A provenance study of mineral matter in coal from Appalachian Basin coal mining

regions and implications regarding the respirable health of underground coal

workers: A geochemical and Nd isotope investigation

Steven J. Schatzel a,⁎,

a National Institute for Occupational Safety and Health, Office of Mine Safety and Health Research, PO Box 18070, Cochrans Mill Road, Pittsburgh, PA 15236, United States

⁎	 Corresponding author.

E-mail addresses: zia6@cdc.gov (S.J. Schatzel), bstewart@pitt.edu (B.W. Stewart).

Brian W. Stewart b

b Department of Geology and Planetary Science, 4107 O'Hara Street, University of Pittsburgh, Pittsburgh, PA 15260, United States

abstract

This study presents geochemical data produced from the analysis of coal and adjacent rock samples retrieved

from coal mining regions in the U.S. to examine mineral matter provenance. Study sites included the Northern Appalachian Basin, where the units of interest were the Lower Kittanning Coal bed, the overlying

Columbiana Shale, and the underlying paleosol (Allegheny Formation). Additional study sites were located

in the Central Appalachian Basin, where sampling was conducted on strata associated with multiple coal

beds of Middle and Lower Pennsylvanian age. The Central Appalachian Basin rock and coal samples were

much lower in overall mineral matter and contained very little carbonate (calcite and siderite) or pyrite mineralogy, which was common at the Northern Appalachian Basin sites. Elemental analysis of rock samples indicated a trend of enrichment in Ca, Mg, Mn, Na, and K cations in the immediate overburden compared to the

underlying rock in the Central Appalachian Basin. A similar trend was observed in coal related strata from the

Northern Appalachian Basin which was attributed to epigenetic marine incursions. Rare earth element (REE)

concentrations were determined in the samples, and showed Eu and Ce anomalies when normalized by

chondritic values. The total REE content of the overburden is generally less than that of the underlying

rock units. Neodymium isotopic analysis of the Lower Kittanning coal, overburden, and paleosol from the

Northern Basin indicate partial resetting of the Sm–Nd system close to the time of deposition. The data indicate a common Appalachian source for the clastic mineral matter in the overburden, underclay, and coal mineral matter. The geochemical findings of this study may provide a viable method for distinguishing respirable

dust sources in both Appalachian Basins. There are potential applications for this research to aid in the respiratory health of underground coal miners.

1. Introduction

Silicosis and coal workers pneumoconiosis (CWP) occur in underground coal mine workers in the U.S. as the result of inhaled mine

dusts. The potential harm of these dusts to human lung tissue is significantly influenced by the geochemistry and mineralogy of the respirable material. A key issue in controlling exposure of mine

workers to ambient dust is identifying the source(s) and specific

components of dust in the mining environment. We present mineralogical, geochemical and neodymium (Nd) isotope data as indicators

of mineral matter provenance in two Appalachian coal basins of the

U.S. Although Nd isotopic methods have been shown to be an important tool in provenance studies of sedimentary rock, very little has

been research has been done to utilize this investigative approach

in coal studies.

This paper follows on earlier work by Schatzel and Stewart (2003)

and Schatzel (2009) which focused on mineral matter origins and

rare earth element (REE) sources in coal and associated rocks. The

2003 study focused on a single bituminous coal bed in the Northern

Appalachian Basin and five sampling sites distributed over hundreds

of kilometers in western Pennsylvania. This research was conducted

due to concern about respirable dust hazards experienced by underground coal miners. One new finding from this study was the correlation between REE some major elements. The total rare earth element

concentrations (TREE) were found to be more strongly related to Si

concentrations than any other elemental component for the coal

and rock samples. The 2009 study focused on geochemical methods

for identifying sources of respirable quartz and silica dust in underground coal mines. Since the sources of respirable silica dust are

thought to be the rock strata immediately above and below the

mined coal bed, the 2009 study was concerned primarily with the

composition of the rock units and respirable dust and less on the analysis of coal samples. The study was conducted in the Central Appalachian Basin, near the intersection of the Kentucky, West Virginia, and
Virginia borders. One finding from the study included the identification of a group of cation concentrations in the parent rock that were

present in similar distributions within the mineral fraction in the produced respirable dust.

Despite advances in engineering controls for enhanced dust reduction during underground coal mining, particularly on continuous

mining sections, and regulatory changes reducing the allowable respirable silica dust concentrations in air, incidence rates for coal

worker's pneumoconiosis (CWP) and silicosis are not improving. In

fact, data show that incidence rates for both diseases are in fact increasing in important segments of the work force since 2000 (Laney

et al., 2010). Although this is a geochemical study, not an analysis of

compromised worker health, mineral and chemical components present in respirable mine dusts have been shown to yield a significant

influence on the inflammatory processes that result in silicosis

(Cecala et al., 2012). Potential applications of this research for the enhanced capture of respirable dust and reduced dust exposures are

reviewed.

2. Background

2.1. Major mineral groups in coal

Mineral matter in coal makes up a highly diverse range of materials. The primary motivations for analyzing coal-bound mineral matter have been coal utilization (Harvey and Ruch, 1986; Mackowsky,

1968; Stach et al., 1982), environmental (Gluskoter et al., 1977;

Ward, 2002), and health interests (Finkelman, 1993; Palmer and

Lyons, 1996). The most common minerals and mineral groups in

coal seams are quartz, clays, feldspars, sulfides, and carbonates.

These major groups were also present at one or both of the coal basins included in this study, and therefore are relevant to the study

findings. Of the clay minerals, kaolinite is quite common (Gluskoter,

1967; Rao and Gluskoter, 1973), as is illite–smectite. Siderite, calcite,

and dolomite are commonly observed carbonate minerals. Pyrite is

the most commonly found sulfide mineral (Gluskoter et al., 1977).

Clastic mineral material is expected to potentially yield information about source rock origins. Clays (primarily illite and kaolinite)

and quartz are common clastic minerals in coal, and in some cases

can account for nearly all of the mineral matter present (Stach et al.,

1982). Eolian transport appears to be a significant means of introducing quartz grains to the coal-forming mire. Detrital aluminosilicates

may be introduced by aqueous or eolian transport modes. Tonsteins

are known in many coal beds and represent another clastic mineral

source, probably of volcanic origin.

Syngenetic quartz is also present in coal seams. Quartz dissolution

has been reported from mire-type environments under reducing conditions (Bennett et al., 1988). Syngenetic quartz and clay may appear

as cell and pore infillings (Ward, 2002), or as coatings or overgrowths

on clastic mineral grains.

Most of the iron present in coal mineral matter is in the form of

syngenetic and epigenetic iron disulfides. Finely dispersed pyrite in

coal generally has syngenetic or early epigenetic origins. Organically

combined sulfur is also common in coal. Iron is also present in coal

beds in the form of siderite, which is usually present in coal as small

(1–2-mm) nodules often associated with vitrinite and clay layers

(Stach et al., 1982). Siderite is thought to form syngenetically or during early diagenesis. Larger nodules of siderite are found in many coal

units and have been known to contain uncompacted plant cell

structures.

Epigenetic mineralization in general includes fracture and cleat

infillings of material (Hower et al., 2001). Clastic dikes are wellreported in the literature in the Appalachian Basin, and generally consist of clays forming continuous veins through cleats, fractures, and

fault planes. Quartz is also formed epigenically in coal seams. Following coal deposition, mobilized silica can enter fracture voids or pore

spaces within the coal horizon. This material can then crystallize to

form epigenetic quartz (Vassilev and Vassileva, 1996).

Sulfides make up a significant portion of the epigenetic mineral

fraction in coal. Pyrite is the dominant sulfide found in coal beds as

a fracture or cleat infill. Carbonates are found as epigenetic minerals

in the form of veins or cleat fillings in coal (Stach et al., 1982). Common epigenetic carbonates can include calcite, ankerite, siderite,

and dolomite. The calcium for mineral authigenesis is thought to be

derived from groundwater (Cecil et al., 1978). Thus, we would expect

more calcium carbonate minerals to be found in coals produced from

topogenous mires than in coals produced by ombrogenous mires

(Cecil et al., 1985; McCabe, 1993).

Phosphate minerals can contain significant amounts of the REE

present in coal (Willett et al., 2000). Apatite is typically the most

common of the phosphate minerals occurring in coals. Other reported

forms of phosphate mineralization include aluminophosphates

(Crowley et al., 1993; Finkelman and Stanton, 1978; Rao and Walsh,

1997, 1999; Ward et al., 1996). Phosphate minerals are present as

cell and pore fillings, in concentrated layers within coal units, and aluminophosphates are known to be associated with tonsteins (Bohor

and Triplehorn, 1993; Hill, 1988).

2.2. Rare earth elements in shale and coal

The rare earth elements (REE), defined as elements with atomic

number 57 (La) through 71 (Lu), are present at the parts per million

(ppm) level in most rocks, and have long been used in petrogenetic

studies of igneous rocks (see Hanson, 1980, for a review). All of the

rare earth elements readily form 3+ ions under earth surface conditions; in addition, Ce4 + can be stable in an oxidizing, lowtemperature environment, and Eu2 + is stable under certain reducing

conditions at high (magmatic) temperatures. This constancy in valence state, when combined with the systematic decrease in ionic radius with increasing atomic number, makes the REE a coherent group

of trace elements for tracking petrogenetic sources and processes. The

high charge/radius ratio of the REE means that these elements are relatively immobile during weathering, erosion, and deposition at the

Earth's surface, and thus can be useful for identifying clastic sediment

sources.

