bda bda bda bdaSiderite bda bda bda bda 18 bda bda bda bda bda bda bda bda bda bdaCalcite bda bda bda bda bda bda bda bda bda bda bda bda bda bda bdaAnkerite bda bda bda bda bda bda bda bda bda bda bda bda bda bda bdaBassinite bda bda bda bda bda bda bda bda bda bda bda bda bda bda bdaOrganic 0 1 2 1 1 5 0 6 1 0 3 0 0 0 4

a Below detection.
ARTICLE IN PRESS

filters were analyzed by the MSHA P-7 standard Fourier transfer infrared (FTIR) analysis for the determination of quartz content in the dust

sample.

To compare dust samples to the rock samples, geochemical and

mineralogical analyses were performed. The elemental composition of

the rock samples was determined by inductively coupled plasmaatomic emission spectroscopy (ICP-AES). Low-temperature ashing

(LTA) techniques were utilized to remove organic matter from the

samples, and preceeded semi-quantitative XRD using internal

standards mixed into the samples. See Appendix A for a detailed

discussion of the analytical methods.

Table 3

Petrographic characterization of coal samples from six coal mine study sites

Mine A A B C C D D E E F F

Sample number arock1coal arock2coal brock2coal crock5coal crock2coal drock7coal drock8coal erock2coal erock7coal frock8coal frock12coal

Basis %

Telinite 21.2 12.6 17.8 5.0 7.0 10.0 19.4 37.2 23.2 21.2 5.4

Collotelinite 23.0 22.0 33.6 27.4 35.2 25.2 36.4 22.4 24.0 45.4 4.6

Vitrodetrinite 0.6 0.6 0.2 0.8 1.0 1.4 1.0 0.6 0.2 0.4 0.0

Collodetrinite 41.8 24.8 21.2 35.4 30.2 23.8 17.4 20.6 32.4 13.8 9.2

Corpogelinite 2.6 3.2 4.2 0.8 0.4 1.6 1.8 2.6 4.6 2.0 2.6

Fusinite 0.6 10.6 4.4 9.4 10.2 19.4 5.2 2.4 3.6 3.4 21.0

Semifusinite 1.6 6.0 5.0 18.6 15.2 6.8 7.6 4.0 4.0 4.6 25.8

Micrinite 1.6 3.2 3.4 1.4 0.8 4.2 2.8 3.8 2.4 1.4 6.0

Macrinite 0.0 0.0 0.0 0.0 0.0 0.0 0.2 0.0 0.0 0.0 0.0

Inertodetrinite 0.2 1.2 0.2 1.2 0.0 1.0 0.8 0.0 0.2 0.2 0.8

Funginite 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0

Secretinite 0.0 0.0 0.0 ta 0.0 0.0 t

a 0.0 ta 0.0 ta

Sporinite 5.6 14.8 8.2 0.0 0.0 5.4 6.8 5.6 4.8 7.0 22.6

Cutinite 0.6 0.6 0.6 0.0 0.0 1.2 0.6 0.2 0.6 t

a 1.0

Resinite 0.6 0.2 1.0 0.0 0.0 ta 0.0 0.6 ta 0.6 1.0

Liptodetrinite 0.0 0.2 0.2 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0

Alginite 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0

Total vitrinite 89.2 63.2 77.0 69.4 73.8 62.0 76.0 83.4 84.4 82.8 21.8

Total liptinite 6.8 15.8 10.0 0.0 0.0 6.6 7.4 6.4 5.4 7.6 24.6

Total inertinite 4.0 21.0 13.0 30.6 26.2 31.4 16.6 10.2 10.2 9.6 53.6

Total maceral groups 100.0 100.0 100.0 100.0 100.0 100.0 100.0 100.0 100.0 100.0 100.0

Rmax 0.90 0.92 0.94 1.17 1.22 0.73 0.80 0.85 0.96 1.01 0.88

Standard deviation 0.07 0.05 0.06 0.17 0.09 0.08 0.06 0.05 0.06 0.05 0.06

Rrandom 0.83 0.85 0.91 1.14 1.19 0.69 0.75 0.83 0.91 0.95 0.83

Standard deviation 0.07 0.05 0.06 0.09 0.09 0.08 0.05 0.05 0.05 0.06 0.06

a Trace.

Fig. 2. The distribution of organic matter maceral groups in petrographic samples from

all six mine sites.

