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mineral formations whose external form does not correspond to their composition and internal structure. When defining pseudomorphs, the name of the original substance is given, as well as the name of the mineral that has replaced it, while preserving the original shape (for example, a pseudomorph of limonite after pyrite or of chalcedony after wood). In some cases pseudomorphs are given their own names, for instance, the pseudomorph of magnetite after hematite is called mouchketovite.
A distinction is made between simple pseudomorphs, in which one mineral crystallizes in place of another, and complex pseudomorphs, formed by an aggregate of several minerals, such as the aggregates of albite and muscovite in spodumene crystals.
Depending on their origin, pseudomorphs are divided into alteration, substitution, filling, encrustation, and negative pseudomorphs. In pseudomorphs of alteration, there is a chemical bond between the new mineral and the one from which the pseudomorph emerged. Examples include pseudomorphs of limonite after siderite, malachite after azurite, and anglesite after cerussite. A particular type of alteration pseudomorph is the paramorph, which has the same composition as the original crystal but a different crystalline structure. Such a change is usually observed in polymorphic transitions (paramorphs of pyrite after marcasite, calcite after aragonite, and sphalerite after wurtzite).
Substitution pseudomorphs are created during various metasomatic processes and are characterized by a lack of apparent relationship between the chemical composition of the newly formed minerals and the preexisting ones, for example, pseudomorphs of quartz after barite or of magnetite after asbestos and all pseudomorphs in organic remains. Filling pseudomorphs occur when the bulk of the material in a body is dissolved out, and the remaining cavity, with a shape characteristic of the body, is later filled by another mineral (pseudomorphs of sandstone after halite).
In the case of encrustation pseudomorphs, one mineral entirely covers and surrounds the crystals of another in the form of crusts (chalcopyrite crusts on magnetite or quartz crusts on fluorite). Negative pseudomorphs are imprints of the crystal of one mineral on the other or cavities from the dissolving of preexisting mineral grains.
The study of pseudomorphs is very important for clarifying the principles of change in minerals and for establishing preexisting conditions and the nature of chemical reactions during the formation of deposits of useful minerals.