The importance of minerals in coal as the hosts of chemical elements: A review

The importance of minerals in coal as the hosts of chemical elements: A review Robert B. Finkelmana,b, Shifeng Daia,c,*, David Frenchd a State Key Laboratory of Coal Resources and Safe Mining, China University of Mining and Technology, China b University of Texas at Dallas, Richardson, TX 75080, USA c College of Geoscience and Survey Engineering, China University of Mining and Technology (Beijing), Beijing 100083, China

concluded that nano-quartz in coal that was being used domestically in Xuan Wei County, Yunnan, Province, China, was the principle cause of the world's highest rate of women's lung cancer. Dai et al. (2008b) showed that exceedingly high levels of quartz (57.6-74.7%, mean 65.7% on a low-temperature ash basis) accounted for the majority of minerals in Xuan Wei coals. Some minerals, sulfides in particular, are the major hosts of toxic elements (such as As and Hg), which have adverse effects on human health (e.g., arsenonsis in Guizhou Province, southwestern China) and the environment.
Although the occurrence of minerals in coal is usually regarded as having deleterious effects in coal utilisation, minerals in coal have some important beneficial aspects. Coal and coal ash are attracting increasing attention as potential sources of critical elements, such as rare earth elements and Y, Li, Ga, Se, Zr, and Nb (Seredin and Finkelman, 2008;Bullock et al., 2018;Dai and Finkelman, 2018;Lin et al., 2018b;Zhao et al., 2018), all of which are in demand in the semi-conductor industry and the production of advanced materials. In a number of countries, rare earth element concentrations are sufficiently high in the coal ashes to make extraction an economically viable option (Serein and Dai, 2012;Hower et al., 2016;Kolker et al., 2017a;Laudal et al., 2018;Lin et al., 2018a;Wagner and Matiane, 2018). High-Al coals have also attracted much attention in recent years in China, because the derived ash has Al2O3 higher than 50 weight percent and thus have been used for Al extraction (Seredin, 2012;Dai et al., 2018d). With a few exceptions (e.g. Ge and U), most of the critical elements are largely hosted in minerals.
In this paper, we identify the most likely minerals or mineral groups that act as hosts for a large number of elements and, when appropriate, we will point out the significance of these relationships. Some of this discussion is based on results from a paper on the quantification of the modes of occurrence of 42 elements in coal (Finkelman et al., 2018). In that paper, the authors concluded that, in bituminous coal, the bulk (>50%) of all elements except Br were associated with minerals. In low rank coals, a greater proportion of the elements are organically associated, but the majority of the elements still have primary inorganic associations. The exceptions are Be, Br, Mg, Se, Na, and U, and possibly Co, Ni, and Sr. We fully recognize that there are many exceptions to these relationships. For instance, coals with ash yields less than 10% many have the bulk of their elements organically associated. Clearly a greater proportion of many elements in low rank coals are associated with the organic matter.
Also, coals formed under unusual conditions or influenced by unusual circumstances (oxidation, epigenesis, igneous intrusion, volcanic ash deposits, marine incursion, etc.) may deviate from these generalizations. However, we contend that these coals are the exceptions and that our observations are relevant to the majority of coals mined around the world. Furthermore, almost all elements associate with more than one mineral or even multiple minerals and mineral groups, making unequivocal associations virtually impossible. We try to make the distinction between those elements that are physically associated with a mineral group, i.e., an element in a mineral that is enmeshed in another mineral phase, and elements that are chemically associated with a mineral or mineral group, i.e. part of the mineral structure.