Rare earth elements in coal appear to consist of a primary fraction

which is associated with clastic and syngenetic mineral matter

(Finkelman, 1982; Palmer et al., 1990; Schatzel and Stewart, 2003).

Another portion of the REE can be externally derived or mobilized

when primary mineral matter is destroyed or modified, and may be

retained in the coal bed but redistributed and incorporated into

other mineral components (authigenic minerals). There still exists

some disagreement about the proportion of primary and redistributed or authigenic REE in coal (Finkelman, 1982; Mariano, 1989;

McLennan, 1989; Ruppert et al., 1993). Although REE are not considered to be highly mobile in low-temperature environments, some researchers have suggested possible mechanisms for mobilizing REE in

coal (Eskenazy, 1987, 1999). Low pH water in the coal-forming mire

can partially or completely dissolve a portion of the REE-bearing mineralogy. The mobilized REE in solution may crystallize in authigenic

minerals forming in the coal bed. Minerals containing REE in coal

are often present in both primary and authigenic forms which can

make it difficult to distinguish between primary and secondary REE.

The fine-grained nature of some REE-bearing minerals can also

make chemical investigations of specific mineral fractions very

challenging.

In addition to their retention in fine-grained phosphates

(Finkelman, 1982; Willett et al., 2000), REE could also be bound to

the surfaces of clay minerals, and may be retained in highly resistant

REE-rich trace minerals such as zircon. Although little work has been

done on REE in sulfides, pyrite and marcasite could contribute significantly to the overall REE budget in coal (e.g., Worrall and Pearson,
2001; Yang and Zhou, 2001; Zhang et al., 2000). Rare earth element

concentrations are generally low in calcium carbonates (Sholkovitz

and Shen, 1995; Vance and Burton, 1999; Zhong and Mucci, 1995),

but little is known about REE associations with siderite (FeCO3). Selective leaching and particle segregation studies of coals suggest

that a portion of the REE may be associated with the organic fraction

of coal (Palmer et al., 1990; Willett et al., 2000).

2.3. Nd isotopes as provenance indicators for sedimentary rock and coal

The Sm–Nd system has proven useful for studies of sedimentary

rocks because the parent samarium (Sm) and daughter Nd, both

rare earth elements, are generally not significantly fractionated by

Earth-surface processes, and isotopic values can be determined as

an indicator of the crustal formation age in the sediment source

area (Patchett et al., 1999). The isotope 147Sm undergoes alpha

decay to 143Nd (a radiogenic isotope of Nd) producing differences in 143Nd/144Nd among minerals over time, based on the starting Sm/Nd

ratio and the amount of time that passes. The half-life of 147Sm decay

to 143Nd is about 106 billion years.

Most of the Sm and Nd in clastic sedimentary rocks are associated

with silicate and certain accessory minerals. Consequently, the Sm–

Nd system has been used as a provenance tool to identify the sources

of clastic material to sedimentary rocks (Bock et al., 1994, 1998; Linn

et al., 1991; Miller et al., 1986; Nelson and DePaolo, 1988).

2.4. Respirable coal dust

Evidence has been forwarded showing that the source of respirable silica dust in coal mines is not the mined seam but rather the adjacent rock units (Dodgson et al., 1971; Hicks and Nagelschmidt,

1943; Ramani et al., 1987). Despite the advances in mining and health

technologies, the incidence of CWP has actually been increasing in

U.S. mines, according to recent statistics (Colinet et al., 2010). Increased exposure to respirable silica dust is thought to be a contributing factor in the observed CWP trends (Laney et al., 2010). Studies

have also shown that variations in pneumoconiosis incidence rates

are produced by a variety of factors, apparently not just the quantity

of respirable silica dust in ventilation air (Robock and Bauer, 1990).

There are multiple mechanisms by which respirable mine dusts

can challenge the health of underground workers, and they all are

influenced by the composition of the dust. For example, respirable silica has also been recognized as a human carcinogen (IARC, 1997).

Quartz particles of submicron size have been documented in a bituminous coal seam and linked to increased cancer rates in rural residences of China (Dai et al., 2008). Some mine dust particles have

been determined to be layered or coated with clay, while clean, crystalline silicate forms have been found to be more damaging to human

lungs than other types of silica mineral dust known to occur in this

environment (Chen et al., 2005; Colinet et al., 2010; Dodgson et al.,

1971; Harrison et al., 2005; Robock and Bauer, 1990). Research has

shown the potential role of coal iron content in contributing to the occurrence of silicosis and coal worker pneumoconiosis in coal mine

workers (Huang et al., 2005). The same authors have also suggested

that increased carbonate content of the dust may diminish the toxic

effects of bio-available iron. Unfortunately these trends have been

demonstrated utilizing coal bed compositional data, not data on the

rock strata adjacent to the mined seam where the respirable dust

fraction is generated.

The current engineering controls that are used to dilute or suppress all coal dust produced during continuous miner operations are

also the primary methods used to address respirable coal and silica

dust. These controls include water sprays on the miner cutting

head, scrubbers on a continuous miner chassis, and ventilation air

which may be either blowing or exhausting on a continuous miner

section (Taylor et al., 2010). Water spray control methods include

spray fans which influence nearby air movement and wet cutting

heads that produce large volumes of dispersed water in the cutter

head-coal face area.

3. Methodology

3.1. Northern Appalachian Basin field study

3.1.1. Site selection

The Lower Kittanning Coal bed is a late-middle Pennsylvanian age

unit which ranges in rank from low volatile bituminous (Ogle) to high

volatile A–B bituminous (West Freedom) in samples from the study

sites. The unit has been extensively mined in the Commonwealth of

Pennsylvania. Samples sites were surface mines or pits for drift

mines operating in the Lower Kittanning Coal bed. The Lower Kittanning Coal is also very laterally extensive, which allows for the investigation of geochemical and other variations over an east-west

range of about 170 km. The sites were chosen to represent coals

with overburdens interpreted to have formed from various depositional environments, from fresh water to marine (Cecil et al., 1985;

Rimmer and Davis, 1986; Williams and Bragonier, 1974).

A total of five study sites were selected (Fig. 1). They were chosen

to encompass the three depositional regimes described for the overlying lithologies (overburden) by Williams (1957). The depositional

environments for the overburden are interpreted as marine (West

Freedom site), brackish (Clearfield, Enon Valley and Homer City

sites), and non-marine (Ogle site). All sites were located at active

coal mine operations.

3.1.2. Coal and rock sampling procedure

At each site, a complete section of Lower Kittanning Coal was selected and the exposed coal on the face was removed to depth of at

least 2.5 cm. Using a handheld rock saw, two vertical slots spaced

about 10 to 15 cm apart were cut in the coal, the underlying clay,

and the shale overburden to form the channel. The section was then

measured and the coal lithotypes described. After description was

complete, the sample sizes and breaks were configured to ensure a

high degree of homogeneity of the coal lithotypes in each sample of

the Lower Kittanning Coal. Samples were then removed with the

aid of hand tools, placed in plastic sample bags, labeled, and returned

to the National Institute for Occupational Safety and Health, Office of

Mine Safety and Health Research (NIOSH/OMSHR) in Pittsburgh for

sample processing. The sampling scheme for the Northern Appalachian Basin is shown in Fig. 2.

3.2. Central Appalachian Basin field study

3.2.1. Study sites

The study area encompasses parts of western Virginia, southern

West Virginia, and eastern Kentucky (Fig. 3). The coal beds include

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. The sampling scheme for the

Central Appalachian basin samples is shown in Fig. 4. Multiple locations within the mine were sampled with rock samples being retrieved primarily from above and below the mined coal bed. Field

sampling techniques and analytical methods for the Central Appalachian Basin portion of the study are discussed in Schatzel (2009).

3.3. Analytical methods

3.3.1. Northern Appalachian Basin samples

Sample preparation methods for the Northern Appalachian Basin

sample suite were discussed in Schatzel and Stewart (2003). The original set of major cation concentrations did not include Mn, which is

an important analyte in this study; consequently the samples were
re-run and new values were determined. The new data were determined by inductively coupled plasma-atomic emission spectroscopy

(ICP-AES) analysis following whole rock Li metaborate and Li tetraborate fusion conducted by Actlabs commercial laboratory, Ontario,

Canada. Following the fusion, samples were dissolved in a solution

of 5% nitric acid and analyzed at Actlabs on a Thermo Jarrell-Ash

ENVIRO II ICP or a Varian Vista 735 ICP. Rock sample crushing and

splitting procedures were unchanged compared to those of Schatzel

and Stewart (2003). Analyses also performed on these samples included organic matter petrography, and instrumental neutron activation analysis (INAA) (Schatzel and Stewart, 2003).

Fig. 1. Lower Kittanning Coal study sites (modified from Rimmer and Davis, 1986). Sample site key: cfd, Clearfield; evy, Enon Valley; hcy, Homer City; ogl, Ogle; and wfm, West

Freedom (from Schatzel and Stewart, 2003).

For Nd isotope analysis, coal ash and crushed rock samples were

digested with acid using a variation of the method described by

Yokoyama et al. (1999). All sample digestions were performed

under clean laboratory conditions at the University of Pittsburgh,

using ultrapure reagents. Samples underwent mixed acid digestions

in concentrated, heated HF and HClO4 in Te on® fl Savillex® vials.