3. Results and discussion

3.1. Mineral analysis

Three XRD analyses were conducted for each mine site (Table 2).

Quartz concentrations determined range widely from 1% in a clay

binder at mine E to 50% in the top rock at mine D (Table 2). Quartz

contents in the top and bottom units shown in Table 2 both average

34%. Illite concentrations exceed kaolinite concentrations and range

from 17% in the bottom of Mine A to 62% in the bottom of Mine C.

Kaolinite and illite were not distinguished in two samples, one from

Mine E, and one from Mine F, where the sum percentage of kaolinite

plus illite is shown. The averaged total aluminosilicate contents

reported (illite plus kaolinite) are slightly higher for the bottom rock

samples compared to the top samples, 50 and 41%, respectively.

Fig. 3. A low R2 value was determined for the trend line fit to the submaceral collotelinite

and liptinite maceral group concentrations.

Reported albite concentrations are quite high, reaching a maximum of 19% in a top sample from Mine A, and a minimum

concentration of below detection limits (∼5 to 10%) in samples from

Mines D, E, and F. The lab reported that albite values could actually

represent concentrations of different or mixed feldspar species, and

thus is referred to as feldspar content in the remainder of this report.

The averaged feldspar content of bottom and top samples shown in

Table 2 is identical, 11%. Pyrite and siderite concentrations were

reported from only one sample from mine C (Table 2). Chlorite is much

more abundant in the top rock samples compared to bottom rock

samples. Chlorite is typically an epigenetic mineral and its mode of
occurrence may range from a fine, highly dispersed, mineral matter to

cleat infillings.

Table 4

ICP-AES major cation analysis data for overlying and underlying rock unit samples

Oxide

SiO2 Al2O3 Fe2O3 CaO MgO MnO Na2O K2O TiO2 P2O5

Mine Sample number Sample type %

A arock4top Top 54.38 20.38 7.42 0.60 2.14 0.097 1.18 4.93 0.931 0.18

A arock7top Top 57.47 18.89 7.16 0.91 2.09 0.094 1.32 4.39 1.009 0.20

A arock9top Top 56.96 21.63 5.98 0.29 1.91 0.059 1.19 5.36 0.931 0.19

A arock11top Top 58.65 19.82 6.24 0.38 1.86 0.074 1.28 4.84 0.878 0.19

A arock11Atop Top (duplicate) 58.46 19.75 6.23 0.37 1.86 0.074 1.29 4.83 0.876 0.18

A arock3bot Bottom 67.62 16.78 2.94 0.08 1.05 0.020 1.49 3.45 0.902 0.04

A arock6bot Bottom 58.59 22.09 4.44 0.12 1.68 0.027 1.07 5.14 1.061 0.04

A arock10bot Bottom 72.91 14.71 2.63 0.09 0.93 0.020 1.79 2.90 0.886 0.06

A arock12bot Bottom 76.44 11.93 1.92 0.23 0.61 0.014 1.91 2.39 0.722 0.18

B brock3top Top 56.38 21.52 6.18 0.27 1.99 0.066 0.48 4.93 0.913 0.14

B brock4top Top 52.29 18.25 11.50 0.69 2.10 0.200 0.59 4.03 0.824 0.18

B brock7top Top 54.86 18.06 10.42 0.72 2.11 0.153 0.23 3.76 0.935 0.22

B brock8top Top 63.65 21.24 2.01 0.08 0.85 0.007 0.35 3.63 1.582 0.03

B brock8Atop Top (duplicate) 62.81 21.00 1.98 0.08 0.84 0.005 0.37 3.57 1.560 0.03

