Mineralogy

As discussed by Robl and Bland (1977), Rice et al. (1994), and Hower et al. (Reference Hower, Andrews, Wild, Eble, Dulong and Salter1994), the tonstein (Fig. 2) mineralogy is dominated by kaolinite with lesser amounts of quartz, K-feldspar, illite, and other minerals (Table 2). The Al₂O₃-SiO₂-K₂O chemistry (Fig. 3) supports the dominance of kaolinite, with most samples clustering along the Al₂O₃–SiO₂ axis. As a caveat concerning the samples, Frank Dulong (Hower et al., Reference Hower, Andrews, Wild, Eble, Dulong and Salter1994; based on a 1991 personal communication with Cortland Eble) noted that sample 4911 might have been transitional to the underclay (sample 4912) resulting in a diminished kaolinite content in the “tonstein” and a relatively high kaolinite content in the underclay. Of the other tonstein samples with <50% kaolinite, 4673 was noted as darkening upwards, perhaps an indicator of an increasing illite content in that part of the rock; 4962 was a parting from a roadcut in the Hindman 7½ˈ quadrangle, thus relatively distant from the other sites; and 4969 was from a weathered roadcut. Basically, the field identification was imperfect and roadcut samples with ~10–15 y of weathering (at the time of the sampling) are imperfect. In this case, the XRD analysis is the final arbiter of the sample type. Setting aside these four gradational (4673 and 4911) and weathered (4962 and 4969) samples, the kaolinite content of the tonstein samples ranges from 59 to >91%. The coal sample included among these samples (41010) contains several flint clay inclusions.

Fig. 2 a Lithologic profile of a coal seam surrounding tonstein sample 4712. b Hand specimen of tonstein (sample 4712), the underlying illitic clay (sample 4713 in Hower et al., Reference Hower, Ruppert and Eble1999), and a portion of the lowermost lithotype of the coal (sample 4714 in Hower et al., Reference Hower, Ruppert and Eble1999). Note centimeter scale to the right of hand specimen. c Polished block corresponding to coal sample 41010. The coal encases kaolinite lenses (K) and a thin tonstein lens (t). The tonstein was present at the top of the polished coal section. Note centimeter scale to the left of polished block

Table 2 Mineral percentages as determined by XRD and Siroquant (%)

Tonsteins in regular type, floor rocks in blue italics, and coal in bold blue italics

KGS Kentucky Geological Survey

Fig. 3 Al₂O₃–SiO₂–K₂O chemistry of the rock and coal samples. The floor samples (4703, 4898, 4880, and 4912) and the weathered tonsteins (4962 and 4969) are identified

Previous electron microscopy studies include the SEM and high-resolution TEM (HRTEM) investigation of the minerals in a high-REE portion of the Dean coal (a Fire Clay coal correlative) in Knox County, Kentucky (Hower et al., Reference Hower, Berti, Hochella and Mardon2018; based on samples from Mardon & Hower, Reference Mardon and Hower2004). Figure 4a shows interlayered monazite and kaolinite. In this case, the kaolinite could represent the diagenetic recrystallization of the volcanic glass. A well crystallized kaolinite, possibly a phenocryst from the volcanic-ash fall, is shown in Fig. 4b,c. The Dean coal site is different from the sites in the current study in that it has no megascopic tonstein. The lower portion of the coal bed does show an enrichment in REE and does contain abundant kaolinite and trace amounts of monazite, however (Hower et al., Reference Hower, Berti, Hochella and Mardon2018).