After the sample residues were heated to dryness, the samples were

fluxed in concentrated, heated HClO4. Dried sample residues were

dissolved in 6 M HCl and evaporated to dryness, then dissolved in

4 M HNO3. Following centrifuging and the removal of any undissolved

material (thought to be fluoride complexes; Yokoyama et al., 1999),

the sample solutions were aliquoted for spiking. Predetermined

amounts of a mixed, ultrapure spike consisting of 147Sm and 150Nd

were added to the aliquoted samples. Spiked samples were evaporated to dryness, redissolved in 1.5 M HCl, centrifuged, and introduced

to a set of cation exchange columns. The REE fraction from the cation

exchange columns was dried down and the residue was dissolved in

0.18 N HNO3, and introduced to a set of REE columns containing

LnSpec® chromatographic resin for separation of Nd and Sm. Total

procedural blanks were b1 ng for Nd and Sm.

The isotopic analyses were performed on a Finnigan MAT 262

thermal ionization mass spectrometer (TIMS) with one fixed and

seven movable cups. For Nd, the concentration and isotopic composition were determined simultaneously on the spiked sample. About

150 ng of Nd and 100 ng of Sm were dissolved in 1 N HCl and loaded

onto double Re filaments. On many of the sample turrets, the La Jolla

Nd standard was run as an internal analytical standard. The Faraday

cups were set to measure masses 142, 143, 144, 145, 146, and 150 simultaneously in static mode. At least 100 ratios were measured, with

the 144Nd beam intensity of 0.4–2.5 V. Interference from Sm (144Sm, 150Sm) was monitored at mass 149. Mass fractionation was corrected
using 146Nd/144Nd= 0.724134 with an exponential law. Our calculated value for 143Nd/144NdCHUR is 0.511847. Estimated external reproducibility is ±0.000015.

Fig. 2. Field sampling configuration at Northern Appalachian Basin sites.

Fig. 3. The location of mine sites included in the Central Appalachian Basin study (from Schatzel, 2009).

3.3.2. Central Appalachian Basin samples

Samples from the Central Appalachian Basin underwent a suite of

analyses, including organic matter petrography, X-ray diffraction

(XRD), X-ray fluorescence (XRF), ICP-AES, inductively coupled

plasma-mass spectrometry (ICP-MS), and (INAA). The methodologies

used are discussed in Schatzel (2009).

4. Results and discussion

4.1. Mineralogical analysis

No XRD-based mineralogical data are available from the Northern

Appalachian Basin samples. The Northern Appalachian Basin samples

included shale and sandy shale overburden, and a clay-rich paleosol

underlying the mined coal bed. Mineralogy descriptions for the

Northern Appalachian Basin are based on petrography, hand sample

descriptions, and observations from sampling sites. Fine pyrite was
visible and abundant in many of the coal samples. Calcite mineralogy

was evident in the coal, and siderite nodules were observed above

and below the mined coal unit and often graded into partings within

the coal bed.

Fig. 4. Field sampling configuration at Central Appalachian Basin sites. Figure shows samples retrieved from mine sites A through F. Samples above or below the coal bed were designated as rock samples (Table 4). Samples vertically aligned above and below the coal bed represent sample pairs that were retrieved from the same location with the mine.

The Central Appalachian Basin strata included in this study are

dominated by sandstones and siliclastic rocks above and sometimes

below the coal beds. X-ray diffraction (XRD) data for Central Appalachian Basin rock samples are shown in Table 1.

Table 1

X-ray diffraction results.a,b,c.

Mine Sample

number

Quartz

Concentration, %

Kaolinite Illite Albite Chlorite Pyrite Siderite Organic

A arock10bot 44 8 17 19 11 bdd bdd 2

A arock11top 30 6 36 12 16 bdd bdd 1

A arock4top 27 4 43 9 18 bdd bdd 0

C crock1bot 16 0 62 14 4 bdd bdd 5

C crock3top 23 0 49 17 10 bdd bdd 1

C crock6top 38 0 19 5 1 19 18 1

D drock3bot 59 2 29 5 bdd bdd bdd 6

D drock5top 50 7 34 6 4 bdd bdd 0

D drock6bot 26 15 48 11 bdd bdd bdd 1

E erock5bot 28 57 13 1 bdd bdd 0

E erock7coal 1 95 bdd bdd bdd bdd bdd 3

E erock6top 37 4 40 9 10 bdd bdd 0

F frock1bot 33 63 bdd bdd bdd bdd 4

F frock2top 37 4 33 10 16 bdd bdd 0

F frock6top 31 4 43 10 12 bdd bdd 0

a Analyses did not detect any hematite, calcite, ankerite, or bassinite in the samples. b Laboratory theoretical detection limits (%) for the analytes were: quartz, 0.1; kaolinite, 0.1; illite, 0.2; albite, 0.2; chlorite, 0.3; pyrite, 0.2; siderite, 0.1; organic matter, 0.1. c Modified from Schatzel, 2009. d Below detection.

Comparisons between

the mineralogical data, cation compositions, and the REE distributions

were made for an indication of provenance from different fractions of

the rock composition. The data show that potential respirable silica

dust sources exist in the overburden and underlying strata in the

form of quartz and clay minerals. The most abundant minerals for

most samples were quartz, kaolinite and illite, and most also contained plagioclase (indicated as albite) and chlorite. Phosphates and

zircon fell below detection limits in all samples, as did hematite,

calcite and ankerite. No carbonates were observed petrographically

or in hand samples. Iron sulfides were rarely observed petrographically and in the hand sample specimens. Mineral matter observed in

the samples included quartz, feldspar, and clays. Other minerals containing REE that were not detected by XRD are expected to be present

in the samples. The coal bed at Mines A and E (Fire Clay Coal bed)

contained a persistent tonstein, which can show very elevated REE

concentrations compared to the host coal unit.

Typically important carrier minerals for Nd, Sm, and other REE are

the clay minerals, phosphates, and zircon (Condie, 1991; Gromet et

al., 1984). Some REE may be associated with organic matter which

was removed by low-temperature ashing. Prior research on REE associations with elemental groups, as determined by ICP-AES, showed

total rare earth element (TREE) concentrations correlated best to Si

content (r2= 0.84), followed by total ash values (r2= 0.83) and Al

(r2= 0.78) (Schatzel and Stewart, 2003). In the Central Appalachian
samples, silicate minerals were the only potential sources of REE

detected by XRD, and were also the most abundant minerals. The

coal parting sample (erock7coal) is composed almost entirely of kaolinite, making it a potential source of silica but not quartz. Other trace

minerals with relatively high REE concentrations may be present

below detection limits.

The XRD data also yield some implications regarding the major

cation analysis. Of the minerals shown in Table 1, the only potential

sources of Mg, Na, and K are chlorite, plagioclase feldspar, and illite,

respectively. Siderite is present in one sample as a potential source

of Ca, but no other Ca mineralogy was detected besides plagioclase

feldspar. It is likely that Mn-bearing minerals are present below conventional XRD detection limits in the Central Appalachian Basin.

4.2. Major cation analysis

Prior research on the Central Appalachian Basin (Schatzel, 2009)

showed that Ca, Mg, Mn, Na, and K were enriched in the strata immediately overlying the mined coal bed, compared to the strata underlying the mined unit. This geochemical trend was used to identify the

sources of respirable silica dust in coal mine continuous miner operations. At four of five sites with a complete set of geochemical data,

the sampled respirable dust was measurably closer in Ca, Mg, Mn,

Na, and K concentration to the overlying rock than to strata beneath

the mined seam.

Major element concentrations for Northern Appalachian Basin

samples are shown in Table 2.

Table 2

Cation concentration determinations for Northern Appalachian Basin samples.

