Pseudomorph, mineral formed by chemical or structural change of another substance, though retaining its original external shape. Although pseudomorphs give the appearance of being crystalline, they are commonly granular and waxy internally and have no regular cleavage; those that are crystalline have optical properties different from those required by their outward form.
Pseudomorphs are formed by substitution, deposition, or alteration. In the formation of a pseudomorph by substitution, the original substance has been gradually removed and simultaneously replaced by another.
A common example of this is petrified wood, in which all the cellulose fibres have been replaced by silica, even those in the bark. Pseudomorphs can be formed by deposition of one mineral on the surface of crystals of another (see also epitaxy).
Pseudomorphs can be divided into three classes of pseudomorphism with six subdivisions:
- A) Pseudomorphism by incrustation
- B) Pseudomorphism by infiltration
- A) Paramorphism
- B) with loss of ingredients
- C) with gain of ingredients
- D) with exchange of ingredients
The process of alteration is similar to that above, except that pseudomorph specimens which undergo alteration have experienced only partial replacement or substitution. This typically occurs in situations where the original mineral undergoes a chemical reaction to another mineral of similar composition – resulting in a transformation of chemical composition, which retains the original crystalline form. These changes often occur on “exposed” surfaces, resulting in a pseudomorph which contains a core of unaltered original material.
Alteration pseudomorphs may be formed in several ways: from a change in internal crystal structure without a change in chemical composition (these pseudomorphs are called paramorphs; e.g., aragonite becomes calcite, and brookite becomes rutile); by the loss of an ingredient from the original compound (e.g., cuprite loses oxygen to form copper); by the addition of an ingredient to the original compound (e.g., anhydrite adds water to form gypsum, and cuprite adds carbon dioxide and water to form malachite); and by an exchange of constituents (e.g., feldspar loses potassium silicate and gains water to become kaolinite). An example of this process is the replacement of wood by silica (quartz or opal) to form petrified wood in which the substitution may be so perfect as to retain the original cellular structure of the wood. An example of mineral-to-mineral substitution is replacement of aragonite twin crystals by native copper, as occurs at the Corocoro United Copper Mines of Coro Coro, Bolivia.
Epimorph, results from a process by which a mineral is coated by another and the encased mineral dissolves. The encasing mineral remains intact, and retains the shape of the original mineral or material. Alternatively, another mineral may fill the space (the mold) previously occupied by some other mineral or material. The original may or may not have dissolved – but if it has, these pseudomorph specimens are sometimes referred to as “casts”.
Paramorphs. Also called allomorphs, paramorphs are pseudomorph specimens which have experienced change only on the molecular level. Paramorphs maintain their original, unaltered form and chemical composition, but with a different molecular structure. Usually, this means replacement of a mineral by one of its dimorphs – a mineral with the same chemical structure, but a different molecule structure. (An example of this would be the pseudomorph Brookite after Rutile – as both have the chemical composition TiO2.)
Often, pseudomorphs exhibit characteristics of more than one of these classes presented.
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