Fig. 4 TEM images of the FIB section. a Monazite (mz) and kaolinite (Kln); the black ribbon is a platinum (Pt) protective layer placed on the surface of the sample; b kaolinite; c selected area electron diffraction pattern (SAED) of the region within the white circle in b. The pattern symmetry and d spacing correspond to kaolinite viewed along $\left(1,\overline{1},0\right)$ . The inset shows the kinematic simulation computed with the software SingleCrystal®4, using the kaolinite structure by Bish and Von Dreele (Reference Bish and Von Dreele1989) which confirms the identification (from Hower et al., Reference Hower, Berti, Hochella and Mardon2018)

In addition to minerals identified by XRD, some trace minerals with contents below the detection limits of XRD analysis were identified by SEM–EDS, including K-feldspar and apatite (Fig. 5); zircon (Fig. 6); Ti oxides (rutile or anatase) (Fig. 7); ilmenite, pyrite, crandallite, florencite, and galena (Fig. 8); and siderite and goethite (Fig. 9). Compared to the trace minerals found in the present study, Weaver (Reference Weaver1963), in his review of heavy minerals in bentonites, noted that hornblende, biotite, zircon, apatite, and titanite were the most common non-opaque heavy minerals in felsic ashes.

Fig. 5 SEM back-scattered electron images of kaolinite, sanidine, and apatite in tonstein samples: a vermicular kaolinite and a matrix kaolinite in sample 4673; b sanidine in a kaolinite matrix in sample 4673; c sanidine in a kaolinite matrix in sample 4777; d sanidine in a kaolinite matrix in sample 4673; e and f apatite in a kaolinite matrix. Key: Kln, kaolinite; Sa, sanidine; Ap, apatite

Fig. 6 SEM back-scattered electron images of zircon in sample 4673

Fig. 7 SEM back-scattered electron images of Ti oxides (rutile or anatase): a Ti oxide distributed in vermicular kaolinite in sample 4673; b authigenic Ti oxide distributed in a kaolinite matrix in sample 4673; c Ti oxides subjected to corrosion and filled with authigenic quartz in sample 4777; d Ti oxides subjected to corrosion and filled with kaolinite in sample 4777. Key: Kln, kaolinite; Qtz, quartz

Fig. 8 SEM back-scattered electron images of trace minerals in sample 4673: a apatite distributed in a kaolinite matrix; b ilmenite in kaolinite; c crandallite and pyrite in kaolinite; d florencite in kaolinite; e euhedral pyrite in kaolinite; f galena. Key: Kln, kaolinite; Ap, apatite; Ilm, ilmenite; Crd, crandallite; Flr, florencite; Py, pyrite; Gn, galena

Fig. 9 SEM back-scattered electron images of siderite and goethite in sample 4673: a well crystallized siderite; b and c siderite subjected to corrosion; d goethite with a bell-emulsion shape. Key: Sd, siderite; Kln, kaolinite; Gt, goethite

As indicated by XRD analysis, kaolinite is the main constituent of tonsteins present in this study. In addition to occurring as matrix for other trace minerals, it occurs in vermicular form (Figs. 5a and 7a), indicating an authigenic origin. Vermicular kaolinite is usually considered as evidence of volcanic ash deposition of the partings with coal seams (Dai et al., Reference Dai, Ward, Graham, French, Hower, Zhao and Wang2017c), mainly because vermicular kaolinite in the partings is indicative of in situ alteration of volcanic minerals, as reported by Triplehorn and Bohor (1981), Ward (Reference Ward2002 Reference Ward2016), and Dai et al. (Reference Dai, Ward, Graham, French, Hower, Zhao and Wang2017c).

The volcanogenic minerals identified in altered volcanic ash layers present in this study that survived post-depositional alteration include mainly sanidine, zircon, apatite, and rutile (or anatase). Sanidine occurs mainly as anhedral, elongate shards distributed in the kaolinite matrix (Fig. 5b,c). Sanidines shown in Fig. 5b,c have well defined shard-like morphologies strongly suggestive of original subhedral to euhedral sanidine micro crystals being torn apart during an explosive volcanic eruption. Some sanidines are in the form of embayed, tabular euhedral crystals (Fig. 5d). The presence of sanidine supports strongly the volcanic-ash origin of the partings. Apatite occurs typically as individual shard-like grains within a kaolinite matrix (Figs. 5e,f and 8a) and the apatite in a few cases is resorbed, all suggesting a volcanic origin (Fig. 5f). The delicate and elongate apatite grains may have fractured as a result of the volcanic explosion or compaction after deposition (Figs. 5c–e and 8a) (Dai et al., Reference Dai, Ward, Graham, French, Hower, Zhao and Wang2017c; Spears, Reference Spears2012).