B brock1bot Bottom 50.25 23.67 4.67 0.09 1.81 0.027 0.37 4.67 0.882 0.04

B brock6bot Bottom 62.56 20.59 2.46 0.09 0.80 0.005 0.35 2.53 1.535 0.04

C crock3top Top 57.99 21.37 5.01 0.2 1.99 0.039 0.21 5.9 0.83 0.09

C crock8top Top 33.9 6.31 31.64 1.48 1.07 1.058 0.5 1.59 0.357 0.3

C crock1bot Bottom 54.27 24.11 2.95 0.15 1.4 0.015 0.3 6.83 0.92 0.04

C crock7bot Bottom 66.28 17.42 3.11 0.11 1.15 0.02 0.18 4.62 0.926 0.04

D drock2top Top 54.2 19.55 8.66 0.39 2.0 0.174 0.24 4.56 0.844 0.14

D drock5top Top 63.23 7.49 14.43 0.73 1.49 0.326 0.71 1.38 0.404 0.09

D drock1bot Bottom 65.81 19.44 1.49 0.09 0.6 0.006 0.18 3.22 1.326 0.04

D drock3bot Bottom 73.97 11.96 1.32 0.05 0.47 0.005 0.18 2.57 1.791 0.03

D drock6bot Bottom 60.7 23.48 1.77 0.1 0.81 0.006 0.21 4.15 1.317 0.04

E erock4top Top 71.09 11.36 4.04 1.77 1.31 0.055 1.34 1.91 0.933 0.19

E erock6top Top 59.56 14.66 8.81 1.1 1.81 0.162 1.12 3.01 0.887 0.16

E erock3bot Bottom 71.61 15.83 1.51 0.04 0.71 0.005 0.3 4.08 1.443 0.04

E erock5bot Bottom 43.53 32.66 0.8 0.22 0.22 0.005 0.37 0.46 0.983 0.1

F frock2top Top 57.00 20.77 6.84 0.44 2.05 0.096 0.73 0.14 0.950 0.14

F frock4top Top 56.68 21.18 5.92 0.26 2.05 0.067 0.60 0.14 0.990 0.14

F frock11top Top 52.23 24.58 6.77 0.26 1.93 0.117 0.66 0.15 0.912 0.15

F frock16top Top 59.31 18.00 7.15 0.51 1.82 0.107 1.07 0.18 1.053 0.18

F frock18top Top 42.68 16.12 19.04 0.84 2.16 0.335 0.52 3.47 0.713 0.20

F frock1bot Bottom 67.15 18.84 1.40 0.13 0.55 0.009 0.30 0.04 1.241 0.04

F frock3bot Bottom 66.95 19.35 1.55 0.08 0.65 0.007 0.25 0.04 1.333 0.04

F frock14bot Bottom 51.09 24.15 4.50 0.10 1.87 0.030 0.38 0.04 0.956 0.04

F frock19bot Bottom 68.92 16.94 1.65 0.14 0.68 0.005 0.17 3.53 1.690 0.03

Analyses of duplicate samples prepared and analyzed by the lab are shown with the same sample number except for an “A” in the name before the top or bottom suffix.

3.2. Coal analysis

Vitrinite is the dominant maceral group with concentrations that

vary widely from 21.8% to 89.2% (Table 3). Liptinite and inertinite

contents also vary widely from 0.0% to 24.6% and from 4.0% to 53.6%,

respectively. Fig. 2 shows the maceral group concentrations for each

mine site. Petrographic data show a complete lack of liptinite macerals

from the Lower Horsepen coal bed sample, Mine C. This sample also has

the highest vitrinite reflectance value which is the most likely cause for

the lack of identifiable liptinite macerals. Liptinite maceral concentrations determined in previous studies correlate well with macerals and

maceral groups associated with high degrees of weathering and

potential surface exposure of the organic material (International

Committee for Coal Petrography, 2001).

Table 5

Dust filter XRF data

Mine A B C C D D D E E E E

Sample no. 421963 422003 458582 458583 483590 483594 483597 483595 483598 483600 483601

Oxide %

SiO2

Al2O3

Fe2O3

CaO

39.91

2.92

1.88

0.33

37.27

2.93

1.73

0.42

31.65

2.08

1.06

0.23

34.07

2.24

1.20

0.22

17.50

0.43

2.63

0.00

17.88

0.52

2.45

0.00

18.07

0.56

2.46

0.00

46.11

2.82

0.90

0.96

53.46

3.25

1.01

1.27

43.35

2.87

0.77

0.78

44.16

2.87

0.80

0.79

MgO

MnO

1.84

0.25

1.64

0.19

1.62

0.00

1.69

0.12

1.55

0.36

1.46

0.31

1.52

0.26

1.30

0.11

1.47

0.12

1.12

0.09

1.13

0.08

Na2O

K2O

TiO2

P2O5

0.91

6.64

1.18

1.42

0.23

5.43

1.33

0.12

0.29

5.03

0.60

0.93

b0.02

6.74

0.68

1.66

1.92

10.84

0.00

6.70

1.52

5.23

0.00

2.99

2.00

9.10

0.00

5.51

0.98

4.75

1.12

0.67

1.19

6.27

1.36

1.17

0.88

4.90

1.23

0.40

0.88

4.94

1.26

0.36

From Fig. 3, it can be seen that

the total liptinite maceral group concentrations are generally inversely

proportional to the collotelinite submaceral group concentrations.