Minerals found in coal
There are a number of methods to detect and identify minerals in coal. Bulk chemical analysis provides clues as to what minerals or mineral groups may be present in the coal sample. Optical petrography and X-ray diffraction (XRD) can help identify specific minerals (Ward, 2016) but provide little to no information on the trace elements associated with the minerals. Further analysis on XRD spectrum using Rietveld-based interpretation software, e.g., Siroquant, can quantitatively determine the mineral percentages in coal and/or low temperature ash (Ward et al., 2001a,b;Ruan and Ward, 2002;Dai et al., 2012b). Selective leaching helps identify mineral groups such as carbonates (leached by HCl), sulfides (leached by HNO3 or HCl), and silicates (leached by HF) and provides information on which elements may be associated with each mineral group rather than specific minerals (Riley et al., 2012;Finkelman et al., 2018;Liu et al., 2015Liu et al., , 2018. Microbeam methods (scanning electron microscopy (SEM), electron microprobe analysis, ion probes, transmission electron microscopes and related instruments) with energy or wavelength dispersive (EDS/WDS) detectors are the most useful method for identifying the host or hosts of the elements in coal (Dai et al., 2012a,b;Etschmann et al., 2017;Hower et al., 2018a,b;Wang et al., 2018;Wei et al., 2018), and in many cases can unequivocally identify a specific mineral unless polymorphs exist. Float-sink density separation combined with XRD, SEM-EDS, and other chemical analyses to determine minerals and trace elements in different density fractions has also been applied (Wagner and Tlotleng, 2012;Tian et al., 2014).
Comparison of results for element affinities determined by density separation and selective leaching techniques applied to the same coal showed good agreement for most elements (Querol et al., 2001).
Statistical analyses, e.g., correlation and cluster analyses, have been used to deduce the modes of occurrenc of major and trace elements in coal and in the mineral hosts (e.g., Spears and Zheng, 1999;Zhao et al., 2019) based on correlations between the concentration of individual major and trace elements in a series of related coal samples. Some authors have pointed out the cuations using statistical analyses for associations between minerals (or ash yield) and elements (e.g., Mraw et al., 1983;Glick andDavid, 1987). Eskanazy et al. (2010) has shown that there are potential problems with this approach if the sample suite contains a wide range of ash yields. Geboy et al. (2013) proposed a mathematical approach to keep consistent interpretations of whole-coal versus ash basis in coal geochemistry. Ward (2016) showed that this approach may be more effective if the nature and quantitative content of the different minerals in coal samples have been independently established (e.g., by XRD analysis on lowtemperature ashes of coal), and this would allow the trace element concentrations to be related more directly to particular minerals in coal. The effectiveness of this integrated approach has been verified by a number of stuides (e.g., Ward et al., 1999;Dai et al., 2012aDai et al., , 2015aZhao et al., 2019).
Some 200 minerals have been observed in coal. Finkelman (1981) contains a list of about 175 minerals that had been reported from coal, and, many more have been recorded in the intervening 40 years. Some of these more recently reported minerals are included by Ward (2016) in a comprehensive list of minerals in coal. Table 1 borrows mainly from these two references, augmented by other recent reports on minerals found in coal. The modes of occurrence and origins of most of these minerals have been comprehensively reviewed by Ward (2002Ward ( , 2016. The minerals identified in coal and LTAs (low-temperature ashes) of coal can be classified as silicate, sulfides, carbonate, oxides and hydroxides, selenides, phosphates and oxalates, based on their elemental compositions and crystal structures; and as common, uncommon, and rare, on the basis of their abundance in worldwide samples. We acknowledge that there are questionable or suspect identifications for some minerals, particularly for those that are rare in coal. Due to limitations of each method, for several minerals, many studies, including this investigation, are constrained to use the generic terms such as 'clays', 'silicates', and 'carbonates'.
In reality, some of the minerals on the current list have not been verified and should be considered as speculative until conformation is forthcoming. The table will be posted on the website of The Society for Organic Petrology (TSOP) and updated periodically. The authors welcome all comments and inputs on the table and anyone wishing to add a new mineral along with supporting evidence, or verified evidence for the minerals that have not be verified in the Table 1, could submit their mineral materials through the entry www.tsop.org.