In many cases, zircon grains observed in samples that were subjected to SEM–EDS analysis do not show well crystallized shapes, but euhedral crystals are observed (Fig. 6a,b,c). The embayments seen in Fig. 6b,c are evidence of magmatic corrosion. The zircon grains shown in Fig. 6d,f are clearly fragments of zircon crystals from an explosive volcanic primary origin while the grain shown in Fig. 6e is a euhedral prismatic crystal with well developed terminations. The fractures shown in Fig. 6d,f are probably the result of this explosive volcanic activity which would have shattered original primary subhedral to euhedral zircon micro-crystals. A single tonstein horizon was suggested by Dopita and Kralik (1977) to have an homogeneous population of euhedral zircons with a limited range of shapes. The various shapes of zircon are probably due to modification resulting from the volcanic eruption and transport in the ash cloud. Larger and rounded zircon grains appear to be older than the normal zircon population (Guerra-Sommer et al., Reference Guerra-Sommer, Cazzulo-Klepzig, Santos, Hartmann, Ketzer and Formoso2008), suggesting that the xenocrysts represent partially resorbed older zircons in the magma (Spears, Reference Spears2012). In addition, some zircon grains contain inclusions (Fig. 6a,b).

Various TiO₂ forms are shown in Fig. 7. Ti oxides in Fig. 7a,b are clearly of authigenic origin, in places (Fig. 7a) clearly intergrown with kaolinite along its cleavage planes. Ti oxides shown in Fig. 7c,d are probably originally magmatic in origin (from the same source as sanidine and zircon grains) and similarly deposited from an explosive volcanic eruption. They would have been primary magmatic Fe-Ti oxides, probably titaniferous magnetite with a well-developed trellis or lattice structure (probably originally magmatic exsolution of Ti oxide in magnetite) and during diagenesis, the Fe was leached out selectively and the Ti oxides have remained behind and now may be the phase ‘leucoxene’.

Ilmenite, observed as a partially corroded grain in kaolinite (Fig. 8b), is a resistate volcanogenic mineral and has also been found in a few tonsteins (Bohor & Triplehorn, 1993; Dai et al., Reference Dai, Ward, Graham, French, Hower, Zhao and Wang2017c; Lyons et al., Reference Lyons, Spears, Outerbridge, Congdon and Evans1994). As with apatite, goyazite-gorceixite-crandallite-florencite minerals are often indicative of volcanic input (Brownfield et al., Reference Brownfield, Affolter, Cathcart, Johnson, Brownfield and Rice2005; Dai et al., Reference Dai, Ward, Graham, French, Hower, Zhao and Wang2017c; Rao & Walsh, Reference Rao and Walsh1997; Wilson et al., Reference Wilson, Sergeant, Young and Harrison1966; Zhao et al., Reference Zhao, Ward, French and Graham2012;) and have been found in a number of tonsteins in coals (e.g. Bohor & Triplehorn, 1993; Kokowska-Pawłowska & Nowak, Reference Kokowska-Pawłowska and Nowak2013; Rao & Walsh, Reference Rao and Walsh1997). Crandallite and florencite were also identified in tonstein samples present in this study. They occur as individual (Fig. 8c) or as elongated grains (Fig. 8d) in kaolinite.

Sulfide minerals such as pyrite (Fig. 8c) and galena (Fig. 8f) have been observed in the samples present in this study and indicate an epithermal solution input. The pyrite seen in Fig. 8e is of syngenetic or authigenic origin. Siderite has been found in tonsteins, but if present, it is probably of secondary origin (Dai et al., Reference Dai, Ward, Graham, French, Hower, Zhao and Wang2017c). Siderite in sample 4763 distributed in kaolinite was subjected to alteration (Fig. 9). Goethite in sample 4673 (Fig. 9d) with a bell-emulsion shape shows an authigenic origin, possibly as a pseudomorph after siderite because of the rhombohedral shape of the original siderite.