However, the R2 value shows a relatively low fit quality between the

trend line and the data. The lack of a more statistically significant inverse

relationship may be the product of grab sampling procedures instead of
ARTICLE IN PRESS

Fig. 4. Titanium-normalized dust concentrations are shown divided by Ti-normalized

rock concentrations.

channel sampling methods or documented transitions of the depositional mires between planar/rheotrophic and raised/ombrotrophic

settings in the coal beds (Cecil et al., 1985; Eble and Grady, 1990;

Martino, 1996; Greb et al., 1999; Bodek and Eriksson, 2005).

3.3. Dust source rock analyses

Although a geochemical analysis is proposed as a method of

analyzing respirable dust contributions from silica sources, there are

certain limitations to the proposed technique. The application of

conventional geochemistry mixing equations has not been sought

since the materials being analyzed at the mine site are not conserved.

For example, a portion of the coal characterized is removed as mined

material. Other non-conservative processes include the disaggregation of rock material, entrainment of dust in ventilation air, followed

by the transport and capture of a portion of the airborne dust in the 1

to 10 μm size fraction on cyclone samplers. Therefore, geochemical

analyses are applied as an indicator or tracer to link the respirable dust

to the parent rock.

Elemental analyses of rock samples (ICP-AES data set, Table 4) and

the dust samples (XRF data set, Table 5) are shown. In an effort to link

weathered, ancient soils to their parent rock, researchers have used

Al or Ti normalization techniques (Maynard, 1992; Ohmoto, 1996;

Rye and Holland, 1998; Bohn et al., 2002). A similar approach has

been used in this study to determine the most appropriate cations to

use in identifying the parent rock for the respirable dust samples. In

Fig. 4, the concentration data for dust samples are divided by the

determined Ti concentrations. These values are then divided by Tinormalized top and bottom rock sample data (Fig. 4). The graph

provides an indicator of how much the dust sample concentrations

are modified from the rock sample concentrations at the time of

deposition in addition to compositional changes occurring during

rock abrasion, dust transport, and sampling.

Problems may exist in applying Fe and Al data to identify the

respirable dust parent rock. In Fig. 4, Al, Fe and P data show the

greatest difference between the Ti-normalized dust and rock data for

either top or bottom rock data. Iron is present in pyrite and siderite

above detection limits in only one rock sample (Table 2). In low

mineral matter coals, excess Fe may be organically combined and

thus not detected by XRF (Tuttle, 1990). Table 6 presents a series of

elemental and oxide ratios calculated for each mine site. The dust

SiO2/Al2O3 ratios bear no resemblance to either the top or bottom

rock and are not representative of typical sandstones or shales (Horn

and Adams, 1966; Gromet et al., 1984). Table 6 also shows TiO2/Al2O3

data where the respirable dust fraction shows an enrichment of Ti

oxide relative to the potential parent rocks and an apparent loss of Albearing mineral matter. Consequently, Ca, Mg, Mn, Na, and K were

chosen for the source rock analysis of respirable dust samples.

Fig. 5 shows dust sample concentrations for Ca, Mg, Mn, Na, and K

normalized using top and bottom rock concentration data. To perform

the normalization, cation concentrations shown for respirable mine

dust were divided by cation concentrations determined for the

retrieved rocks samples (Fig. 4a through e). Also included is a

reference line at y=1 where the dust to rock ratio is one, an ideal

correlation between the rock and dust data. Error bars are included in

plots for the 5 mine sites. The Mine A data is shown in Fig. 5a. The Ca,

Mg, and Mn data for the top rock samples are all closer to ideal

correlation line than the bottom rock sample data although only the

Mn data values are significantly different, based on error bar

dimensions. The Mg/Ca ratios for Mine A show just a slightly stronger

correspondence between the dust and top rock values. The Na/Ca data

show a close correspondence between the dust and the top rock at all

mine sites.

