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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 identical close-packed planes, but differ in the stacking sequence in the third dimension perpendicular to these planes. It is most common in phyllosilicates, eg. Micas, chlorides, etc., also some hydroxycarbonates like the hydromagnesite 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.


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