Analyte symbol SiO2 AI2O3 Fe2O3 MnO MgO CaO Na2O K2O TiO2 P2O5

Sample number %

ocfd08401040

ocfd00720795

ocfd00000165

ccfd00000070

ccfd00210290

ucfd00530640

oevy00200450

oevy00000200

cevy00000140

cevy00253351

uevy00730870

uevy02670000

ohcy00000185

chcy00000110

chcy00110162

chcy00690840

uhcy16451855

uhcy21552355

uhcy9000190

oogl00000150

cogl00951050

cogl00630900

uogl00110124

uogl00124130

owfm00000195

cwfm00730142

cwfm00427560

uwfm00780820

57.1

48.2

49.6

36.1

10.9

60.4

52.9

44.9

39.1

54.2

54.0

54.3

24.1

54.2

52.0

16.3

64.5

56.3

51.2

65.7

39.7

47.1

47.8

63.2

46.5

47.7

40.6

51.4

18.2

17.0

20.9

16.1

10.9

21.8

21.0

20.5

21.9

21.4

25.7

27.3

8.8

24.6

28.8

13.4

16.5

16.0

25.0

15.0

30.1

35.4

23.8

17.6

22.8

25.0

32.3

26.5

8.2

15.0

7.8

29.3

65.2

2.8

9.0

5.8

24.7

8.8

1.6

1.7

18.8

8.4

6.4

53.5

4.4

13.1

2.9

1.4

12.6

3.0

3.8

3.1

4.5

15.0

11.9

1.8

0.1

0.3

0.1

0.0

0.0

0.0

0.1

0.0

0.01

0.01

0.01

0.00

0.08

0.03

0.00

0.02

0.03

0.06

0.01

0.01

0.07

0.01

0.02

0.02

0.02

0.01

0.01

0.00

1.5

1.0

1.4

0.4

0.1

0.9

1.7

1.4

0.6

0.3

0.4

0.4

0.4

1.0

0.6

0.3

0.5

0.7

0.5

0.6

0.3

0.6

0.3

0.5

1.2

1.1

0.4

0.7

0.4

0.8

0.2

0.7

1.5

0.1

0.5

0.3

0.5

1.3

0.3

0.3

0.2

0.2

0.6

1.5

0.1

0.2

0.1

0.1

1.5

2.0

0.1

0.1

0.3

0.8

1.8

0.8

0.2

0.3

0.2

0.1

0.1

0.2

0.2

0.2

0.2

0.2

0.1

0.1

0.1

0.2

0.1

0.2

0.1

0.2

0.2

0.2

0.6

0.2

0.1

0.2

0.2

0.2

0.2

0.2

3.5

2.6

3.8

1.0

0.1

3.3

3.6

3.3

1.9

1.1

1.5

1.3

1.5

3.1

1.7

0.4

2.4

2.6

2.0

2.7

0.3

2.2

1.2

1.9

3.4

3.1

0.9

2.6

0.9

0.9

0.8

1.5

0.1

1.10

0.96

0.80

0.76

2.02

1.63

1.64

0.41

1.39

1.18

0.34

0.97

0.96

1.39

0.88

1.36

1.59

1.66

1.58

0.93

0.92

0.74

1.24

0.2

0.1

0.2

0.7

0.1

0.1

0.2

0.1

0.15

2.06

0.09

0.07

0.1

0.07

0.5

0.03

0.03

0.03

0.08

0.05

0.23

0.19

0.11

0.06

0.13

0.14

1.17

0.54

From this dataset, alkali, alkaline

earth, and Mn concentration data were reviewed and plotted together

to assess the relationship observed in the Central Appalachian Basin

samples (Fig. 5). As no dust samples were available from these mine

sites, no comparisons can be made to the respirable dust fraction.

However, major element data can be analyzed to evaluate the potential for the same geochemical trends that were present in the Central

Appalachian Basin rocks. In the Central Appalachian Basin, rock sampling was conducted at a minimum of two sampling sites within a

mine where samples of the overburden and the underlying rock unit

were analyzed (a minimum of four samples). Only one site was sampled at each mine in the Northern Appalachian Basin.

Concentrations of Ca, Mg, and Mn from the Clearfield mine site

(Fig. 5a) show relative enrichment in the overburden while Na and

K concentrations are very similar for both overlying and underlying

rock units. Additional discussion of Northern Appalachian Basin sample cation compositions is given in Schatzel and Stewart (2003). Calcite and other carbonates present in the Lower Kittanning Coal most

likely produced the high Ca concentrations in the coal ash. Calcite

mineralization was observed in the many of the coal and rock samples from the Northern Appalachian Basin.

At the Enon Valley mine site (Fig. 5b), Mg, Mn, and K are clearly

enriched in the overburden compared to the underlying rock unit.

Lower Kittanning Coal ash data show a similar pattern to the Clearfield mine site data, with Ca and K being the most concentrated cations. Major cation data from the Homer City site (Fig. 5c) show

enrichment of Mn in the overburden, but the remaining four analytes

are essentially equal in the overburden and the underclay.

At the Ogle mine site, the overburden and underclay have roughly

the same concentrations of the elements plotted (Fig. 5d). Coal ash

data show the same high concentrations of Ca and K in the coal ash

compared to the other analytes in the plot. At the West Freedom

mine site, the overburden is slightly enriched in all plotted elements

relative to the underclay, except Ca (Fig. 5e); as with all of the sites,

Ca and K concentrations in the coal are the highest of all elements

plotted (Fig. 5e). For the analyzed Northern Appalachian Basin samples, the overburden is enriched in the majority of alkali, alkaline

earth, and Mn cations of interest at all sites except Homer City. Consequently, the data generally replicate the same geochemical trends

observed in the Central Appalachian Basin for the coal overburden

and the underlying rock (Schatzel, 2009).

4.3. Rare earth element data

Rare earth element concentrations for Northern Appalachian Basin

samples are presented and discussed by Schatzel and Stewart (2003).
a b 

Clearfeild top
 

Clearfeild coal
 

Clearfeild bottom
 

100000 100000


10000 10000

1000 1000

100 100

10 10

1 

Ca Mg Mn Na K Ca Mg Mn Na K 

1 

Enon Valley top 

Enon Valley coal 

Enon Valley bottom 

c d 

Homer City top
 

Homer City coal
 

Homer City 


bottom
 

100000 100000


10000 10000

1000 1000

100 100

10 10

1 

Ca Mg Mn Na K Ca Mg Mn Na K 

1 

Ogle top 

Ogle coal 

Ogle Bottom 

e 

100000 

10000 

1000 

100 

10 

1
 

Ca Mg Mn Na K
 

West Freedom top 

West Freedom coal 

West Freedom 

bottom 

Fig. 5. Northern Appalachian Basin rock and coal sample ICP analyses showing average alkali, alkali earth, and Mn concentrations.

Sample concentrations were compared to the North American Shale

Composite (NASC) sedimentary rock standard (Gromet et al., 1984;

McLennan, 1989). All of the data from the Northern Appalachian

Basin were produced by INAA analysis and therefore show only La,

Ce, Nd, Sm, Eu, Tb, Y, and Lu concentrations, making it somewhat

more difficult to discern Ce and Eu anomalies.

Fig. 6a shows REE data from the Northern Appalachian Basin. A

different plot of this data was given in Schatzel and Stewart (2003).

This graph is shown for comparison to a similar plot of REE data

from the Central Appalachian Basin. In general, the Northern Appalachian Basin samples show light REE (LREE) enrichment compared to

chondrites, and a relatively flat pattern of heavy REE (HREE, Er to

Lu). The plot shows that the samples from the underlying rock unit

generally contain higher REE concentrations than the overlying sample REE suites. The exception to this trend is the Clearfield site, where

the overlying rock samples are more enriched in REE than the underlying rock unit.

The REE concentrations determined for Central Appalachian Basin

samples are presented in Table 3, and plotted in Fig. 6b, which shows

the immediate coal bed overburden, the underlying strata, and NASC

for comparison. All samples exhibit enrichment in LREE compared to

chondrites, as well as negative Eu anomalies. The ICP-MS data from

the Central Appalachian Basin produced Eu anomalies ranging from

0.58 to 0.68 for the overburden and 0.36 to 0.64 from the underlying

strata. These patterns would appear to be reflective of a common or

very similar sediment source and mineral matter composition.
These data are also very similar to the pattern produced by the NASC,

which should be typical for North American sedimentary rock, with

an Eu anomaly of 0.69. At the Central Appalachian Basin sites, all samples show elevated concentrations of REE in the underlying rock strata compared to the overlying strata, except for Mine e, in which the

overlying and underlying strata have roughly equal REE concentrations.

Most of the overlying and underlying strata from both sites have small

negative Ce anomalies, indicating some loss of Ce as Ce4+ from oxidative conditions during weathering and/or transport. Cerium anomalies

have been used as indicators of marine depositional environments by

some researchers although wide variations exist in the application of

this concept and in the interpretation of findings (McLennan, 1989;

Wilde et al. 1996).

Fig. 7c shows averaged data produced from three coal samples

taken from the Lower Kittanning Coal bed, Northern Appalachian

Basin. Some differences between the REE patterns produced from

the sites are visible. The coal samples (ash) produced REE chondrite

normalized with patterns that are quite similar to the rock patterns

in the LREE range. However, in the HREE range, the coal patterns

show higher concentrations than the rock samples analyzed from either basin. A possible exception is the West Freedom site of the

Northern Appalachian Basin, where the bottom rock REE concentrations were elevated. However, the West Freedom site shows relatively high REE concentrations throughout the elemental suite, not just in

the HREE range. The coal Eu and Ce anomalies are similar to those determined for the rock samples (Fig. 7c). Comparing the coal REE patterns to the NASC in the figure also shows relative enrichment of

HREE concentrations for the coal samples.

c 

a 

rock top cfd 

rock top evy 

rock top ogl 

rock top wfm 

NASC 

100 

rock bottom cfd 

rock bottom evy 

rock bottom ogl 

rock bottom wfm 

Northern Appalachian 

Basin 

[REE]sample/[REE]chon 

10 

La Ce Nd Sm Eu Tb Yb Lu 

b 

Central Appalachian 

Basin 

rock top a rock bottom a 

rock top b rock bottom b 

rock top c rock bottom c 

rock top d 

rock top e 

rock bottom d 

rock bottom e 

NASC 

100 [REE] sample/[REE] chon

10 

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

cfd evy ogl 

wfm NASC 

Northern Appalachian 

Basin coal averages 

100 [REE]sample/[REE]chon 

10 

La Ce Nd Sm Eu Tb Yb Lu 

Fig. 6. Rare earth element concentration patterns for the Northern and Central Appalachian sits. (a) Rare earth element concentrations of overburden and underclay for the

Northern Appalachian Basin sample sites. (b) Rare earth element concentrations of

overlying and underlying formation at the Central Appalachian Basin sample sites.

4.4. Samarium–neodymium isotope systematics, Northern Appalachian

Basin

Samarium–neodymium concentration and Nd isotopic data from

the Northern Appalachian Basin samples are presented in Table 4.

The 143Nd/144Nd ratios are also shown in ε notation, where the εNd

value at time T is given by:

143Nd 144 4 = Ndð Þ T ε ð Þ1⁄4 10 SAMPLE Nd T 1 143Nd 144 = Ndð Þ T CHUR

Here the 143Nd/144Nd(T)CHUR is the chondritic uniform ratio (assumed to be approximately the same as the bulk earth value) at

time T, where T is some time in the past (in years before present),

typically the age of formation of the rock unit, when known. εNd(0)

is the present-day (measured) value for the sample.

