Journal of Geochemical Exploration
Geochemistry of Rare Earth Elements in Lower
Gondwana Coals of the Talchir Coal Basin, India
Vivek Mishra a,b,⁎ , Sanchita Chakravarty b , Robert B. Finkelman c , Atul Kumar
Varma a
a Coal Geology and Organic Petrology Laboratory, Department of Applied Geology,
Indian Institute of Technology (ISM), Dhanbad 826004, India
b Coal Characterization Laboratory, CSIR – National Metallurgical Laboratory,
Jamshedpur 831007, India
c Geosciences Department, University of Texas at Dallas, Richardson, TX 75080,
USA
ABSTRACT
• This study examined the concentration, distribution, and modes of rare earth elements in coal from the Talchir coal
basin in Odisha State. The coals are medium to high volatile, high ash, and low sulphur. The average value of rare
earth elements and yttrium (REY) in 34 coal samples ranged from 29.6 ppm to 179.4 ppm, with an average of 91.0
ppm. The Barakar Formation had a lower total REY value (53.6 ppm) than the Karharbari Formation (127.4 ppm). The
Karharbari Formation had a higher ratio of light to heavy rare earth elements (LREE/HREE) than the Barakar
Formation (4.4). Coals from both Formations showed negative europium anomalies, while only coals from the
Karharbari Formation showed a positive cerium anomaly. EPMA analysis revealed REY-bearing phosphates in clay
minerals, while monazites were found in the Mg aluminosilicate matrix. Ag, Ba, and Zr were found as replacements or
substitutions in the REY phosphates.
1. Introduction
• The 17 lanthanides, or rare earth elements (REEs), are valued for their unique magnetic, phosphorescent, and catalytic
properties in various products and processes, including high-strength magnets, car batteries, catalysts, defense and
consumer electronics. The major controls on the geological and cosmochemical behavior of REEs are their size,
coordination number, redox potential, volatility, and ionic behavior. Geochemists divide REEs into light rare earth (LREE)
and heavy rare earth (HREE), with Y being commonly grouped with HREE and referred to as REY.
• In recent years, almost 90% of the world's supply of REE comes from China, leading countries to seek secure sources of
these valuable and strategically important elements. One potential source is coal and the products of coal combustion.
Numerous publications from the U.S., China, and Russia have investigated the concentration, distribution, mode of
occurrence, and origin of REEs in coal.
• Finkelman et al. (2018) concluded that 70% of the LREE in bituminous coals were in phosphates, 20% in clays, and
20% in other forms such as organic association and carbonates. About 50% of the HREE occurred in phosphates, 20%
in clays, and 30% in organic association and other forms. In contrast, in low rank coals, 60% of the LREE were in clays,
only 20% in phosphates, and 20% in other forms, and the HREE were primarily in clays (50%), with 25% both in
phosphates and other forms.
• Vinokurov et al. (2002) presented a comparative study of metalbearing coal and their source rocks in the Zyryanka coal
basin of Russia to elucidate the anomalous concentration, mode of formation, and distribution of the REEs. They
observed a significant positive relationship between the REEs and ash yield and concluded that negative Eu anomalies
in coals are typical for source rocks of acidic composition.
• However, there are no publications describing the concentrations, distributions, and modes of occurrence of REEs in
Indian coal, despite India having a huge coal deposits and being the world's second-largest producer of coal. This
paper aims to determine the concentration, distribution, and modes of occurrences of REEs in the Lower Gondwana
coal of the Talchir Coal Basin, India. A set of 34 coal samples from one borehole were used for various analytical tests,
including proximate, ultimate, and ash analyses, X-ray diffraction (XRD), microprobe analysis (EPMA), and inductively
coupled plasma mass spectrometry (ICP-MS).
2. Geological settings
• The Talchir coal basin, located in Odisha, spans 1810 km2 and is situated north of the Mahanadi River and occupies the
valley of the Brahmani River, which drains directly into the Bay of Bengal. It is a detached elliptical basin surrounded by
Precambrian age metamorphic rocks and is the southern member of the Lower Gondwana basin within the Mahanadi
valley graben. The basin is well-known for its coal mines, which have operated since 1875. The Gondwana sediments in
the Talchir coal basin are represented by the succession of Talchir, Karharbari, Barakar, Barren measures, and the
Kamthi Formation.
