Pseudomorphs,

Pseudomorphs, in Mineralogy, applied to minerals which assume the crystalline form of other species. Pseudomorphs result from the action upon minerals of water containing carbonic acid, oxygen, and other reagents in solution. The internal structure of a pseudomorph has no relation to the external form of the crystal. Two kinds of pseudomorphs are recognised: (a) alteration and (b) substitution pseudomorphs. Alteration pseudomorphs are the result of the chemical metamorphosis of the original mineral either by loss or gain, or exchange of constituents. Substitution pseudomorphs are minerals formed in the moulds or vacant spaces left by the total removal in solution of previously existing minerals.