Definition of polymorph
A mineral whose crystal structure is different from another mineral of identical chemical composition. For example, Al2(SiO4)O crystallizes in nature with three different crystal structures as the minerals kyanite, andalusite, and sillimanite. Quartz, tridymite, cristobalite, coesite, and stishovite are all polymorphs of SiO2.
Polymorphism is the property of a substance to exhibit different crystal structures. Some of these differences relate to distinct pressure, temperature, or other conditions during crystallization that have been replicated in the laboratory or documented in nature, while the reasons for polymorphism among other minerals remain poorly understood. Polymorphism limited to two or three crystal structures is dimorphism or trimorphism, respectively. Individual species are polymorphs (dimorphs, trimorphs).
Allotropy refers specifically to polymorphism among single-element minerals, for example graphite, diamond, chaoite, and lonsdaleite, all allotropes of carbon.
Polytypism refers to variations in stacking of identical layer structures resulting in different arrangements of the unit cell (the fundamental module of a mineral's crystal structure). Most polytypes are not considered separate species. Wurtzite exhibits numerous polytypes, and sphalerite could be considered one.
In polysyngony, polymorphs vary only slightly in atomic bond angles, for example: acanthite and argentite. It is synonymous with displacive polymorphism. Displacive polymorphism describes structures that differ only slightly in their atomic positions/bond angles. Transitions between them are generally reversible, meaning that the crystal will spontaneously transition between the two structures due to changes in physical conditions such as temperature and pressure. This can occur because no strong bonds are broken or formed during the transition. The transition from alpha-quartz to beta-quartz is an example of displacive polymorphism and occurs at 573°C at ambient pressure. This is known as "quartz inversion" and is important in the production of ceramics.
Enantiomorphs are polymorphs that differ in being mirror images of each other; see enantiomorphism. Quartz is a well-known example of enantiomorphism, where well-formed crystals may exhibit forms indicative of whether that crystal is left- or right-handed.
Adj: polymorphic or polymorphous (dimorphic/dimorphous, trimorphic/trimorphous)
Polymorphism is the property of a substance to exhibit different crystal structures. Some of these differences relate to distinct pressure, temperature, or other conditions during crystallization that have been replicated in the laboratory or documented in nature, while the reasons for polymorphism among other minerals remain poorly understood. Polymorphism limited to two or three crystal structures is dimorphism or trimorphism, respectively. Individual species are polymorphs (dimorphs, trimorphs).
Allotropy refers specifically to polymorphism among single-element minerals, for example graphite, diamond, chaoite, and lonsdaleite, all allotropes of carbon.
Polytypism refers to variations in stacking of identical layer structures resulting in different arrangements of the unit cell (the fundamental module of a mineral's crystal structure). Most polytypes are not considered separate species. Wurtzite exhibits numerous polytypes, and sphalerite could be considered one.
In polysyngony, polymorphs vary only slightly in atomic bond angles, for example: acanthite and argentite. It is synonymous with displacive polymorphism. Displacive polymorphism describes structures that differ only slightly in their atomic positions/bond angles. Transitions between them are generally reversible, meaning that the crystal will spontaneously transition between the two structures due to changes in physical conditions such as temperature and pressure. This can occur because no strong bonds are broken or formed during the transition. The transition from alpha-quartz to beta-quartz is an example of displacive polymorphism and occurs at 573°C at ambient pressure. This is known as "quartz inversion" and is important in the production of ceramics.
Enantiomorphs are polymorphs that differ in being mirror images of each other; see enantiomorphism. Quartz is a well-known example of enantiomorphism, where well-formed crystals may exhibit forms indicative of whether that crystal is left- or right-handed.
Adj: polymorphic or polymorphous (dimorphic/dimorphous, trimorphic/trimorphous)