Lignite, also called brown coal, is a soft, brown, combustible sedimentary rock formed from compacted peat. It has a low carbon content and energy value compared to other types of coal, like bituminous coal or anthracite and is primarily used for electricity generation. In 1934, a lignite deposit was accidentally discovered when a “brown substance” mixed with water gushed out from an artesian well on T. M. Jambulingam Mudaliar’s 620-acre farm in Neyveli, located in the Cauvery basin of Tamil Nadu. Today, the Neyveli mines are Asia’s largest opencast lignite mines, producing around 30 million tonnes annually, and are used primarily for power generation locally.
A new study by researchers from Anna University, Chennai, Kristu Jayanti College, Bangalore, University of Wyoming, USA and China University of Mining and Technology, China, has examined this vast lignite deposit more closely, not only for its energy potential but also to learn about the mine’s distant past. By analysing the organic (carbon-based) and inorganic (mineral-based) parts of lignite, they hope to understand better how the environment looked millions of years ago and how much energy it has today. Their comprehensive study merges chemistry, geology, palaeontology and environmental science to understand the Neyveli lignite deposits.
The researchers began by collecting pieces of lignite from different parts of the Neyveli mine. They selected twenty samples to represent the deposit as a whole, ensuring a variety of layers and characteristics. The samples were carefully wrapped up to avoid contamination and transported to the lab. Here, they were crushed into very fine particles, each less than 212 microns in size. This step is crucial as it standardises the sample material for precise chemical tests.
To measure the energy potential of lignite, the team performed two types of chemical analysis: a proximate analysis, where they measured factors like moisture, ash, and fixed carbon, and an ultimate analysis, which measured the carbon, hydrogen, nitrogen, and sulfur contents. They also measured the gross calorific value (GCV), which tells us how much heat the lignite can produce if burned. They found that moisture varied widely between about 7.5% to 25%, which can affect how easily lignite burns. Ash content remained relatively low on average, indicating that the lignite deposits were not overly mixed with dirt and minerals. Carbon content ranged from around 52% up to 78% in some samples, meaning some layers are pretty rich in carbon.
Next, the researchers used X-ray techniques, like X-ray Fluorescence, or XRF and high-powered microscopes called Field Emission Scanning Electron Microscopes ( FE-SEM) to see what minerals and elements were present besides carbon. They detected minerals containing silica, iron, calcium, magnesium, and trace elements such as zinc, chromium, and strontium. The researchers also tried to figure out how much the original rocks had weathered or broken down using ratios of minerals, like aluminium oxide (Al2O3) to titanium dioxide (TiO2), in the sample. Because some elements correlate strongly with each other, the ratio suggests how certain minerals were preserved or how they leached out due to water moving through the deposits. They concluded that the source rocks mostly had a felsic origin or a mix between felsic and intermediate igneous rocks. Felsic are silicate minerals, magma, and rocks that are rich in lighter elements such as silicon, oxygen, aluminium, sodium, and potassium, while igneous rocks are formed from magma and contain minerals like feldspar.
By combining chemical data with what they saw under the microscope, the team inferred that the Neyveli lignite deposits likely formed in a deltaic environment, like a large river delta. The study suggests that the mines were formed during the late Cretaceous to early Tertiary period, somewhere around 70–50 million years before now.
Lignite is among the earliest stages of coal formation, containing the remains of ancient plants that have only partially decomposed. This makes it an excellent recorder of conditions from millions of years ago. By studying which minerals exist together or by measuring how much carbon is in the lignite, scientists can guess how warm, wet, or oxygen-rich the region might have been in the past.
The study paints a picture from 70 million years ago when rivers carried mud, sand, and plant material to swampy peatlands. Over time, these layers were buried, and the plant matter transformed into today’s lignite deposits of Neyveli. The study also found that the lignite here often contains a lot of water, sometimes up to 25% or more, which reduces its heating value and can make it less economical to transport or burn without further processing.
The study, however, is conducted on just 20 samples from the region. Variability across layers of lignite deposits, the potential for pollutants in the deposits to skew the results and Incomplete fossil records in the lignite mean we only get a partial reconstruction of the paleoenvironment. To get a more accurate picture, more samples may need to be collected and examined.
Nevertheless, their findings highlight that while the lignite deposits of Neyveli can help meet local energy needs, they contain too much water. The relatively high moisture and sulfur levels in some layers hint that specialised methods might be needed for clean and efficient energy use. At the same time, the unique geochemical fingerprint preserved in the lignite also helps the researchers piece together the deep geologic and climatic history of Neyveli.