Willett et al. (2000) reported Mn to be primarily associated with

carbonate with lesser amounts in aluminosilicates, sulfides and

organic matter. In the same study, Ca was found to occur dominantly

within the carbonate mineralogy. For the other alkali and alkaline

earth elements, Willett et al. (2000) reported Mg occurs in carbonates

and silicates, Na is present in aluminosilicates and exchangeable

cations, and K is dominantly present in aluminosilicates. Although no

carbonate mineralogy was detected by XRD, low concentrations of

carbonate that are widely dispersed could be present below XRD

detection limits. Ward (2002) reported that carbonate and clay

components of coal mineralogy may be commonly present as

epigenetic minerals. Late phase epigenetic mineralization may

account for the presence of clay and low concentrations of carbonate

with associated alkali and alkaline earth minerals plus Mn with less

efficient fluid transport to the bottom rock strata underlying the coal

beds in this study (Schatzel, 2001; Schatzel and Stewart, 2003).

Data from Mine B are shown in Fig. 5b. The Ca and Mn data for the

top rock samples are much closer to the ideal correlation line than the

bottom rock samples and a significant difference is shown between

the two sources of rock sample data. The Mg data for the top rock

sample show enrichment compared to the bottom rock sample data

(closer to the y= 1) but are not statistically different. Data for Mine C

do not show the Ca and Mn dust concentrations to be similar (near

y= 1) to either the top or bottom rock samples (Fig. 5c). The Ca and

Table 6

Elemental and oxide ratios for rock and dust samples

Sample site Sample type Mg/Ca Na/Ca SiO2/Al2O3 TiO2/Al2O3

Mine A Top 3.10 2.37 2.82 0.0465

Bottom 6.93 12.5 4.21 0.0545

Dust 4.75 2.87 13.7 0.404

Mine B Top 3.11 0.803 2.50 0.0462

Bottom 11.2 4.31 2.69 0.0610

Dust 3.30 0.579 12.7 0.455

Mine C Top 1.54 0.439 3.32 0.0429

Bottom 8.27 1.92 2.90 0.0444

Dust 6.22 0.662 15.2 0.297

Mine D Top 2.63 0.88 4.34 0.0462

Bottom 6.61 2.47 3.65 0.0221

Dust bda bda 35.3 bda

Mine E Top 0.917 0.890 5.02 0.0699

Bottom 3.02 2.67 2.37 0.0500

Dust 1.05 1.01 15.8 0.422

Mine F Top 3.66 1.85 2.70 0.0457

Bottom 7.03 2.54 3.21 0.0658

Dust ndb ndb ndb ndb

Average sandstone valuesc 0.665 0.326 3.83 0.0414

Average shale valuesd 0.556 0.123 3.36 0.0551

a Below detection. b Not determined. c Horn and Adams (1966). d Gromet et al. (1984).
ARTICLE IN PRESS

Mn data for top and bottom rock are shown to be statistically different

but about equidistant from y= 1 and thus indeterminate regarding the

likely source of the dust. The Mg, Na, and K data differences between

the top and bottom rock samples do not exceed the calculated errors.

Fig. 5. Elemental ratios for Ca, Mg, Mn, Na and K determined from rocks and dust samples retrieved from Mines A through E. Vertical scales are variable.

Continuous miner downtime limited dust sampling at Mine C. Also,

the evidence of bottom rock being cut in the production section could
account for a mixed top and bottom rock source for the sampled dust.

Table 6 shows the Mg/Ca ratios for the bottom rock at Mine C to be

closer to the dust values although the Na/Ca data show a close

correspondence between the dust and the top rock.

Fig. 6. Quartz and elemental silicon concentrations are shown from respirable dust

samples as determined by P-7, P-7 alternate and XRF techniques.

For Mine D, the continuous miner production problems also affected

the dust data set such that no Ca data are included in the plot (Fig. 5d). As

in Fig. 5a and b, the plot shows Mn data for the top rock samples are

much closer to the ideal correlation line than the bottom rock samples

and the error calculations show these to be distinct data sets. The Mg, Na,

and K data show the top and bottom rock samples to be not statistically

distinct but the Mg and Na data from the top rock samples are closer to

the y=1 line (Fig. 5d). In Fig. 5e, Mine E data shows the Ca, Mg, Mn and

Na for the top rock samples results all to more closely approach the ideal

correlation line than the bottom rock values, although only the Ca and

Mn rock data sets are statistically distinct.

No Mine F dust cassettes were preserved for XRF elemental analysis.