• The Karharbari and Barakar Formations are the major coal-bearing formations in the Talchir coal basin. The Karharbari
Formation is composed of sandstones with different textures and structures, grey and carbonaceous shales, and one
coal seam. The presence of tabular and trough crossbeddings in large quantities suggests that the likely origin of the
Karharbari Formation is from braided streams.
• The Barakar Formation is underlain by the Karharbari Formation and lies between a conglomerate horizon (Boulder
Gravel Unit). The Barakar Formation is composed of both matrix and clast-supported oligomictic conglomerate and
boulder gravel, with a thickness of 300 m. The seams are cut by numerous faults along which they are disturbed and
dislocated.
3. Sampling, distribution and analytical
methods
• The Talchir coal basin was analyzed using coal and shaly coal samples from boreholes in the Barakar Formation
and Karharbari Formation. Thirty-four drill core composites were prepared from the coal and shaly coal bands
using the standard method in India, as described by Mishra et al. (2016). The samples were air-dried, milled, and
split, and stored in a sealed container to prevent contamination and weathering.
• The 34 samples were used for evaluating the physical and chemical properties of the Talchir coal.
Thermogravimetric Analyser (TGA-1000) was used to analyze the technological properties, while an elemental
analyzer and sulphur analyzer were used to determine the mineralogical phases. X-ray diffraction (XRD) was used
to determine the mineralogical phases of the raw coal
• Approximately 2 g of coal sample was mixed with epoxy resin to prepare pellets for analysis by an electron
probe micronalyser. X-ray fluorescence spectrometry was performed on the 200 mesh size samples to
determine the ash composition. Inductively coupled plasma mass spectrometry (ICP-MS) was used to
determine the concentration of rare earth elements after microwave digestion in the Analytical Chemistry
Division of CSIR-National Metallurgical Laboratory, Jamshedpur. The digestion was performed using a
Microwave Digestion System (Milestone) with ten vessels on 0.1 g samples. The internal temperature and
pressure were monitored using a vessel equipped with a sensor unit. The microwave programme consisted of
two steps:
• First step: Power 1000 W, temperature 100 °C–150 °C
Holding time: 2 min, 5 min, and 15 min.
• Second step: Power 1200 W, temperature 150 °C–220 °C
Holding time: 5 min and 20 min
4. Results and discussion
• 4.1. Elemental properties
Table 1 presents the result of the proximate and ultimate analysis with gross
calorific values (GCV) of the studied samples. The volatile matter of all 34 coal
samples ranged from 39 wt% to 52 wt% on a dry ash free basis, moisture ranged
from 2.42 wt% to 6.09 wt% on an air dry basis and the ash yield ranged from 29.2
wt% to 51.7 wt% on a dry basis. The samples are characterized by high carbon
value i.e. from 65.7 wt% to 78.7 wt% and sulphur contents from 0.84 wt% to 1.28
wt %. Calorific values range from 3121 to 5154 cal/g which affirm that the rank of
studied coal samples varies from lignite to sub-bituminous type (ASTM D388–12).
• 4.2. Chemistry and mineralogy
• Table 2 presents the major element analysis results, revealing a predominant presence of alumino-silicates, mainly
clay minerals and quartz. SiO2 and Al2O3 constitute over 80 wt% of the ashes, while Fe2O3 is typically 1.94-8.07
wt% and K2O is around 1-2 wt%. SO3 retention in the ashes varies from 0.27 to 1.35 wt%, likely from pyrite,
sulphates, and organic sulphur. SiO2 and Al2O3 vary between 59.6 and 73.3 wt%, while P2O5 and CaO are between
0.06 and 1.29 wt%. Silica and alumina are negatively correlated, suggesting that most silica is present as quartz
instead of clays or other aluminosilicates. Calcium and phosphate are positively correlated, possibly indicating the
presence of apatite in the coal. XRD results show quartz and clay minerals like kaolinite, illite, and montmorillonite,
as well as pyrite and goethite.