Silicates as hosts of chemical elements
The silicates are the largest, most complex, and generally the most abundant group of minerals in coal (Table 1). Not surprisingly the silicates are the hosts of many elements found in coal, particularly of major elements including Si and Al, and to a lesser extent, K, Ca, Na, Mg, and Fe.
The silicates include the clay minerals, the most diverse and generally most abundant mineral group in coal, and quartz, perhaps the most common mineral in coal. Other important silicates are micas, analcime (Finkelman, 1988;Wang et al., 2018), and various feldspars.
It is highly likely that the clays, relative to other silicates, are the primary hosts of major, and, in particular, a substantial number of trace elements in coal. For example, quartz, chalcedony, and cristobalite in coal tends to be low in most elements with exceptions of Si, O and possibly Li. Zircon and tourmaline have been found in some coals and are hosts of a limited number of trace elements, e.g., Zr, Hf, REE, Nb, Ta, Th, and U (Zircon); and Li, B, and F (tourmaline). A number of studies have shown strong correlations between of many trace elements and clay minerals (e. g., Finkelman, 1981). This is because clays, usually negatively charged in nature, have high surface to volume ratio, which enable trace elements, usually positively charged, to be adsorbed on its surface. Also, some clays have interlayer space, where cation exchange may take place. Kuhn et al. (1980) showed that at least 20 trace elements are associated with clay minerals based on the investigation of 27 coals from eight areas in USA. Some studies have shown that some elements, usually occurring at a low concentration level in coal and as adsorbed forms, could be the major component of clay minerals. For example, Li-bearing minerals, cookeite [(Al2Li)Al2(AlSi3O10)(OH)8], has been identified in an anthracite in the Jinchen deposit in China, and was derived from the reaction of previously-formed kaolinite with Li ions (Zhao et al., 2018). In the Guanbanwusu Al-Ga-REY coal-hosted deposit in China, chlorite phase has a composition intermediate between chamosite and a Li-bearing cookeite component (Dai et al., 2012a). Another such case is V-bearing mineral, roscoelite, K(V 3+ ,Al)2(AlSi3O10)(OH)2, has been identified in a late Permian coal in the Moxinpo Coalfield in southwestern China (Dai et al., 2017).
In addition to anatase, rutile, and ilmenite, clay minerals (such as kaolinite and illite) may host a large proportion of Ti in some coals (Minkin et al., 1979;Ward et al., 1999;Dai et al., 2015b).
Two modes of Ti occurrence were observed in the kaolinite in coal: substituting for Al in the crystal lattice of the kaolinite and as fine-grained discrete particles in kaolinite. About 1.5% Ti was suggested to substitute for Al in the kaolinite in the coal from the Gunnedah Basin, Australia . Huggins et al. (2000) analyzed four coals using XAFS spectroscopy and a selective leaching protocol supplemented by SEM. They found that both methods indicate two principal forms of Cr in the bituminous coals: the major occurrence of chromium is associated with the macerals as the oxyhydroxide CrOOH, whereas a second, lesser occurrence, is associated with the clay mineral, illite, which was subsequently confirmed by ion microprobe analysis.
An interesting aspect is that the primary elements that are associated with the silicates are largely benign, that is, with several exceptions they do not cause technological, environmental, or human health problems and are not on the critical element list. Sodium may appear to be an exception as it does contribute to boiler fouling but it appears that organically-bound and non-mineral-bound Na, is primarily to blame (Finkelman, 1988). Some critical elements, such as Al and Ga, are the other exceptions as they have been industrially extracted from Al-Ga-rich fly ash derived from the coals in the Jungar deposit, Inner Mongolia, China (Seredin, 2012;Dai and Finkelman, 2018;Dai et al., 2018d). Another exception is Mg in coal, which has been recovered from fly ash derived from low-rank coals in southeastern Australia, using a combined hydrometallurgical/thermal reduction process (Dai and Finkelman, 2018).

Sulfides and selenides as hosts of chemical elements
A wide range of sulfide minerals have been found in coal (Table 1; Fig. 1) with pyrite being, far and away, the most common. Without question pyrite has greatest impact of any mineral in coal. Among the many problems caused by pyrite are: • Oxidation of pyrite results in costly acid mine drainage problems (Campbell et al., 2001;Weber et al., 2006;Shahhosseini et al., 2017;Stewart et al., 2017).
• The iron and sulfur from pyrite are major contributors to boiler slag (Bool III et al., 1995;Brink et al., 1994;Regina et al., 2004).
• Pyrite is likely a major contributor to Coal Workers Pneumoconiosis (Black Lung Disease) (Huang et al., 2006).
• Proper disposal of pyrite and products of pyrite decomposition (coal cleaning wastes, boiler slag, fouling deposits, bottom ash, FGD, fly ash, etc.) adds costs to the utilization of coal.