Rock samples from Mine F analyzed by XRD (Table 2) show potential

quartz dust sources in both the top and bottom rock strata. Coal height at

the working face was measured to be 0.85 m but the mining height away

from the face was 1.4 to 1.9 m. Nearly all non-coal material being removed

at these sites was from the top strata. Since 0.65 to 0.85 m of rock material

is removed from the top strata and minimal or no rock removal from the

bottom strata, the top rock is considered the most likely source rock for

respirable silica dust at Mine F.

Fig. 6 shows quartz and Si dust compositional data associated

with the P-7 analysis, P-7 alternate analysis, and XRF. The P-7 and P-7

alternate analysis results show the quartz contents determined on

the filters. Silicon concentrations for the quartz mineral matter

fraction were calculated from the P-7 and P-7 alternate analysis. The

XRF data provide determinations of Si concentrations from all

mineral matter sources present on the filters. For the Mine A

samples, the P-7 method averaged about 6% quartz and the P-7

alternate method yielded about 10% quartz for duplicate samples

splits (Fig. 6).

The graph in Fig. 6 suggests a good correspondence between total Si

concentrations determined in the mineral matter on the dust filter and

Si concentrations as respirable quartz. This result may be surprising

since a variety of Si sources are present in the top and bottom rock

samples (Table 2). The data shown in Fig. 6 are derived from all six mines

which include a range of stratigraphic settings. The Fire Clay coal bed

(Mines sites A and E) contains a prominent tonstein ash layer which

altered during peat development and coal diagenesis (Hower et al.,

1999).

To further explore the relationship between the total silica and

quartz silica origins in the respirable dust samples, elemental data for

the top, bottom and dust samples are plotted in Fig. 7. The dust data

produce a relatively flat slope and a relatively high R2 value of 0.79.

The relatively flat slope and high R2 suggest significant contributions

of Si are present in addition to quartz and that they are contributing to

the respiratory dust fraction at a relatively consistent rate at the

different mine sites.

The apparent loss of Al-bearing minerals shown (Fig. 4, Table 6)

is thought to be produced by the breakage, entrainment, and

capture of the respirable dust particles. This process is likely to be

influenced by the mineralogical composition of the Al- and Sibearing dust fraction. Quartz and feldspar grains are harder than

clays and tend to form equant fragments upon breakage whereas

kaolinite, illite and possibly chlorite can form long or platy

fragments and are not mechanically strong. The systematic loss of

Al minerals from respirable dust may have played a significant role

producing the observed trend between total silica and quartz silica

dust (Fig. 7).

A determination is given on the degree of Al loss from the parent

rock in the respirable dust samples. Based on elemental ratios, the Al

loss from the top rock concentrations relative to Si concentrations is

about 81%. Compared to Ti, elemental the loss of Al is about 87% from

top rock concentrations. Since illite, kaolinite, and chlorite contain

Si in additional to Al, the loss of these minerals also results in some

loss of Si; meaning the Ti value is a better indicator of changes

having occurred in the respirable dust fraction. It is noted that the

XRF and P-7 analyses performed on the dust filters contained

organic matter in the form of coal dust.

Fig. 7. Comparison of respirable dust, top and bottom rock total Si concentrations to dust,

top and bottom rock quartz Si concentrations. Additional lines for rocks of 100% quartz

and 50% quartz, 50% clay are shown for comparison. Dust data refers to respirable-sized

particle samples.

In addition to the dust and rock Si concentration lines, Fig. 7

shows two additional calculated Si rock concentration plots. The

first shows the total mineral matter Si plot versus the quartz mineral

matter Si plot for a pure quartz rock. Also shown is a calculated rock

mineralogy with a 50% quartz and 50% clay composition. For this

rock, the clay composition was assumed to be 4:1 illite to kaolinite

since the XRD data suggested the clay fraction was mostly illite. The

plot shows the top rock trend line to have a steeper slope than the

pure quartz line. The bottom rock trend line plots very close to the

100% quartz line. Unfortunately, both the top and bottom rock plots

produce trend lines with relatively low R2 values. The quartz Si-total

mineral Si line has a slope of 0.28 suggesting about 28% of the total

mineral Si is present in quartz. An average top rock composition line

produces a slope of about 0.88, very similar to the 50% quartz and

50% clay composition line. It is not clear why the respirable dust

samples were determined to have a much smaller concentration of

quartz than the potential parent rocks. The uncertain instrument
response to mixed composition dust particles (e.g., clay and quartz)

by FTIR and XRF is one potential source of error.