4.3. REE parameters and distribution
• The (REY) content in 34 coal samples varies significantly with depth, with an average value of 91.0 ppm. This is slightly
higher than the average total REY content in US coals and worldwide bituminous coal and anthracite, but lower than that in
the average total of Chinese coals. The average REY of the Barakar Formation is considerably lower than the average value
of the Karharbari Formation, which has a higher concentration of LREE, specifically La and Ce. Samples B2 and K33 from
the Barakar Formation and Sample K34 from the Karharbari Formation show LREE-enriched type patterns, suggesting the
source region may be of felsic composition.
• The average value of La/Yb (La/Yb)N of all 34 coal samples is 1.4, indicating relative enrichment of LREE compared to
Upper Continental Crust (UCC). The average value of La/Yb in the Karharbari Formation is twice that of the Barakar
Formation. Vertical variation of LaN/YbN shows an increment of LREE in shallow depth and at a depth of about 613 m.
• The negative Eu anomaly (EuN/EuN*) ranges from 0.02 to 1.6, indicating that the source region is of felsic composition
with a depleted Eu anomaly. Eu mobility during coalformation is another possible explanation for negative anomalies.
Strongly reducing environments with low temperatures favor the mobility of Eu during coal formation. However, Sample
K34 from the Karharbari Formation has a EuN/EuN* value > 1, implying possible occurrences of hydrothermal fluids
having a temperature > 200 °C within the source area or presence of Ca-rich minerals like apatite and plagioclase
feldspar.
• The coal samples also display negative Ce anomalies (CeN/CeN*) values ranging from 0.09 to 1.5 with an
average value of 0.30. However, the Karharbari Formation does not show any Ce anomalies with an average
(avg. CeN/CeN* value of 1.1), while the Barkar Formation shows a Ce negative anomaly (avg. CeN/CeN*
value is 0.16).
• The depositional conditions for the coals of the two Formations are different. During the Barakar Formation,
marine incursion is the probable reason for thiscerium negative anomaly. However, felsic rocks also give
negative Ce anomalies, which contribute to an increment of Ce anomalies. Weathering and oxidation of Ce+3
to Ce+4 and its relative immobility and in-situ precipitation lead to negative anomalies of Ce in coal.
4.4. Modes of occurrences
• EPMA analysis helps determine the association of minerals and the modes of occurrences of elements in coal. REYbearing phosphates are found in clay minerals, while monazites are found in the Mg aluminosilicate matrix. The
positive correlation of the rare earth element (REY) with phosphorus is observed through EPMA elemental mapping.
Monazite is also observed in chlorite, suggesting that they are formed due to hydrothermal alteration during
coalification. Ag, Ba, and Zr are found as replacements or substitutions in REY phosphates. Authigenic minerals, such
as clays and phosphates, constitute a small portion of the total rare earth elements present in the samples. REY
minerals undergo phase changes and alterations, and some may be externally modified or redistributed within coal or
incorporated into authigenic other minerals. The total REY content is positively correlated with the total ash yield, but
SiO2 is negatively correlated with ƩREY content. Significant correlations of LREE with Al2O3, CaO, Fe2O3, and P2O5
indicate the derivation of REY phases from terrigenous detrital sources. The positive correlation of HREE with Al2O3
supports a detrital source, while a negative correlation exists between HREE and SO3.
5. Conclusion
• The study analyzed rare earth elements (REE) concentration and modes of occurrence in Indian coals from a
borehole in the Talchir Coal Basin, India. The average REY value is 91 ppm, slightly higher than U.S and worldwide
bituminous coals but lower than Chinese coals. The Barakar Formation coals have a lower average REY value (53.6
ppm) than the Karharbari Formation coals (127 ppm), possibly due to higher ash yield and light REE concentration.
The negative correlation with sulfate and positive correlation with ash yield and lithophile elements suggests a
detrital source for REE. A strong negative correlation with SiO2 suggests quartz was derived from a different source.
Negative Eu anomalies in Barakar coals indicate a felsic source, while most Karharbari coals do not have negative
Ce anomalies. These findings suggest different depositional conditions and possible marine intrusions during
Barakar time.