All measured εNd(0) values from the Lower Kittanning Coal and adjacent units are negative (Table 4), with an average of −11.4, typical of

time-averaged growth of 143Nd/144Nd from a LREE-enriched crustal

source. There does not appear to be a consistent trend in εNd(0) between coal ash and the underlying or overlying strata. However, as

noted previously (Schatzel and Stewart, 2003), the coal ash is somewhat enriched in REE, including Nd and Sm, relative to the bounding

units. In a plot of εNd(0) vs. Nd/Si ratio (Fig. 7), the coal ash defines a

field that is enriched in [Nd] but with a greater range of εNd values

than in the combined overlying and underlying strata. This enrichment

of [Nd] in coal is also seen Nd/Al, Sm/Si, and Sm/Al ratios (not shown).

As both Si and Al are associated with clastic mineral matter and are

thought to be relatively immobile during diagenesis, this suggests possible addition of Nd to the coal in excess of the clastic components. Although the dominant mode of most REE in coal is reported to be

associated with mineral matter, a portion of the REE content has been

found to be combined with coal organic matter (Eskenazy, 1987;

Finkelman, 1982; Palmer et al., 1990; Willett et al., 2000). This could account for some differences between the REE concentrations of coal and

rock samples. Differences between the Lower Kittanning Coal overburden and the coal and underclay patterns may also be the product of epigenetic mineralization. One possibility is that Nd is captured by

vegetation in the coal mire, and later incorporated with the organic

matter or pyrite.

To determine the origin of Nd in the coal bed, it is necessary to correct for decay of 147Sm since the time of deposition of the mineral
matter. In Fig. 8, the data are plotted in a standard isochron diagram.

The combined coal ash and adjacent strata from the four sites define a

linear trend with a slope that is consistent with the Westphalian D

depositional age (~308 Ma; Fig. 8) of the Kittanning Coal unit (Cecil

et al., 1985). The actual linear correlation of the data (R2= 0.83) corresponds to a calculated Sm–Nd “age” of deposition of 318±81 Ma,

with an initial εNd(T) of −8.9, consistent with previous work on Paleozoic clastic sediments in eastern North America (Bock et al.,

1994, 1998; Cullers et al., 1997; Patchett et al., 1999). While the scatter is significant (as is typical for sedimentary rocks), and well outside

of the limits required for accurate age determination, the correlation

suggests a partial resetting of the Sm–Nd system at or near the time

of deposition. Similar effects were observed by Bock et al. (1994,

1998) in the Middle Ordovician Austin Glen Member of the Normanskill Formation in New York State. Samarium and Nd could have been

redistributed along with the rest of the REE during epigenetic

formation of pyrite or during early coalification processes, as suggested by Schatzel and Stewart (2003). Most of the higher Sm/Nd

samples defining the linear trend in Fig. 8 are coal, consistent with

the higher HREE observed in coal relative to the underclay and overburden (Schatzel and Stewart, 2003).

Table 3

Rare earth element concentrations determined in samples from the Central Appalachian Basin.

Sample

number

Elemental concentration, ppm

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

arock4top 57.4 122 nd 60.0 9.0 2.00 nd 1.7 nd nd nd nd 4.5 0.69

arock7top 60.7 131 nd 53.0 9.8 2.10 nd 1.5 nd nd nd nd 4.8 0.72

arock9top 63.2 127 nd 63.0 9.2 2.00 nd 1.4 nd nd nd nd 4.6 0.70

arock11top 55.3 120 nd 50.0 9.1 1.60 nd 1.2 nd nd nd nd 4.3 0.65

arock11Atop 57.1 124 nd 57.0 9.1 2.10 nd 1.2 nd nd nd nd 4.7 0.73

arock3bot 84.6 178 nd 81.0 13.4 1.90 nd 1.7 nd nd nd nd 4.9 0.75

arock6bot 89.9 182 nd 83.0 11.9 2.10 nd 1.7 nd nd nd nd 5.9 0.90

arock10bot 122.0 250 nd 122.0 21.3 3.00 nd 2.4 nd nd nd nd 5.2 0.79

arock12bot 147.0 290 nd 136.0 23.8 2.50 nd 3.1 nd nd nd nd 7.1 1.08

brock3top 44.9 102 nd 40.0 7.8 1.70 nd 1.3 nd nd nd nd 4.3 0.65

brock4top 53.0 112 nd 49.0 8.0 1.80 nd 1.1 nd nd nd nd 4.1 0.61

brock7top 51.8 113 nd 50.0 8.1 2.00 nd 1.2 nd nd nd nd 4.5 0.68

brock1bot 55.0 108 nd 45.0 6.9 1.40 nd -0.5 nd nd nd nd 4.1 0.60

brock6bot 49.1 120 nd 47.0 12.2 3.20 nd 2.0 nd nd nd nd 6.2 0.93

crock3top 47.6 94.6 12.23 41.6 8.2 1.64 7.2 1.1 6.1 1.2 3.5 0.52 3.2 0.48

crock8top 20.8 42.9 5.71 20.2 4.4 1.02 4.5 0.7 3.8 0.8 2.3 0.34 2.2 0.33

crock1bot 60.2 111.9 14.86 49.4 8.3 1.12 5.5 0.9 5.3 1.1 3.4 0.53 3.5 0.53

crock7bot 44.5 88.2 11.51 40.9 8.7 1.46 7.0 1.1 6.6 1.3 3.9 0.56 3.4 0.52

drock2top 44.6 88.6 11.5 40.8 8.2 1.70 7.4 1.1 6.6 1.2 3.7 0.54 3.4 0.52

drock5top 21.9 46.0 6.06 21.0 4.1 0.95 4.0 0.6 3.6 0.7 2.3 0.34 2.2 0.34

drock1bot 58.7 110 14.0 46.9 8.7 1.69 8.0 1.4 8.0 1.6 4.7 0.70 4.4 0.70

drock1bot 57.2 108 14.1 48.1 9.1 1.69 7.7 1.3 7.5 1.6 4.7 0.71 4.6 0.75

drock3bot 39.1 75.1 9.86 34.8 7.0 1.51 7.4 1.3 7.8 1.6 5.2 0.80 5.1 0.84

drock6bot 48.1 86.3 10.7 35.6 6.5 1.32 6.0 1.0 6.2 1.3 4.0 0.60 3.9 0.61

erock4top 69.4 143 18.7 64.3 12.3 2.09 11.2 1.7 9.6 1.8 5.8 0.88 5.7 0.90

erock6top 46.4 92.9 12.3 43.0 8.6 1.73 7.9 1.2 6.9 1.3 4.0 0.59 3.6 0.57

erock3bot 51.6 94.9 12.1 41.1 7.7 1.58 7.4 1.3 7.4 1.5 4.7 0.71 4.4 0.69

erock5bot 69.0 149 19.7 67.7 13.9 0.81 11.7 1.9 9.9 1.7 4.6 0.64 3.6 0.47

nd—not determined.

Fig. 7. Nd/Si ratios plotted against εNd(0) values for the northern Appalachian sample

suite. The εNd(0) values represent the present-day chondrite normalized 143Nd/144Nd

ratios. The highest Nd/Si ratios and the greatest range of εNd(0) were found in the

coal samples compared to the rock samples retrieved from overlying and underlying

strata.

Despite the partial resetting, it can still be possible to obtain useful

data about the provenance of the source material for the clastic sediments and coal mineral matter. In Table 4 we show the calculated depleted mantle model age (TDM) for each sample, which can be

interpreted as the time that the continental crust material constituting the clastic source was derived from the Earth's mantle (Bock et

al., 1994, 1998; DePaolo et al., 1991). For these calculations we use

the depleted mantle evolution curve of DePaolo (1981). As indicated,

the samples yield a wide range of model ages, from 1.35 to 4.18 Ga.

There is a clear disturbance of the TDM systematics, as exemplified

by Clearfield coal sample ccfd00360500, which never converges

with the mantle value. The theoretical isotopic evolution of these

samples and their intersection with the depleted mantle are shown

in Fig. 9. While a number of the samples give model ages around

1.5 Ga, several are significantly offset to higher values; most of these

are coal samples (solid lines). The convergence of these evolution

curves at around the deposition age of ~308 Ma also indicates that

Sm/Nd ratios were significantly altered at this time, with many of

the coal samples becoming enriched in Sm relative to Nd near the

time of deposition. Those samples that appear to be least affected

by the Sm/Nd redistribution yield model ages in the range of 1.35 to

1.62 Ga. This is consistent with a sedimentary source consisting of

Grenville age rock mixed with an older crustal component, as observed by Bock et al. (1998) for the Normanskill Formation. Most of

the variation observed in present-day εNd values appears to have

resulted from the partial resetting and disturbance of Sm/Nd ratios

at the time of deposition, rather than from large isotopic differences

in source material among the underclay, coal mineral matter, and

overburden. The key compositional differences between the three

units are expected to be in the syngenetic and authigenic fractions
in each unit and the addition of large volumes of preserved organic

matter in the coal unit.

The Sm–Nd isotope data from this study suggest that the Lower

Kittanning coal and adjacent units are derived from a source region

with relatively uniform isotope signatures. Thus the mineralogy of

dust-producing materials in lower Kittanning coal mines should not

show significant stratigraphic variability. For application of 143Nd/ 144Nd as a tool to discriminate dust sources, a dust source signature

would thus be most effective when there are extraneous sources of

Nd in the coal mine system. One example of this would be tonsteins

of volcanic origin. The clastic portion of coal mineral matter has an

isotope signature reflective of its weathering source, while volcanic

ash could have sources that are quite variable, from suboceanic mantle (positive εNd values) to Precambrian lower crust (highly negative

εNd values). In this case, εNd would be very effective at differentiating

between tonstein-derived dust and other sources.