Geochemistry

The chemical analyses of the tonstein, floor rock, and coal are shown in Tables 3, 4, 5, 6, 7, and 8. Both the CAER and CUMT major oxides results are given. The CAER major oxides data were used in the plotting of the figures.

Table 3 a Sample; as-determined moisture, ash, and total S; ash-basis major oxides determined at CAER. b Sample, ash-basis (%) major oxides determined at CUMT

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Tonsteins in regular type, floor rocks in blue italics, and coal in bold blue italics

Table 4 Sample; ash basis (µg/g) minor elements from Li through Nb; World coal average (ash basis), 2 × World coal average, and 5 × World coal average at bottom of table (after Ketris & Yudovich, Reference Ketris and Yudovich2009).

KGS Kentucky Geological Survey

Tonsteins in regular type, floor rocks in blue italics, and coal in bold blue italics

Table 5 Sample; ash basis (µg/g) minor elements from Mo through U; World coal average (ash basis), 2 × World coal average, and 5 × World coal average at bottom of table (after Ketris & Yudovich, Reference Ketris and Yudovich2009).

KGS Kentucky Geological Survey

Tonsteins in regular type, floor rocks in blue italics, and coal in bold blue italics

Table 6 Sample; ash basis (µg/g) REE plus Sc and Y; World coal average (ash basis), 2 × World coal average, and 5 × World coal average at bottom of table (after Ketris & Yudovich, Reference Ketris and Yudovich2009)

KGS Kentucky Geological Survey

Tonsteins in regular type, floor rocks in blue italics, and coal in bold blue italics

Table 7 Sample; UCC-normalized REE values (after Taylor & McLennan, Reference Taylor and McLennan1985)

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Tonsteins in regular type, floor rocks in blue italics, and coal in bold blue italics

Table 8 Sample; Light (L) vs. Heavy (H) distribution, EuN/EuN*, CeN/CeN*, GdN/GdN*, total REE, REE + Y (REY), REY + Sc (REYSc, Light REE/Heavy REE (LREE/HREE)

KGS Kentucky Geological Survey

Tonsteins in regular type, floor rocks in blue italics, and coal in bold blue italics

Major Oxides

The major-element oxides in all samples are dominated by SiO₂ and Al₂O₃ (Table 3) with traces of TiO₂, Fe₂O₃, MgO, CaO, MnO, Na₂O, K₂O, and P₂O₅. Among the tonsteins, the amounts of each major element oxide in the two data sets based on CUMT and CAER analyses (Table 3) are very close to each other. The data for major-element oxides are also consistent with the mineral components in the samples as presented in Table 2. Samples with large illite contents also have large K₂O contents.

Rare Earth Elements

In the present study, REE were used to describe the lanthanide elements, REY for REE + Y, and REYSc for REY + Sc with light REE (LREE) defined as La through Sm and the heavy REE (HREE) defined as Eu through Lu (Hower et al., Reference Hower, Ruppert and Eble1999; Seredin, Reference Seredin1996). Following the normalization of REE abundances to the Upper Continental Crust (UCC) averages (indicated by the subscript suffix ‘N’) (after Taylor & McLennan, Reference Taylor and McLennan1985), L-type (light type: La_N/Lu_N > 1), M-type (medium type: La_N/Sm_N < 1, Gd_N/Lu_N > 1), and H-type (heavy type: La_N/Lu_N < 1) enrichment patterns are delineated (Seredin & Dai, Reference Seredin and Dai2012). Building upon the UCC corrections (Taylor & McLennan, Reference Taylor and McLennan1985), Ce, Eu, and Gd are decoupled from the other REE in the distribution patterns (Bau & Dulski, Reference Bau and Dulski1996; Dai et al., Reference Dai, Graham and Ward2016b, Reference Dai, Xie, Jia, Ward, Hower, Yan and French2017a, Reference Dai, Xie, Ward, Yan, Guo, French and Grahamb):