4. Summary and conclusions

A methodology is proposed based on alkali, alkaline earth and

Mn concentrations to distinguish mine top rock strata contributions

to respirable quartz dust from mine bottom rock contributions. The

most consistent and pronounced cation enrichment in top rock

samples was observed for Ca and Mn data. Late phase epigenetic

mineralization may account for the presence of clay and lesser

amounts of carbonate-bound alkali and alkaline earth minerals plus

Mn with less efficient fluid transport to the bottom rock strata

underlying the coal beds.

Silicon concentrations determined in respirable quartz dust were

closely related to the total silicon concentrations from all mineral

matter making up about an average of 28% of the sample mass. A

similar relationship was not found between silicon concentrations

determined in quartz and silicon concentrations from all mineral

matter in the top and bottom rock. Silicate mineral matter was found

to be present in the top and in the bottom rock at many sites

including quartz, feldspars, illite, kaolinite and chlorite. The

calculated SiO2/Al2O3 and TiO2/Al2O3 ratios show that the respirable

dust composition has been significantly modified from the potential

parent rocks. This loss of Al in the dust samples is most likely due to a

disproportionately high loss of clay minerals and possibly chlorite.

The preferential loss of Al-bearing minerals may account for the

observed relationship between quartz Si and all mineral matter Si in

respirable dust samples.

Acknowledgements

We would like to thank the following people for their valuable

contributions to this research: Mr. Thomas Mal (NIOSH/PRL) for dust

sample gathering and weighing and assistance at all field sites; Mr.

George Persetic (NIOSH/PRL, retired) for assistance in preparing rock

samples for subsequent analyses; Mr. Jay Colinet (NIOSH/PRL) for

helpful input on respirable silica dust problems in coal mines; and Dr.

Allan Kolker (USGS), Dr. Anthony Iannacchione (University of

Pittsburgh), the late Dr. William Wallace (NIOSH/HELD), Dr. James

Hower, University of Kentucky, CAER, and an anonymous reviewer for

technical manuscript reviews.

Appendix A

Study sites

The three underground mining operations included in the study

within West Virginia are mining the Williamson, the Lower Cedar

Grove, and the Powellton coal beds (Table 1). All three of the coal beds

are in the Kanawha Formation (Middle Pennsylvanian) within the

Pottsville Group. In general, the Kanawha Formation coal beds contain

limited mineral matter (on the order of a median value of 5% ash) and

sulfur-bearing minerals (Martino, 1996).

Two Kentucky underground mining operations are included in the

study. At both sites, the operators are mining the Fire Clay or Hazard

No. 4 coal bed. The Fire Clay coal bed is part of the Middle

Pennsylvanian Hyden Formation within the Breathitt Group. A

tonstein layer (volcanic ash) is present in the lower bench of Fire

Clay coal bed and is a prominent feature in the unit. Averaged data

(n= 24) for the Fire Clay coal bed from eastern Kentucky sites gives an

ash yield of 20.22% and a sulfur content of 1.20% in the lower bench

(Greb et al., 1999). Averaged data from the upper bench produced

values of 7.87% for ash yield (n= 39) and 1.09% sulfur content (n=44).

One coal mine site in Virginia was included in the study. This

operator is mining the Lower Horsepen coal bed (Lower Pennsylvanian) which is part of the Lee or New River Formation. Abundant

sandstones associated with the New River/Lee Formation coal beds are

derived from eroded Alleghenian Highlands clastic matter (Cecil,

1990; Bodek and Eriksson, 2005).

Analytical methodology

All work performed on samples undergoing ICP-AES (inductively

coupled plasma-atomic emission spectroscopy) including preparation

and analysis was conducted by Activation Laboratories, Ltd. Entire

samples were crushed to a nominal minus 10 mesh (1.7 mm),

mechanically split (riffle) to obtain a representative sample and then

pulverized to at least 85% minus 200 mesh (75 μm). Samples of less

than 5 kg were crushed, split and pulverized with mild steel to

produce samples of about 100 g. A portion of the older samples were

prepared at NIOSH/PRL using a tungsten carbide shatter box, and

divided into aliquots using a riffle splitter. Activision Laboratories

utilizes a lithium metaborate/tetraborate fusion-ICP-AES-whole rock

analysis method to dissolve the crushed samples.

