What type of sedimentary rock is lignite coal?

Lignite coal is actually an early stage of coal formation and is classified as the precursor to bituminous coal. It is predominantly derived from decomposed plant material, which undergoes a transformation through heat and pressure over time, typically in swampy areas. During its formation process, lignite represents a point in the sequence of coal progression from peat to higher-grade coals.

Describing lignite as peat is correct because peat itself is an accumulation of decayed vegetation and organic matter, and lignite can be considered as a more compacted and altered form of peat. This makes it a low-grade coal that retains many of the characteristics associated with the organic material from which it derives. As the organic material becomes increasingly compressed and undergoes changes due to heat and pressure, it progresses through various stages to ultimately form anthracite coal, the highest grade of coal.

The other types of rocks—metamorphic and igneous—are formed through completely different geological processes. Metamorphic rocks arise from the alteration of existing rocks under heat and pressure without the rock melting, while igneous rocks form from the cooling and solidification of magma or lava. Bituminous coal, although another stage of coal formation, is not a precursor to lignite