Table 4

Sm–Nd isotope data for Lower Kittanning Coal, overburden, and underlying strata.

Mine/sample [Nd]a [Sm]a 147Sm/144Ndb 143Nd/144Ndc εNd(0)c,d εNd(T)

(308 Ma)

T e DM

ppm Ga

Clearfield

ocfd00000165 24.9 8 0.1205 0.511280±0.000005 −11.08±0.10 −8.08 1.59

ccfd00000070 87.3 17.5 0.1214 0.511267±0.000009 −11.33±0.18 −8.37 1.62

ccfd00007015 60.8 13.5 0.1345 0.511217±0.000008 −12.31±0.16 −9.87 1.99

ccfd00360500 46.8 15.4 0.1986 0.511415±0.000008 −8.44±0.16 −8.52 n/a

ucfd00530640 30 4.8 0.0965 0.511213±0.000007 −12.39±0.14 −8.45 1.35

Enon Valley

oevy00000200 12.1 2.2 0.1083 0.511186±0.000010 −12.91±0.20 −9.44 1.54

cevy00000140 41.6 10.3 0.1497 0.511279±0.000009 −11.10±0.18 −9.25 2.33

cevy00351451 43.5 12.6 0.175 0.511340±0.000011 −9.91±0.21 −9.06 3.82

cevy00549730 46.6 13.7 0.1776 0.511342±0.000007 −9.87±0.14 −9.12 4.18

uevy00026700 10.3 2 0.117 0.511221±0.000009 −12.23±0.18 −9.1 1.62

Homer

chcy00000110 34 6.1 0.1087 0.511152±0.000009 −13.58±0.18 −10.12 1.59

uhcy11151305 11.8 2.8 0.1431 0.511285±0.000004 −10.98±0.08 −8.87 2.09

Ogle

cogl00000100 47.1 11.9 0.1523 0.511288±0.000010 −10.92±0.20 −9.18 2.41

cogl00574840 107.4 20.6 0.1158 0.511230±0.000009 −12.05±0.18 −8.87 1.59

cogl01020110 32.7 7 0.1288 0.511257±0.000013 −11.53±0.25 −8.86 1.78

a Determined by isotope dilution. b Uncertainty ≤0.3% of measured value. c In-run uncertainty shown. d Normalized to 143Nd/144NdCHUR= 0.511847. e Calculated relative to the depleted mantle model of DePaolo (1981).

Fig. 8. Pseudo-isochron plot for the Lower Kittanning Coal bed, the overlying Columbiana Shale, and the underlying paleosol, Northern Appalachian Basin. The calculated

“age” from this plot is broadly consistent with the depositional age of the sediments,

suggesting partial resetting of the Sm–Nd system at the time of deposition.

Fig. 9. Depleted mantle model age evolution diagram for Northern Appalachian Basin

samples. Solid lines are coal mineral matter, dashed lines are underclay samples, and

dotted lines are overburden samples. The convergence of the lines around the

308 Ma deposition age suggests partial resetting of the Sm–Nd system at that time

with an εNd value of about 9.0 (gray circle). Model ages older than ~1.6 Ga are probably

spurious, caused by redistribution of Sm/Nd during resetting. Note that most of these

data are from coal mineral matter.

4.5. Implications on respirable dust composition from this study

Results from this research were produced from rock samples

taken from two different coal basins with very different tendencies

in producing silicosis or CWP. The Lower Kittanning Coal bed shows

one set of compositional characteristics, and the Central Appalachian

units display similar compositions to each other and very different

mineralogy compared to the Central Appalachian Basin. Samples retrieved from the Central Appalachian Basin represent a region with

the most severe incidence rates for CWP in the US (Huang et al.,

2005). The Northern Appalachian Basin, including the Lower Kittanning stratigraphy, has not produced unusually high incidence rates

of silicosis or CWP. It is also well established that, although there

can be variations in overburden and underlying rock units associated

with coal seams due to facies changes, the Central Appalachian Basin

is dominated by sandstones and siliciclastic rocks above and
sometimes below the coal beds (Cecil, 1990). The Northern Appalachian Basin sites typically included shale or sandy shale overburden

and an underlying paleosol.

The mineralogy and geochemistry data from this study offer some

additional explanation for the differing silicosis and CWP incidence

rates for the basins. The Northern Appalachian Basin contains significant pyrite and therefore relatively high quantities of bio-available

iron (Huang et al., 2005). Both overburden and coal samples from

this study, associated with the Lower Kittanning Coal bed contained

visibly abundant quantities of pyrite and carbonate, as observed in

the samples and shown by ICP-AES analyses (Schatzel and Stewart,

2003). Although research shows the high pyrite content would exacerbate CWP and silicosis tendencies, the carbonate content tends to

have a buffering effect and neutralizes, to a degree, the health hazard

posed by the high iron content (Huang et al., 1998). A Chinese study

found nano-quartz particles in a bituminous coal seam and in the

lungs of rural residents who were burning this coal in their homes

(Dai et al., 2008). Although the studies focused on the particles as a

carcinogen, the authors documented a mechanism for respirable silica produced from coal becoming embedded in victim's lungs. In a petrographic characterization of organic matter, no nano-quartz particles

were observed in coal samples from either Appalachian basin. However, the submicron particle size observed in the Chinese samples

was near the limit of resolution for optical mineralogy. Any nanoquartz that was present would have been included in the digestion

and bulk analysis techniques used in this study.

In the Central Appalachian Basin samples, both iron and carbonates contents were very low. Both hand sample examination and

XRD analysis failed to find significant quantities of iron sulfides or

carbonates (Table 1). However, the high Si and quartz content produces dust particles that are silica rich during the mining process,

which present a respiratory hazard to the miners (Schatzel, 2009).

The Central Appalachian Basin portion of this study generally includes

small, near surface mines, which typically require only the minimum

ventilation air quantities to remove noxious gases. The airflow quantities measured at the mines’ last open crosscut were about 14 m3

/s;

the minimum quantity allowed by federal statutes to remove noxious

gases produced by the mining process. The limited airflow typical of

this mining region restricts the amount of air available to dilute respirable dust.

Data from the Nd isotope analyses show that the primary mineral

matter in the Lower Kittanning Coal bed, Northern Appalachian Basin,

is produced from the same source as the clastic mineral matter in the

rock units above and below the coal bed. A wealth of research has

produced compositional data on coal samples, but far less information is available on the geochemistry and mineralogy of rock units adjacent to coal beds. For Northern Appalachian Basin coal beds, it is

anticipated that primary mineralogy will have a common source

throughout the Pennsylvanian period of coal deposition, due to unchanging source terrains and distribution patterns (Patchett et al.,

1999). Although Nd analysis was not done in the Central Appalachian

Basin, the apparent constancy of erosional patterns distributing material from source terrains to basins in this region during the Pennsylvanian suggests the primary mineral source was the same between the

mined coal bed, overburden, and underlying unit.

Data from the Central Appalachian Basin samples show that respirable dust samples were most likely produced from the rock overlying the mined coal beds (Schatzel, 2009). A similar geochemical

trend is present in the rock samples from the Northern Appalachian

Basin. Although no dust samples were collected in this basin, it is proposed that respirable dust samples can be linked to their source on

the basis of sample geochemistry. Due to the relative commonality

of continuous miner practices, it is expected that the roof source of respirable dust will not change, although this will require validation

(Fig. 10).

Fig. 10. Continuous mining machine producing mine dust.

It is suggested that continuous miner water sprays can be

modified to more directly address roof dust sources if the parent

rock for respirable dust is shown to be consistently overlying rock

at all sites. This knowledge could be very valuable for the configuration of water sprays for dust control on continuous miners. A water

spray block directed at the mine roof is undergoing trials at an Illinois

Basin mine, based in part on the findings from the published study by

Schatzel (2009) (Chugh et al., 2011).

5. Summary and conclusions

This research combines analyses from multiple prior studies and

new analysis to investigate mineral matter provenance utilizing REE

and Nd isotopes. To complete this study, 11 mine sites in the Northern and Central Appalachian Basins of the United States were visited

and sampled. The methods in the earlier studies were not identical,

but the geochemical data presented here offer a range compositional

analyses and insights for the sample suite. Of the geochemical

methods utilized in this study, some techniques are conventional

practices routinely used by commercial laboratories. Other geochemical techniques employed were novel for coal research, and were only

possible in a laboratory research environment.

The mineralogy of rock samples retrieved from the Central Appalachian Basin was characterized by XRD. Quartz and clays (illite and

kaolinite) were the most abundant minerals detected in the sample

suite. These three minerals accounted for the great majority of the

sample mineralogies, with albite (plagioclase feldspar) and chlorite

being the next most abundant minerals. The abundance of these minerals in the samples is quite typical of bituminous rank, Pennsylvanian age coal. Carbonate and the iron-bearing minerals pyrite and
siderite are rare in the Central Appalachian Basin, and were rarely

found in hand samples and in the XRD results. The ICP-AES data

reflected these mineral matter compositions.

Samples from the Northern Appalachian Basin showed significant

carbonate and iron sulfide mineral content. Carbonates were present

in the form of siderite and calcite. Abundant pyrite was noted in the

coal and overburden samples. The ICP-AES data showed a tendency

toward enriched Ca, Mg, Mn, Na, and K in the overburden above the

mined coal seams in both basins when compared to the underlying

rock. It has been hypothesized that this geochemical tendency may

have been produced by late stage epigenisis associated with marine

incursions. This geochemical trend was observed in both basins, despite the significant differences between the basins in the mineralogy

of the coal beds and adjacent rock units.