(1) ${\mathrm{Eu}}_N/{\mathrm{Eu}}_N\ast ={\mathrm{Eu}}_N/\left({0.67\mathrm{Sm}}_N,+,{0.33\mathrm{Tb}}_N\right)$

(2) ${\mathrm{Ce}}_N/{\mathrm{Ce}}_N\ast ={\mathrm{Ce}}_N/\left({0.5\mathrm{La}}_N,+,{0.5\mathrm{Pr}}_N\right)$

(3) ${\mathrm{Gd}}_N/{\mathrm{Gd}}_N\ast ={\mathrm{Gd}}_N/\left({0.33\mathrm{Sm}}_N,+,{0.67\mathrm{Tb}}_N\right)$

In addition to the tuffaceous, detrital, infiltrational/leaching, and hydrothermal modes of REE enrichment in coals (Seredin & Dai, Reference Seredin and Dai2012), peat and low-rank coals would have had organic associations, with HREE having a greater affinity for organics than the LREE (Aide & Aide, Reference Aide and Aide2012; Davranche et al., Reference Davranche, Grybos, Gruau, Pédrot, Dia and Marsac2011; Eskenazy et al., Reference Eskenazy, Mincheva and Rousseva1986; Eskenazy, Reference Eskenazy1978, Reference Eskenazy1987a, Reference Eskenazyb, Reference Eskenazyc, Reference Eskenazy1999, Reference Eskenazy2015; Pédrot et al., Reference Pédrot, Dia and Davranche2010). As the chelating functional groups diminish in importance in bituminous coals (Given, 1984; Hatcher & Clifford, Reference Hatcher and Clifford1996), organic associations are lost from the coal with low LREE/HREE being a ghost signature of the original organic association (Hower et al., Reference Hower, Eble, Backus, Xie, Liu, Fu and Hood2020). The formerly organic-bound REE could be leached from the coals, bound to clays (Eskenazy, Reference Eskenazy1995, Reference Eskenazy1999; Seredin, Reference Seredin1996), or incorporated in secondary carbonates and phosphates. Leaching of LREE from intra-seam and bordering rocks (Dai et al., Reference Dai, Ren, Chou, Li and Jiang2006, Reference Dai, Li, Chou, Zhao, Zhang, Ren, Ma and Sun2008) and hydrothermal mobilization of HREE-depleted phosphates and HREE- and Y-enriched organic compounds (Dai et al., Reference Dai, Zhang, Seredin, Ward, Hower, Wang, Li, Song, Zhao, Kang, Zheng and Zhou2013a, Reference Dai, Zhang, Ward, Seredin, Hower, Li, Song, Wang, Kang, Zheng, Wang and Zhoub, Reference Dai, Chekryzhov, Seredin, Nechaev, Graham, Hower, Ward, Ren and Wang2016a) contribute to the redistribution of light and heavy REEs. According to Williams-Jones et al. (Reference Williams-Jones, Migdisov and Samson2012), hydrothermal-influenced deposits tend to be enriched in LREE.

The UCC-normalized REE for all of the samples are shown in Figs. 10 and 11. As noted above, several of the highlighted samples are complicated, being weathered tonsteins (sample 4673 with a distinct negative Eu anomaly; samples 4962 and 4969 do not have a distinct Ce anomaly), floor rock (sample 4703 without a distinct Ce anomaly; sample 4880 with a distinct Ce anomaly), gradational between the tonstein and the floor rock (4911 without a distinct Ce anomaly), or a coal with kaolinite inclusions (41010 without a distinct Ce anomaly). All of the latter samples comprise the highest normalized Lu values. Basically, all of the other samples have similar spider plots, varying in their normalized REE but not in the distribution patterns. Most of the samples have L-type distributions (the exceptions being 4703, 4823, 4962, and 41010) and negative Eu anomalies (the exceptions being 4703 and 4962; Figs. 10 and 11).

