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Pseudomorph
Mineral or mineral compound that appears in an atypical form / From Wikipedia, the free encyclopedia
In mineralogy, a pseudomorph is a mineral or mineral compound that appears in an atypical form (crystal system), resulting from a substitution process in which the appearance and dimensions remain constant, but the original mineral is replaced by another. The name literally means "false form".
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![Thumb image](http://upload.wikimedia.org/wikipedia/commons/thumb/2/2a/Goethite-Pyrite-44341.jpg/320px-Goethite-Pyrite-44341.jpg)
Terminology for pseudomorphs is "replacer after original", as in brookite after rutile.