Lignite

Lignite, commonly called brown coal, is the lowest rank of coal, representing an early stage in the transformation of plant material into harder, more carbon-rich coals. It is brown to brownish-black, relatively soft, and often shows visible woody plant textures.

With a high moisture content (often 30 to 60 percent) and lower carbon content than higher-rank coals, lignite has a comparatively low energy value. It tends to crumble on drying and can spontaneously combust if stockpiled improperly.

Despite its low rank, lignite is abundant and widely used for power generation near where it is mined.

Lignite forms from peat, the accumulated remains of plants in waterlogged, oxygen-poor swamps and bogs. As peat is buried beneath later sediments, increasing pressure and modest heat drive off water and volatile compounds, gradually concentrating carbon in a process called coalification.

Lignite represents the first true coal stage, having advanced beyond peat but not yet reaching sub-bituminous or bituminous rank, which require deeper burial and higher temperatures. Major deposits, often in relatively young (Cretaceous to Tertiary) basins, occur in Germany, Australia, the United States (North Dakota, Texas), and Eastern Europe.

Lignite is identified by its brown to brownish-black color, low weight, and softness; it can be scratched with a fingernail and often leaves a brown streak and dirties the hands. It frequently preserves recognizable woody or layered plant textures.

On drying it cracks and crumbles. Compared with bituminous coal it is duller, softer, browner and less dense; compared with peat it is more compact and less fibrous. It burns readily with a smoky flame. Its brown streak, woody texture and low density distinguish it from harder black coals.

Lignite is used mainly as a fuel for electricity generation in power plants located close to the mines, since its high moisture and low energy density make long-distance transport uneconomical. It is also processed into briquettes and used as a source of synthetic gas and chemicals.

Lignite is a source of humic acids and montan wax, and is used as a soil conditioner in agriculture and horticulture. It has no jewelry use, though the related, more compact material jet (a form of lignite) is carved into ornaments and mourning jewelry.