Fig. 10 Upper continental crust-normalized REE distributions for all of the studied samples. Normalization after Taylor & McLennan (Reference Taylor and McLennan1985)

Fig. 11 Upper continental crust-normalized REE distributions for the samples with the highest normalized Lu. Normalization after Taylor & McLennan (Reference Taylor and McLennan1985)

While the Ce _N /Ce _N ^* vs. Eu _N /Eu _N ^* and Gd _N /Gd _N * vs. Eu _N /Eu _N ^* trends show flat, non-significant distributions for the tonstein samples, the Gd _N /Gd _N * vs. Ce _N /Ce _N ^* (Fig. 12) plot shows a positive correlation (r² = 0.59 with the inclusion of sample 4746 (a weathered tonstein; significant at the 1% level); r² = 0.36 (below the r² = 0.37 point for being significant at the 5% level). Removing the weathered and gradational tonsteins from consideration yields an r² = 0.46 which is significant at the 5% level. Sample 41010, the coal with tonstein-like inclusions, and the coal samples from the 4754–4765 series (data from Hower et al., Reference Hower, Ruppert and Eble1999) exhibit a non-significant distribution to the low-Gd _N /Gd _N * side of the tonstein distribution indicating that the tonstein has a more pronounced M-type-distribution tendency than the coals.

Fig. 12 Gd _N /Gd _N * vs. Ce _N /Ce _N ^* distribution showing the locations of coal data from Hower et al. (Reference Hower, Ruppert and Eble1999)

Principal components analysis (PCA) (JMP® Pro 17.0.0, ^© JMP Statistical Discovery LLC, Cary, North Carolina, USA) for all of the samples shows that the first Eigenvector, contributing to 42.71% of the variation, comprises Al₂O₃/TiO₂, LREE/HREE, and Gd _N /Gd _N * with a less significant contribution by Zr + Nb (Fig. 13a; supporting statistics in Table 9). The second Eigenvector, contributing to 25.80% of the variation, is composed of Al₂O₃/TiO₂ and REE (ppm; ash basis) with a less significant contribution of Zr + Nb. The most significant outliers are 4703, 4746, 4806, 4969, and 4880, the weathered tonsteins and floor samples noted above (Fig. 13b). Without the floor and coal samples, the first Eigenvector, contributing to 42.96% of the variation, is comprised of Zr + Nb, LREE/HREE, and Gd _N /Gd _N * (Fig. 14a; supporting statistics in Table 10). The second Eigenvector, contributing to 34.02% of the variation, consists of Al₂O₃/TiO₂, Zr + Nb, and REE. The outliers are the weathered tonsteins (Fig. 14b).

Fig. 13 a Principal components analysis (PCA) distribution for selected variables for all of the samples. Note that the vectors for LREE/HREE and Gd _N /Gd _N * overlap at this scale. b Outlier distribution for all samples

Table 9 Principal components analysis parameters for the analysis of all samples

Fig. 14 a Principal components analysis (PCA) distribution for selected variables for the tonstein samples. Note that the vectors for LREE/HREE and Gd _N /Gd _N * overlap at this scale. b Outlier distribution for all samples

Table 10 Principal components analysis parameters for analysis of the tonstein

In previous studies, the relationship between Al₂O₃/TiO₂ and REE (Dai et al., Reference Dai, Nechaev, Chekryzhov, Zhao, Vysotskiy, Graham, Ward, Ignatiev, Velivetskaya, Zhao, French and Hower2018) was used to discern a relationship, e.g. the REE distribution and the detrital minerals (represented, in part, by TiO₂). The Al₂O₃/TiO₂ vs. REE relationships is shown in Fig. 15 but it does not show a significant distribution among the tonstein samples. Samples 4703 and 4969, distinct outliers, are weathered tonsteins.

