Читать книгу Earth Materials - John O'Brien - Страница 126

Reconstructive transformations involve the conversion of one polymorph (or mineral) into another by processes that require bond breakage so that a significant change in structure occurs. Such transformations require large amounts of energy, and this requirement tends to slow or inhibit their occurrence. In the transformation of diamond to graphite, a large amount of energy is required to break the strong bonds that hold carbon atoms together in the isometric diamond structure, so that they can rearrange into the more open, hexagonal structure of graphite. This inhibits the transformation of diamonds into graphite as diamonds find themselves in lower pressure and lower temperature environments near Earth's surface. Minerals such as diamond that exist under conditions where they are not stable are said to be metastable. All polymorphs that require reconstructive transformations have the potential to exist outside their normal stability ranges as metastable minerals. This allows them to preserve important information about the conditions under which they, and the rocks in which they occur, were formed and, in the case of diamonds, to grace the necks and fingers of people all over the world.