A common belief suggests that coal transforms into diamonds under immense pressure. This notion is a misconception. While both coal and diamonds are carbon-based, their formation processes are vastly different, occurring under distinct geological conditions that prevent coal from naturally becoming a diamond. This article explores the science behind the formation of these two carbon materials.
The Diamond Formation Process
Natural diamonds originate deep within the Earth’s mantle, at depths of 150 to 200 kilometers (about 90 to 120 miles) below the surface. These extreme environments are characterized by immense pressures, 45 to 60 kilobars (650,000 to 850,000 pounds per square inch), and high temperatures, between 900 and 1300 degrees Celsius (1650 to 2370 degrees Fahrenheit). Under these conditions, carbon atoms bond together in a rigid, crystalline lattice structure, forming diamonds.
The carbon source for natural diamonds is not coal, but carbon-bearing fluids or ancient organic matter subducted deep into the Earth. Once formed, these diamonds are brought closer to the Earth’s surface through rare, deep-source volcanic eruptions. These eruptions create distinctive geological structures known as kimberlite and lamproite pipes, which act as conduits for the rapid ascent of diamonds from the mantle. Most commercial diamond deposits are found within these vertical, igneous rock formations.
The Formation of Coal
Coal is a sedimentary rock that forms from the accumulation and burial of plant matter over millions of years. This process, called coalification, occurs in ancient swamp environments where dead vegetation is submerged, preventing decay. Over time, layers of sediment cover the plant material, subjecting it to increasing pressure and moderately elevated temperatures.
As burial depth increases, the plant material undergoes a series of transformations, progressing through stages such as peat, lignite, sub-bituminous coal, bituminous coal, and finally, anthracite. Each stage involves the removal of water and volatile compounds, leading to a higher concentration of carbon. Coal forms in shallow sedimentary basins under significantly lower pressure and temperature conditions.
Why Coal Doesn’t Become Diamond
The primary reason coal does not transform into diamonds lies in the vastly different geological conditions required for their formation. Diamonds necessitate extreme pressures and high temperatures found deep within the Earth’s mantle, while coal forms under much lower pressures and temperatures in shallow sedimentary basins.
The carbon source also differs; diamonds form from deep mantle carbon or subducted organic material, whereas coal originates from terrestrial plant matter. Coal contains various impurities, including hydrogen, oxygen, sulfur, and other elements, which are not conducive to forming a pure carbon crystalline structure like diamond. Despite sharing carbon as a common element, coal lacks the necessary geological environment and chemical purity to naturally transform into diamonds.