Fig. 15 REE vs. Zr + Nb (ppm; ash basis) for all samples

Other Elements

Zircons can contribute REE and Y to a coal as a detrital or tuffaceous mineral. Zircons were noted in the Fire Clay tonstein by Hower et al. (Reference Hower, Andrews, Wild, Eble, Dulong and Salter1994) and in the present study. The relationship between Zr + Nb (ppm; ash basis) and Al₂O₃/TiO₂ (after Dai et al., Reference Dai, Nechaev, Chekryzhov, Zhao, Vysotskiy, Graham, Ward, Ignatiev, Velivetskaya, Zhao, French and Hower2018), with the TiO₂ being an indicator of detrital contributions and a trace mineral in tuffaceous deposition (Hower et al., Reference Hower, Andrews, Wild, Eble, Dulong and Salter1994), is shown in Fig. 16. Within the tonstein population, excluding the weathered samples (4703, 4962, and 4969), there is a clustering of the points but there is no significant relationship, with the Zr + Nb showing a greater relative variation than the Al₂O₃/TiO₂. Zircons can contain trace amounts of Y but the relationship between Zr and Y (Fig. 17) is not significant for these samples.

Fig. 16 Al₂O₃/TiO₂ vs. Zr + Nb (ppm; ash basis) for all samples

Fig. 17 Zr vs. Y (both ppm; ash basis) for all samples

Compared with the world hard coal average (after Ketris & Yudovich, Reference Ketris and Yudovich2009) (admittedly, not necessarily the best baseline for the tonstein and floor samples, but they are within or adjacent to coal), few elements exceed even the 2 × -world coal average level. Lithium reaches its highest concentrations at several Hazard South and Vicco quadrangle locations. Aside from Ag, with only one tonstein sample (weathered sample 4962) not reaching the 2 × level, only Zr and Ta exceed the 2 × level in five of the 27 tonstein samples. The greatest Ta concentrations correspond with some of the highest Nb samples (Fig. 18), with samples 4647, 4806, and 4895 all being from Vicco quadrangle sites. The largest Zr value corresponds with the largest Y value (weathered sample 4969; Fig. 17). The same sample also has the highest REE concentrations (except for Eu) and the largest Th and U contents of any of the tonstein samples. The high Zr, Y, REE, and Th + U in sample 4969 and, to a lesser extent, in sample 4962, might be a function of an enhanced concentration of zircons and REE-bearing minerals, such as monazite, in the weathered tonstein.

Fig. 18 Nb vs. Ta (both ppm; ash basis) for tonstein, weathered tonstein, floor, and coal

Discussion

The tonstein rock type, after Winchester and Floyd (Reference Winchester and Floyd1977) and later utilization of the defining geochemical parameters, is largely trachyandesite to trachyte (Fig. 19) and has a relatively constant Zr/TiO₂ value but Nb/Y values which change significantly, especially as there is little change in the designated fields, suggesting that tonsteins present in this study have all come from a very similar source.

Fig. 19 Rock classification based on Nb/Y vs. Zr/TiO₂ after Winchester and Floyd (Reference Winchester and Floyd1977) and Vergunov et al. (Reference Vergunov, Arbuzov, Soktoev, Ilenok and Chekryzhov2022)

The tonstein mineralogy is generally dominated by kaolinite (59–91% of the unweathered samples) with illite > quartz comprising the remainder of the major minerals. Much of the kaolinite is a secondary mineral after volcanic glass, as evidenced not only by its vermicular texture derived from other original volcanic minerals (e.g. mica; Dai et al., Reference Dai, Ward, Graham, French, Hower, Zhao and Wang2017c) but also by its matrix occurrence for other minerals rather than detrital modes of occurrence (Fig. 5) and in Hower et al. (Reference Hower, Berti, Hochella and Mardon2018), although phenocrysts of kaolinite suggest a primary deposition of those forms. Minor amounts of clinoptilolite, siderite, pyrite, gypsum, chlorite, and anatase were detected and quantified by XRD + Siroquant. Sanidine, apatite, zircon, ilmenite, pyrite, crandallite, florencite, galena, siderite, and goethite were detected by SEM among these samples. Monazite and kaolinite phenocrysts were noted in the electron microscopy study of the correlative Dean coal (Hower et al., Reference Hower, Berti, Hochella and Mardon2018); Hower et al. (Reference Hower, Andrews, Wild, Eble, Dulong and Salter1994) found sanidine (some with alteration to quartz and kaolinite), β-quartz, magnetite, magnetite with an ilmenite core, anatase, rutile, and weathered titanite; and Robl and Bland (1977) noted the presence of kaolinite pseudomorphs after β-quartz and sanidine. Mineralogical evidence for a volcanic-ash deposit includes vermicular kaolinite, sanidine, β-quartz, euhedral zircons, ilmenite, apatite, and phosphates including crandallite-group minerals, monazite. and rutile (or anatase). The possibility that some of the phosphates and TiO₂ minerals could be of secondary origin cannot be discounted. The Fe-Ti oxides shown in Fig. 7c,d give strong evidence for the magmatic origin of at least some of the Fe-Ti oxide/TiO₂ suite.

