It is possible for two minerals with exactly the same chemical composition to have different internal structures and, hence, different external forms. Minerals of this type are called polymorphs (poly 5 many, morph 5 form). Graphite and diamond are particularly good examples of polymorphism because, when pure, they are both made up exclusively of carbon atoms. Graphite is the soft gray mineral from which pencil “lead” is made, whereas diamond is the hardest-known mineral. The differences between these minerals can be attributed to the conditions under which they form. Diamonds form at depths that may exceed 200 kilometers (nearly 125 miles), where extreme pressures and temperatures produce the compact structure shown in Figure 1A.
Graphite, on the other hand, forms under comparatively low pressures and consists of sheets of carbon atoms that are widely spaced and weakly bonded (Figure 1B). Because the carbon sheets in graphite easily slide past one another, graphite has a greasy feel and makes an excellent lubricant.
Scientists have learned that by heating graphite under high confining pressures, they can generate synthetic diamonds. Because human-made diamonds often contain flaws, they are generally not gem quality, but due to their hardness, they have many industrial uses.
Further, because diamonds form in environments of extreme pressure and temperature, they are somewhat unstable at Earth’s surface. Fortunately for jewelers, “diamonds are forever” because the rate at which diamonds change to their more stable form, graphite, is infinitesimally slow.
The transformation of one polymorph to another is an example of a phase change. In nature, certain minerals go through phase changes as they move from one environment to another. For example, when a slab of ocean crust composed of olivine-rich basalt is carried to great depths by a subducting plate, olivine changes to a more compact, denser polymorph with the same structure as the mineral spinel.
Recall that oceanic lithosphere sinks because it is colder and more dense than the underlying mantle.
It follows, therefore, that during subduction, the transformation of olivine from its low- to high-density form would contribute to plate subduction. Stated another way, this phase change causes an increase in the overall density of the slab, thereby enhancing its rate of descent.