In most cases, the crystal structure or form taken by the mineral is strongly influenced by the environment in which it forms. Polymorphs therefore record important information concerning the environments that produced them. Many polymorphs belong to very common and/or economically significant mineral groups, such as the examples summarized in Table 4.13.
The polymorphs of carbon can be used to illustrate how environmental conditions during growth determine which crystal structure a chemical compound possesses. Figure 4.36 is a phase stability diagram for systems composed of pure carbon. This phase stability diagram clearly indicates that diamond is the high pressure polymorph of carbon, whereas graphite is the low pressure polymorph. If we add geotherms, lines showing the average temperature of Earth at any depth, to this diagram, we can infer that diamonds are the stable polymorph of carbon at pressures of more than 3.5 GPa, corresponding to depths of more than 100 km below the surface whereas graphite is the stable polymorph of carbon at all shallower depths. Inferences must be tempered by the fact that Earth's interior is not pure carbon and temperature distributions with depth are not constant. Nonetheless, it is widely believed that most natural diamonds originate at high pressures far below the surface of old continental shields in which they most commonly occur. If graphite is the stable polymorph of carbon at low pressures, why do diamonds occur in deposits at Earth's surface where pressures are low? Obviously, as diamonds rise toward Earth's surface into regions of substantially lower pressure, something keeps the carbon atoms from rearranging into the graphite structure. What keeps the transformation from unstable diamond to stable graphite from occurring?
Table 4.13 Important rock‐forming mineral polymorphs.
| Chemical composition | Common polymorphs |
|---|---|
| Calcium carbonate (CaCO3) | Calcite and aragonite |
| Carbon (C) | Diamond and graphite |
| Silica (SiO2) | α‐quartz, β‐quartz, tridymite, cristobalite, coesite, stishovite |
| Aluminum silicate (AlAlOSiO4) | Andalusite, kyanite, sillimanite |
| Potassium aluminum silicate (KAlSi3O8) | Orthoclase, microcline, sanidine |
| Iron sulfide (FeS2) | Pyrite, marcasite |
Figure 4.36 Phase stability diagram showing the conditions under which graphite, the low pressure polymorph of carbon, and diamond, the high pressure polymorph of carbon, are stable beneath continental lithosphere.
Reconstructive transformations
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.
Displacive transformations
Some polymorphs are characterized by structures that, while different, are similar enough that the conversion of one into the other requires only a rotation of the constituent atoms into slightly different arrangements without breaking any bonds. Transformations between polymorphs that do not require bonds to be broken and involve only small rotations of atoms into the new structural arrangement are called displacive transformations and tend to occur very rapidly under the conditions predicted by laboratory experiments and thermodynamic calculations. Polymorphs involved in displacive transformations rarely occur as metastable minerals far outside their normal stability ranges and so may preserve less information about the conditions under which they and the rocks in which they occur originally formed.
Alpha quartz (low quartz) is generally stable at lower temperatures than beta quartz (high quartz). Although α‐ and β‐quartz have different structures, the structures are so similar (Figure 4.37) that the conversion of one to the other is a displacive transformation. It is not at all unusual, especially in volcanic rocks, to see quartz crystals with the external crystal form of β‐quartz but the internal structure of α‐quartz. These quartz crystals are interpreted to have crystallized at the elevated temperatures at which β‐quartz is stable and to have been diplacively transformed into the α‐quartz structure as they cooled, while retaining their original external crystal forms.
Figure 4.37 The closely similar structures of α‐ and β‐quartz.
Source: Courtesy of Bill Hames.
Other transformations between silica polymorphs are reconstructive. For example, the transformations between the high‐pressure minerals stishovite and coesite and between coesite and quartz are reconstructive. Therefore, both stishovite and coesite can be expected to exist as metastable phases at much lower pressures than those under which they are formed. Their preservation in rocks at low pressures allows them to be used to infer high pressure conditions, such as meteorite impacts, long after such conditions have ceased to exist.
Order–disorder transformations
Many polymorphs differ from one another only in terms of the degree of regularity in the distribution of certain ions within their respective crystal structures. Their structures can range from perfectly ordered to a random distribution of ions within structural sites (Figure 4.38). The potassium feldspar minerals (KAlSi3O8) provide many examples of such variation in regularity or order in the distribution of aluminum ions within the structure. In the feldspar structure, one in every four tetrahedral sites is occupied by aluminum (Al+3), whereas the other three are occupied by silicon (Si+4). In the potassium feldspar high sanidine, the distribution of aluminum cations is completely random (high disorder); the probability of finding an aluminum