Rare earth element data were reviewed for rock samples from the

Northern and Central Appalachian Basins. Predominantly negative

anomalies for Eu and Ce were present in the samples when compared

to chondrite data. The negative Ce anomalies may be indicative of oxidative loss of Ce4+ during weathering and/or transport. The TREE totals for the samples are inferred from the graphs and do show a

tendency towards enrichment in the underlying rock when compared

to the overburden. For the Northern Appalachian samples, the coal

mineral matter is generally enriched in REE, especially HREE, compared to underlying and overlying units.

Neodymium isotopic studies have been performed on sedimentary rocks for provenance studies, but are extremely rare in coal science. Determinations of Nd isotopic ratios were made for Northern

Appalachian Basin samples. For the Lower Kittanning Coal bed, the

overlying rock unit and underlying rock strata, measured εNd(0)

values range from −13.6 to −8.4, with no clear trends between the

coals and the under/overlying rock units. The εNd(T) (time T, the

time of deposition, estimated at ~308 Ma) fell within a much narrower range, averaging −8.9. We interpret these results as indicating

partial re-equilibration of the samples at the time of deposition, with

a greater spread in Sm/Nd produced during deposition or diagenesis.

While calculated depleted mantle model ages are variable due to this

resetting, we suggest that the source region was a relatively restricted

mixture of Grenville and older continental crust. The findings of this

study are consistent with those of Patchett et al. (1999), which

showed an Appalachian sedimentary source terrain throughout the

Pennsylvanian of eastern North America.

There are implications related to respiratory health for underground coal mine workers based on the study findings. The compositional data from coal measure rocks associated with both basins are in

agreement with CWP and silicosis incidence rates for underground

coal mine workers. Findings from recent studies have shown that respirable dust composition plays an important role in determining its

toxicity. If a parent rock source can be demonstrated to be in the overlying rock (roof) in all continuous miner operations, dust controls on

miner can be reconfigured to preferably address overhead dust

sources. The data presented here can be used in part to validate parent source rocks for respirable dust in mines.

Disclaimer

The findings and conclusions in this report are those of the author

and do not necessarily represent the views of the National Institute

for Occupational Safety and Health.

References

Bennett, P.C., Melcer, M.E., Seigel, D.I., Hassett, J.P., 1988. The dissolution of quartz in dilute aqueous solutions of organic acids at 25o C. Geochimica et Cosmochimica Acta

52, 1521–1530.

Bock, B., McLennan, S.M., Hanson, G.N., 1994. Rare earth element redistribution and its

effects on the neodymium isotope system in the Austin Glen Member of the

Normanskill Formation, New York, USA. Geochimica et Cosmochimica Acta 58,

5245–5253.

Bock, B., McLennan, S.M., Hanson, G.N., 1998. Geochemistry and provenance of the

Middle Ordivician Austin Glen Member (Normanskill Formation) and the Taconian

Orogeny in New England. Sedimentology 45, 635–655.

Bohor, B.F., Triplehorn, D.M., 1993. Tonsteins: altered volcanic ash layers in coalbearing sequences. Geological Society of America Special Paper 285, pp. 44

Cecala, A.B., O'Brien, A.D., Schall, J., Colinet, J.F., Fox, W.R., Franta, R.J., Joy, J., Reed, W.R.,

Reeser, P.W., Rounds, J.R., Schultz, M.J., 2012. Dust Control Handbook for Industrial

Minerals Mining and Processing. NIOSH RI 2012–112.

Cecil, C.B., 1990. Paleoclimate controls on stratigraphic repetition of chemical and silicate rocks. Geology 18, 533–536.

Cecil, C.B., Stanton, R.W., Allshouse, S.D., Finkelman, R.B., 1978. Geologic controls on

mineral matter in the Upper Freeport coal bed. Proceedings: Symposium on coal

cleaning to energy and environmental coals, U.S. Environmental Protection Agency, 600017-79-098a, 1, pp. 110–125.

Cecil, C.B., Stanton, R.W., Neuzil, S.G., Dulong, F.T., Ruppert, L.F., Pierce, B.S., 1985.

Paleoclimate controls on late Paleozoic sedimentation and peat formation in

the Central Appalachian Basin, USA. International Journal of Coal Geology 5,

195–230.

Chen, W., Hnizdo, E., Chen, J.-Q., Attfield, M.D., Gao, P., Hearl, F., Lu, J., Wallace, W.E.,

2005. Risk of silicosis on cohorts of Chinese tin and tungsten miners and pottery

workers (I): an epidemiological study. American Journal of Industrial Medicine

48, 1–9.

Chugh, Y.P., Mondal, K., Kollipara, V.K., Gurley, H., Yerram, R., Relangi, D.D., 2011. Characterization of coal and quartz and dust in Interior Basin coal mines. SME Annual

Meeting Preprint 11-132. pp. 7.

Colinet, J.F., Rider, J.P., Listak, J.M., Organiscak, J.A., Wolfe, A.L., 2010. Best practices for

dust control in coal mining Pittsburgh, PA: U.S. Department of Health and

Human Services, Public Health Service, Centers for Disease Control and Prevention,

National Institute for Occupational Safety and Health, DHHS (NIOSH) Publication

No. 2010-110, Information Circular 9517, pp. 1–76.

Condie, K.C., 1991. Another look at rare earth elements in shales. Geochimica et Cosmochimica Acta 55, 2527–2531.

Crowley, S.S., Ruppert, L.F., Belkin, H.E., Stanton, R.W., Moore, T.A., 1993. Factors affecting

the geochemistry of a thick, sub-bituminous coal bed in the Powder River Basin: volcanic, detrital, and peat forming processes. Organic Geochemistry 20, 843–853.

Cullers, R.L., Bock, B., Guidotti, C., 1997. Elemental distributions and neodymium isotopic compositions of Silurian metasediments, Western Maine, USA. Redistribution

of the rare earth elements. Geochimica et Cosmochimica Acta 61 (9), 1847–1861.

Dai, S., Tian, L., Chou, C.L., Zhou, Y., Zhang, M., Zhao, L., Wang, J., Yang, Z., Cao, H., Ren,

D., 2008. Mineralogical and compositional characteristics of Late Permian coals

from an area of high lung cancer rate in Xuan Wei, Yunnan, China: Occurrence

and origin of quartz and chamosite. International Journal of Coal Geology 76,

318–327.

DePaolo, D.J., 1981. Neodymium isotopes in the Colorado Front Range and crust–

mantle evolution in the Proterozoic. Nature 291, 193–196.

DePaolo, D.J., Linn, A.M., Schubert, G., 1991. The continental crustal age distribution:

Methods of determining mantle separation ages from Sm–Nd isotopic data and application to the southwestern United States. Journal of Geophysical Research 96,

2071–2088.

Dodgson, J.G., Hadden, G.G., Jones, C.O., Walton, W.H., 1971. Characteristics of the airborne dust in British coal mines. In: Walton, W.H. (Ed.), Inhaled Particles III.

Unwin Brothers Ltd., Surrey, England, pp. 757–781.

Eskenazy, G.M., 1987. Rare earth elements and yttrium in lithotypes of Bulgarian coals.

Organic Geochemistry 11 (2), 83–89.

Eskenazy, G.M., 1999. Aspects of the geochemistry of rare earth elements in coal: an

experimental approach. International Journal of Coal Geology 38, 285–295.

Finkelman, R.B., 1982. The origin occurrence, and distribution of the inorganic constituents in low rank coals, DOE sponsored workshop on Basic Coal Science, pp. 69–90.

Houston, TX, December 8–9, 1981.

Finkelman, R.B., 1993. Trace and minor elements in coal. In: Engles, M.H., Macko, S.A.

(Eds.), Organic geochemistry: Principles and Applications. Plenum, New York,

pp. 593–607.

Finkelman, R.B., Stanton, R.W., 1978. Identification and significance of accessory minerals from a bituminous coal. Fuel 57, 763–768.

Gluskoter, H.J., 1967. Clay minerals in Illinois coals. Fuel 44, 285–291.

Gluskoter, H.J., Ruch, R.R., Miller, W.G., Cahill, R.A., Dreher, G.B., Kuhn, J.K., 1977. Trace

elements in coal: occurrence and distribution. Illinois State Geological Survey Circular, 499. pp. 154.

Gromet, P.L., Dymek, R.F., Haskin, L.A., Kortev, R.L., 1984. The North American shale

composite: its compilation, major and minor trace element characteristics. Geochimica et Cosmochimica Acta 48, 2469–2482.

Hanson, G.N., 1980. Rare earth elements in petrogenetic studies of igneous systems.

Annual Review of Earth and Planetary Sciences 8, 371–406.

Harrison, J.C., Chen, J.-Q., Miller, W., Chen, W., Hnizdo, L.J., Chisolm, W., Keane, M., Gao,

P., Wallace, W., 2005. Risk of silicosis on cohorts of Chinese tin and tungsten

miners and pottery workers (II): workplace-specific silica particle surface composition. American Journal of Industrial Medicine 48, 10–15.

Harvey, R.D., Ruch, R.R., 1986. Mineral matter in Illinois and other US coals. In: Vorres,

K.S. (Ed.), Mineral Matter in Ash and Coal: American Chemical Society Symposium

Series, 301, pp. 10–40.

Hicks, D., Nagelschmidt, G., 1943. Special Report of the Service Medical Research Council, 244, pp. 153–186.

