Definition of polytype
A mineral that differs from another only in the stacking of similar structural units in its atomic structure.
Polytypes are a special case of polymorphs, where multiple close-packed crystal structures differ in one dimension only. Polytypes have structures containing identical close-packed layers, but differ in the stacking sequence of these layers. It is most common in phyllosilicates, eg. Micas, chlorites, kaolinite-serpentine group, etc., also some hydroxycarbonates like the hydrotalcite group, some elements eg. graphite, some carbides, eg moissanite, and sulphides with layered structures like molybdenite, pyrrhotite, etc.
Normally polytypes are not defined as separate species, but there are exceptions, eg. Sphalerite/wurtzite and kaolinite/dickite/nacrite.
See also polytypism.
Polytypes are a special case of polymorphs, where multiple close-packed crystal structures differ in one dimension only. Polytypes have structures containing identical close-packed layers, but differ in the stacking sequence of these layers. It is most common in phyllosilicates, eg. Micas, chlorites, kaolinite-serpentine group, etc., also some hydroxycarbonates like the hydrotalcite group, some elements eg. graphite, some carbides, eg moissanite, and sulphides with layered structures like molybdenite, pyrrhotite, etc.
Normally polytypes are not defined as separate species, but there are exceptions, eg. Sphalerite/wurtzite and kaolinite/dickite/nacrite.
See also polytypism.