If a crystal changes chemically or structurally, yet keeps the shape of the original, it is called a pseudomorph or
"false form"; it looks like a crystal of one species but is composed of another
(Sinkankas). Pseudomorphs include the following:
Paramorphs are the result of polymorphism, the existence of a chemical compound or element in two or more
crystal structures
(KL).
Polytypes are a special case of paramorphs 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, for example micas
and chlorides, also some hydroxycarbonates like the hydromagnesite group,
some elements, such as graphite, some carbides, such as
moissanite, and sulphides with layered structures like
molybdenite and pyrrhotite.
Normally polytypes are not defined as separate species, but there are exceptions, such as
sphalerite/wurtzite and
kaolinite/dickite/nacrite
(Mindat).
Alteration pseudomorphs result from processes involving chemical reactions
(KL).
Replacement pseudomorphs involve complete or partial solution and chemical precipitation of a new substance
(KL).
Epimorphs occur where an original mineral is encrusted by a second one, but the core of the first mineral remains unchanged
(KL).
Perimorphs occur where an original mineral is encrusted by a second and then the first mineral is leached away leaving
a hollow shell in the form of the original mineral
(KL).
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