The tonstein geochemistry is dominated by Al₂O₃ and SiO₂, a function of the dominance of kaolinite. The total REE content, exceeding 400 µg/g in one case (sample 4997) and < 100 µg/g at many of the sites, is, perhaps, deceptively small as the REE in the coal is often reported on an ash basis (e.g. Hower et al., Reference Hower, Eble, Dai and Belkin2016, Reference Hower, Eble, Backus, Xie, Liu, Fu and Hood2020). The tonstein, with REE and REY-bearing minerals such as apatite, crandallite, monazite, florencite, and zircon, was, indeed, a primary source of the REE in the surrounding coal. That influence may have been through direct deposition of REY-bearing minerals, as in coal sample 41,010, or via remobilization of the REY in the tonstein by leaching.

Among the tonsteins, some of the highest REY concentrations occur in tonsteins that were possibly gradational with non-tonstein lithologies (samples 4673 and 4911; noted in both the megascopic appearance and in the <40% kaolinite content) and in weathered outcrops (samples 4962 and 4969). The reasons for this could not be explored further in this study because crushed whole-rock samples were being analyzed. The character of these samples is seen in the upppermost UCC spidergram plots (Figs. 10 and 11), with the gradational and weathered tonsteins, the floor rock, and the coal sample having the greatest and, in some cases, the most significant, negative Eu anomalies. In the PCA analyses (Figs. 13b and 14b), the weathered tonsteins are among the outliers. In the assessment of REE vs. Zr + Nb (Fig. 15), Al₂O₃/TiO₂ vs. Zr + Nb (Fig. 16), and Zr vs. Y (Fig. 17), weathered sample 4969 stands out as being distinctly different from the other tonstein samples. Both weathered samples are on the low-Ta and, to a lesser extent, the low-Nb end of the Nb vs. Ta plot (Fig. 18). Transitional sample 4673 is at the low-Nb/Y and high-Zr-TiO₂ end of the Winchester and Floyd (Reference Winchester and Floyd1977) rock classification (Fig. 19).

What are the implications of the outliers? For the transitional tonsteins, the rock type was gradational between the tonstein, as represented by the apparently non-gradational samples with 59–92% kaolinite, and a relatively illite-rich lithology. In the deposition of the partings, the passage from an illitic clay to the kaolinite-rich tonstein can be seen (Fig. 2a,b); the boundary between the two is transitional, as seen by the dark streaks in the lower quarter of the tonstein (Fig. 2b). The illitic clay, presumed to have been a terrigenous depositional event, is an influence on the REE chemistry of the coal underlying the tonstein, with the coal below the illitic clay having a lower REE content than coal directly underlying the ash-fall tonstein (Fig. 2c) (Hower et al., Reference Hower, Ruppert and Eble1999, Reference Hower, Eble, Backus, Xie, Liu, Fu and Hood2020). Weathering of the tonstein preferentially removes kaolinite from the rock, leaving behind REE-bearing phosphates (apatite, crandallite, florencite, monazite) and Y-bearing zircon, the latter causing a relative enrichment in the REY.