Hill, P.A., 1988. Tonsteins of Hat Creek, British Columbia: a preliminary study. International Journal of Coal Geology 10, 155–175.
Hower, J.C., Williams, D.F., Eble, C.F., Sakulpitakafon, T., Moecher, D.P., 2001. Brecciated

and mineralized coals in Union County, western Kentucky. International Journal of

Coal Geology 47, 223–234.

Huang, X., Fournier, J., Koenig, K., Chen, L.C., 1998. Buffering capacity of coal and its acid

soluble Fe2+ content, possible role in coal workers’ pneumoconiosis. Chemical Research in Toxicology 11, 722–729.

Huang, X., Li, W., Attfield, M.D., Nadas, A., Frenkel, K., Finkelman, R.B., 2005. Mapping

and prediction of coal worker's pneumoconiosis with bioavailable iron content in

the bituminous coals. Environmental Health Perspectives 113, 964–968.

IARC, 1997. IARC monographs on the evaluation of carcinogenic risks to humans: silica,

some silicates, coal dust and para-aramid fibrils, Vol. 68. World Health Organization, International Agency for Research on Cancer, Lyon, France.

Laney, A.S., Petsonk, E.L., Attfield, M.D., 2010. Pneumoconiosis among underground bituminous coal miners in the United States: is silicosis becoming more frequent?

Occupational and Environmental Medicine 67, 652–656.

Linn, A.M., DePaolo, D.J., Ingersoll, R.V., 1991. Nd–Sr isotopic provenance analysis of

Upper Cretaceous Great Valley fore-arc sandstones. Geology 19, 803–806.

Mackowsky, M.Th., 1968. Mineral matter in coal. In: Murchison, D.G., Westoll, D.S.

(Eds.), Coal and Coal Bearing Strata. Oliver and Boyd, London, pp. 309–321.

Mariano, A.N., 1989. Economic geology of rare earth minerals. In: Lipin, B.R., McKay,

G.A. (Eds.), Geochemistry and mineralogy of rare earth elements: Mineralogical

Society of America Reviews in Mineralogy, 21, pp. 309–334.

McCabe, P.J., 1993. Sequence stratigraphy of coal bearing strata. AAPG short course,

AAPG New Orleans, LA, April 25–28, 1993, pp. 81.

McLennan, S.M., 1989. Rare earth elements in sedimentary rocks: influence of provenance and sedimentary processes. In: Lipin, B.R., McKay, G.A. (Eds.), Geochemistry

and mineralogy of rare earth elements: Mineralogical Society of America Reviews

in Mineralogy, 21, pp. 169–200.

Miller, R.G., O'Nions, R.K., Hamilton, P.J., Welin, E., 1986. Crustal residence ages of clastic sediments, orogeny and crustal evolution. Chemical Geology 57, 87–99.

Nelson, B.K., DePaolo, D.J., 1988. Comparison of isotopic and petrographic provenance

indicators in sediments from Tertiary continental basins of New Mexico. Journal

of Sedimentary Petrology 58, 348–357.

Palmer, C.A., Lyons, P.C., 1996. Selected elements and major minerals from bituminous

coals as determined by INAA: implications for removing environmentally sensitive

elements from coal. International Journal of Coal Geology 32, 151–166.

Palmer, C.A., Lyons, P.C., Brown, Z.A., Mee, J.S., 1990. The use of trace element concentrations in vitrinite concentrates and companion whole coals (hvA bituminous) to

determine organic and inorganic associations. Geological Society of America Special Paper 248, 55–62.

Patchett, P.J., Ross, G.M., Gleason, J.D., 1999. Continental drainage in North America

during the Phanerozoic from Nd isotopes. Science 283, 671–673.

Ramani, R.V., Mutmansky, J.M., Bhaskar, R., Qin, J., 1987. Experiments, Results and Analysis. Contract Report H058031. : Fundamental Studies on the Relationship Between Quartz Levels in the Host Material and the Respirable Dust Generated

During Mining, Volume 1. U.S. Bureau of Mines.

Rao, P.D., Gluskoter, H.J., 1973. Occurrence and distribution of minerals in Illinois coal.

Illinois State Geological Survey, Circular, 476. pp. 56.

Rao, P.D., Walsh, D.E., 1997. Nature and distribution of phosphorous minerals in Cook

Inlet coals, Alaska. International Journal of Coal Geology 33, 19–42.

Rao, P.D., Walsh, D.E., 1999. Influence of environments of coal deposition on phosphorus accumulation in a high latitude, northern Alaska coal seam. International Journal of Coal Geology 38, 261–284.

Rimmer, S.M., Davis, A., 1986. Geologic controls on the inorganic composition of the

Lower Kittanning Coal Seam. In: Vorres, K.S. (Ed.), Mineral Matter in Coal, American Chemical Society Symposium Series No. 301, pp. 41–51.

Robock, K., Bauer, H.D., 1990. Investigations into the specific fibrogenicity of mine dusts

in hardcoal mines of counties of the European Community. Proc. VIIth International

Pneumoconiosis Conference, Part 1: Pittsburgh, 1988: DHHS (NIOSH) publication

No. 90-108, Part 1, pp. 280–283.

Ruppert, L.F., Neuzil, S.G., Cecil, C.B., Kane, J.S., 1993. Inorganic constituents from samples of a domed and lacustrine peat, Sumatra, Indonesia. In: Cobb, J.C., Cecil, C.B.

(Eds.), Modern and ancient coal forming environments: Geological Society of

America Special Paper, 286, pp. 83–96.

Schatzel, S.J., 2009. Identifying sources of respirable quartz and silica dust in underground coal mines in southern West Virginia, western Virginia, and eastern

Kentucky. International Journal of Coal Geology 78, 110–118.

Schatzel, S.J., Stewart, B.W., 2003. Rare earth element sources and modification in the

Lower Kittanning coal bed, Pennsylvania: implications for the origin of coal mineral matter and rare earth element exposure in underground coal mines. International Journal of Coal Geology 54, 223–251.

Sholkovitz, E.R., Shen, G.T., 1995. The incorporation of rare earth elements in modern

coral. Geochimica et Cosmochimica Acta 59, 2749–2756.

Stach, E., Mackowsky, M.-T.h., Teichmüller, M., Taylor, G.H., Chandra, D., Teichmüller,

R., 1982. Stach's Textbook of Coal Petrology. Gebrüder Borntraeger, Berlin. 535 pp.

Taylor, C.D., Chilton, J.E., Goodman, G.V., 2010. Guidelines for the control and monitoring of methane gas on continuous mining operations. DHHS (NIOSH) Publication

No. 2010-141, Report of Investigations 9523, pp. 1–75.

Vance, D., Burton, K., 1999. Neodymium isotopes in planktonic foraminifera: a record of

the response of continental weathering and ocean circulation rates to climate

change. Earth and Planetary Science Letters 173, 365–379.

Vassilev, S.V., Vassileva, C.G., 1996. Occurrence, abundance and origin of minerals in

coals and coal ashes. Fuel Processing Technology, 48, pp. 85–106.

Ward, C.R., 2002. Analysis and significance of mineral matter in coal seams. International Journal of Coal Geology 50, 134–168.

Ward, C.R., Corcoran, J.F., Saxby, J.D., Read, H.W., 1996. Occurrence of phosphorus minerals in Australian coal seams. International Journal of Coal Geology 30, 185–210.

Wilde, P., Quinby-Hunt, M.S., Erdtmann, B.D., 1996. The whole-rock cerium anomaly: a

potential indicator of eustatic sea-level changes in shales of the anoxic facies. Sedimentary Geology 101, 43–53.

Willett, J.C., Finkelman, R.B., Mroczkowski, S., Palmer, C.A., Kolker, A., 2000. Semiquantitative determination of the modes of occurrence of elements in coal: results

from an international round robin project. In: Davidson, R.M. (Ed.), Modes of

occurrence of trace elements in coal. : Reports from an International Collaborative

Programme. IEA Coal Research, London, UK. CD-ROM.

Williams, E.G., 1957. Stratigraphy of the Allegheny series in the Clearfield Basin. PhD.

Thesis, The Pennsylvania State University, Vol. 1 and 2, pp. 582.

Williams, E.G., Bragonier, W.A., 1974. Controls of early Pennsylvanian sedimentation in

western Pennsylvania. In: Briggs, G. (Ed.), Carboniferous of Southeastern United

States: Geological Society of America Special Paper, 148, pp. 135–152.

Worrall, F., Pearson, D.G., 2001. Water-rock interaction in an acidic mine discharge as

indicated by rare earth element patterns. Geochimica et Cosmochimica Acta 65,

3027–3040.

Yang, J.-H., Zhou, X.-H., 2001. Rb–Sr, Sm–Nd, and Pb isotope systematics of pyrite: Implications for the age and genesis of lode gold deposits. Geology 29, 711–714.

Yokoyama, T., Makishima, A., Nakamura, E., 1999. Evaluation of the coprecipitation of

incompatible trace elements with fluoride during silicate rock dissolution by acid

digestion. Chemical Geology 157, 175–187.

Zhang, S., Shan, X.Q., Li, F., 2000. Low molecular weight organic weight acids as extractant to predict plant bioavailability of rare earth elements. International Journal of

Environmental Analytical Chemistry 76 (4), 283–294.

Zhong, S., Mucci, A., 1995. Partitioning of rare earth elements (REEs) between calcite

and seawater solutions at 25 °C and 1 atm, and high dissolved REE concentrations.

Geochimica et Cosmochimica Acta 59, 443–453.