pseudomorph

American Heritage® Dictionary of the English Language, Fourth Edition

1. n. A false, deceptive, or irregular form.
2. n. A mineral that has the crystalline form of another mineral rather than the form normally characteristic of its own composition.

Century Dictionary and Cyclopedia

1. n. A deceptive, irregular, or false form; specifically, in mineralogy, a mineral having a definite form belonging, not to the substance of which it consists, but to some other substance which has wholly or partially disappeared. Sometimes quartz is found in the form of fluor-spar crystals, the fluor-spar having been changed by a process of substitution into quartz. Such crystals are pseudomorphs by substitution; another illustration is that of tinstone, cassiterite, after orthoclase feldspar. A more common and important class of pseudomorphs includes those formed by the chemical alteration of the original mineral: these are illustrated by pseudomorphs of native copper after the oxid cuprite, where there has been a simple loss of one ingredient, in this case oxygen; also, of gypsum after anhydrite, where the anhydrous calcium sulphate has been changed by assumption of water to the hydrous sulphate; or, still more important, where there has been a more or less complete exchange of constituents, as of the lead carbonate cerusite after the lead sulphid galena, or of serpentine after chrysolite, or of kaolin after feldspar, etc. Pseudomorphs are also formed by molecular change without change of chemical substance, as of calcite after aragonite, or rutile after brookite; these last are also called paramorphs. (See paramorphism.) Pseudomorphs very commonly have a noncrystalline waxy structure, but this is not necessarily the case.

Wiktionary

1. n. geology, mineralogy A mineral that formed by replacement of an existing mineral (or organic matter) such that the new mineral has the appearance and dimensions of the original.

GNU Webster's 1913

1. n. An irregular or deceptive form.
2. n. (Crystallog.) A pseudomorphous crystal, such as a crystal consisting of quartz, but having the cubic form of fluor spar, the fluor crystal having been changed to quartz by a process of substitution.