Coal petrography analysis was performed by University of Kentucky,

Center for Applied Energy Research (CAER). The maceral content of one

or two coal samples from each mine was determined by organic

petrography. All samples were completely crushed in a tungsten carbide

shatterbox, and divided into aliquots using a riffle splitter. All sample

preparation conducted at NIOSH was begun with a procedure very

similar to American Society for Testing and Materials (ASTM) D-2013-86

(2004a). Coal petrographic samples were prepared in accordance with

the requirements specified in ASTM procedure D-2797-85 (2004b).

Petrographic methods are consistent with ASTM methods (2004c).

Polished mounts of coal samples from the six mine sites were

analyzed using reflected-light microscopy. Analyses were performed

on a Leitz (Leica) MPV compact petrographic microscope in reflected

light with an oil immersion lens at a magnification of 500 times.

Maceral descriptions include point counts of 1000 points on two

surfaces of the polished coal pellets. The organic matter analysis

included maceral point counts in accordance with current International Committee for Coal Petrography (ICCP) classification (1998,

2001). Coal rank determinations were made petrographically using

vitrinite maximum and random reflectance measurements. The

analyses were performed under a NIOSH contract by Dr. James Hower.

The experimental design included FTIR (Fourier transform infrared

analysis) analysis of respirable dust samples by two methodologies.

The analytical methods included were the MSHA P-7 protocol (Mine

Safety and Health Administration, 1989) and the NIOSH 7603

procedure (National Institute for Occupational Safety and Health,

1994). The two methods were specified for a comparison of the results

produced by the two techniques although the number of samples

included was small. The MSHA P-7 analysis was performed by RJ

LeeGroup, Inc. These measurements were performed on a Bruker

Optik gmbH FTIR utilizing the Bruker Opus software version 4.2 to

analyze the data.

The samples which had been planned for analysis by the NIOSH

7603 were performed by DataChem Laboratory. The samples were

analyzed on a Perkin-Elmer Paragon 1000PC FTIR. To address an

instrumentation irregularity, the laboratory performing the analysis

modified the standard experimental method. The analytical methodology differed from NIOSH 7603 in that: samples contained on the

PVC filters were treated with 9% vol/vol HCl and isopropyl alcohol;

37 mm filters instead of a 47 mm filters were used; and ashing was

done in a stepwise manner instead of the described 600 °C approach.

The resulting method is referred to as the P-7 alternative technique.

Since the P-7 alternative results did not conform to longstanding

laboratory protocols, the results may be less dependable than the

MSHA P-7 data.

The X-ray fluorescence analysis was conducted by RJ LeeGroup, Inc.

The samples were run on a BRUKER S4 Explorer wavelength dispersive
X-ray fluorescence (WD-XRF) unit with a gas proportional detector.

The resulting spectra were analyzed using the Bruker SpectraPlus

software. Respirable dust samples were analyzed by mounting filters

with a Teflon backer. Raw elemental data (%) from the XRF scans were

then calibrated by pure element standards (converting % to μg/cm2

).

All X ray diffraction (XRD) analysis was performed by RJ LeeGroup.

The samples were run on a PANalytical X'Pert Pro MPD XRD unit is

equipped with copper radiation and an X'Celerator detector from 4–

64° 2-theta with a step size of 0.033°, with an overall scan time of

20 min (using the PANalytical X'Pert Pro). As previously mentioned, a

low temperature ashing (LTA) procedure was performed prior to the

XRD analysis.

Rock samples were removed from the field grab samples and were

submitted to the lab for sample preparation and for XRD analysis.

Previous research suggests that the use of LTA of coal can modify

mineral matter present in typical coal samples during heating

(Vassilev and Vassileva, 1996; Vassilev and Tascon, 2003). Samples

were ground and mixed with a fluorite (CaF2) known amount (∼10%)

internal standard. The diffraction patterns are collected using

standardized scan conditions and the integrated intensities for a

number of phase peaks are measured and plotted versus concentration in order to obtain a standardization factor for different diffraction

peaks of the phase. A sample containing unknown proportions of

mineral phases is then spiked with ∼10% CaF2, and the calibration

coefficients are then used to quantify the particular mineral phase in

an unknown sample. Clay analyses were done in accordance with the

procedure described by Kahle et al. (2002).

The diffraction patterns were analyzed using X'Pert HighScore and

DataViewer software utilizing the International Center for Diffraction

Data (ICDD) database PDF2. A few of the older samples were run on a

Philips XRD unit equipped with copper radiation and a graphite

monochromator. These older samples were analyzed using TRACES

software utilizing the ICDD database PDF2.

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