Diamonds are known for their exceptional hardness and beauty. This often leads to questions about their resilience, particularly concerning extreme conditions like high temperatures. Many wonder if a diamond can actually melt when subjected to intense heat.
The Unique Structure of Diamond
The remarkable properties of diamond stem from its unique atomic arrangement. Each carbon atom within a diamond crystal is strongly bonded to four other carbon atoms, forming a rigid, three-dimensional network known as a giant covalent lattice. This structure is responsible for diamond’s unparalleled hardness and resistance to high temperatures, requiring significant energy to disrupt its bonds.
What Happens When Diamond is Heated?
Diamond does not melt in the conventional sense, where it transitions from a solid to a liquid, under normal atmospheric pressure. Instead, when heated to extremely high temperatures in an oxygen-free environment, diamond undergoes sublimation, transforming directly from a solid into a gas. This sublimation typically occurs at temperatures ranging from approximately 3500°C to 4000°C.
In the presence of oxygen, a different process occurs: diamond will burn. Since diamonds are composed of carbon, they react with oxygen to produce carbon dioxide gas. This combustion can happen at much lower temperatures, generally between 700°C and 900°C. When a diamond burns, it essentially vanishes, leaving behind no ash or residue.
Under specific conditions of high temperature and pressure, and without oxygen, diamond can also transform into graphite. This transformation can occur at temperatures above 400°C to 700°C, with rapid conversion happening around 4227°C. True melting of diamond, into a metallic fluid, has only been observed under extraordinary pressures, such as 10 gigapascals, at temperatures exceeding 4000°C to 5000°C.
Conditions for Diamond’s Transformation
The conditions required for a diamond to undergo these changes are quite specific and extreme. For diamond to burn, oxygen must be present, and temperatures must reach at least 700°C to 900°C. This chemical reaction converts the solid carbon of the diamond into gaseous carbon dioxide.
Sublimation demands even higher temperatures, typically above 3500°C, and a complete absence of oxygen, such as in a vacuum or inert gas environment.
The transformation of diamond into graphite is thermodynamically favorable at standard pressures but is usually extremely slow at room temperature. Faster graphitization happens at elevated temperatures, with some changes observed above 400°C and more significant conversion occurring at 700°C or higher. Achieving a liquid phase of diamond requires pressures exceeding 10 gigapascals and temperatures above 4000°C, conditions not found naturally on Earth’s surface.
Diamond’s Stability in Everyday Scenarios
Despite the scientific possibilities of diamond transformation, these gems exhibit remarkable stability in common situations. Diamonds are highly resistant to nearly all acids and chemicals.
In a house fire, where temperatures can reach up to 1100°C, a diamond may experience changes. While it will not melt, its surface can oxidize and develop a cloudy or whitish appearance. This surface damage can often be removed by re-polishing the stone. Sudden and extreme temperature fluctuations, rather than sustained heat, pose a greater risk, as they can induce thermal shock and potentially cause new fractures or exacerbate existing ones within the diamond.