2018).
Leaching coals with nitric acid (Finkelman et al., 2018), microprobe analysis  and Laser ablation ICP-MS (Kolker et al., 2017a) showed that sulfides, likely pyrite, are the primary host of As and Hg, with pyrite commonly containing up to several weight percent As.
Arsenopyrite has been reported in coal (Belkin et al., 1997;Kolker, 2012), but this mineral is exceedingly rare and is not a major host of As. Other elements that are likely associated with the sulfide minerals are Te, Tl, Ag, and Bi.
Geological Survey's WoCQI database (Bragg et al., 1997) showed that As is the most abundant minor constituent in Fe-disulfides in coal and elements including Se, Ni, and other minor constituents are less-commonly present with lower concentrations than As. Fe-disulfides with different generations (different formation stages) or different origins may have different abundance of trace elements (Kolker, 2012). For example, framboidal pyrite in some instances shows preferential Ni enrichment with respect to other co-occurring pyrite with other modes of occurrence (e.g., cleat-or vein-filling pyrite or marcasite; Kolker, 2012). Using highresolution time-of-flight secondary ion mass-spectrometry, Dai et al. (2003) investigated the abundance of trace elements in different-form pyrites, such as bacteriogenic, framboidal, massive, cell-filling, fracture-filling, and nodular pyrites. They found that relative to other form pyrite, bacteriogenic pyrites are rich in Cu, Zn, and Ni, and this is consistent with bacterial complexing of metals in anoxic sediments (Kolker, 2012).
When trace elements such as As, Se, and Sb are present in pyrite, they usually substitute for S of pyrite, whereas transition metals, such as Hg and Pb, are thought to substitute for Fe of pyrite (Kolker, 2012). However, a recent study by Etschmann et al. (2017) showed that As has a more complex speciation pattern than expected. Arsenic may have several valence states such as As(III), As(V), and As(−I/+II) in solid solution in sulfides in coal. Arsenic may occur in anionic and cationic forms, i.e., it shows both the common substitution for S and the substitution for Fe.
The fly ash derived from a Chinese Al-Ga-REY coal-hosted ore deposit (Jungar Coalfield, Dai et al., 2006aDai et al., , 2012c has been utilized for Al and Ga extraction because it contains >50% Al2O3 and ~100 ppm Ga. One of the major hosts for Al and Ga is oxyhydroxide (e.g., boehmite and diaspore; Dai et al. 2006aDai et al. , 2012c.
In addition to Ca, Ba, Sr, P, Al, and U, the phosphates are also an important host of REE and Y and silico-rhabdophane, the major light rare earth element hosts, have been identified in many coals (Dai et al., 2014b). Chlorine and Br may be present in gas-liquid inclusions of detrital and authigenic apatite (Vassilev et al., 2000); however, as pointed out by Yudovich and Ketris (2006b), Cl with such modes of occurrence seems to be very minor.

Sulfates as hosts of chemical elements
Sulfates are not uncommon in coal and most are secondary oxidation products. The most common syngetic sulfates in coal are gypsum (Ward, 2002(Ward, , 2016Liu et al., 2018) and barite (Finkelman et al., 2018).
Sulfates are the hosts of some major elements in coals (Table 1) Seredin and Finkelman (2008) reported many unusual mineral phases, including native W, Au and Ag, (BiPb)3FeCdMoS, BiMo(Cu)S, various Au and Pt phases, primarily in Russian coals. Hower et al. (2018b) found in the Blue Gem coal in Kentucky grains containing Co-Ge and Ag-Cd-Bi, the latter with a more evident S association than the former, metallic Bi, Ni3Sn, and silver cadmium. These strange and interesting phases may have economic and geochemical significance, but are not likely to represent significant common modes of occurrence of these elements in coal. Other interesting but rare mineral occurrences include:

Uncertain associations and issues that need further attention
Although modes of mineral occurrence of a number of elements have been widely investigated, there are some elements whose associations and particularly association mechanism with minerals are, to a degree, uncertain or even largely unknown and deserve further attention.
These include but not limited to Be, Bi, B, Br, Cs, Co, Au, I, In, Mo, Ni, Nb, the platinum group elements, Ag, Ta, Te, Tl, Th, Sb, W, and V.

Conclusions
Minerals are the most important components of inorganic matter in coal and, in most cases, play the most significant role in affecting the utilization of coal. Minerals are also the major hosts of the vast majority of toxic, benign, and critical elements present in coal. Although a number of elements and their hosting minerals in coal have been widely investigated, some issues require further investigation to evaluate more fully the modes of occurrence of elements in coal: • The associations of some elements with minerals are uncertain or even largely unknown (for examples but not limited to Be, Bi, Br, Co, I, In, Mo, Nb, Ta, Te, W, and V) and deserve further investigation using integrated approaches as mentioned in the text above.
• The association mechanism of many elements with minerals are also unknown, e.g., conditions of toxic elements (As, Hg, Tl) substitute for major ions of sulfides in coal.
• Quantitative analysis of elements associated with specific minerals rather than with generic terms such as 'clays', 'silicates', and 'carbonates' needs new technologies for more fully understanding modes of occurrence of elements.
• As mentioned in Section 3.8 a number of interesting and rare phases have been found in coal (see Fig. 4) confirming that additional detailed mineralogical investigations of coal are entirely justified.     Table 1. Minerals reported in coal and coal low temperature ash (LTA). The data is taken primarily from Finkelman (1980) and Ward (2016) but with additional information from various sources. The minerals in bold have been confirmed by X-ray diffraction, a unique chemistry, or multiple observations. The validity of those in italics has not been confirmed.