When a mineral presents itself under a form which does not belong to it, there is then what I shall call pseudomorphism.

The substance from which the mineral borrows its form may be of any kind—inorganic or even organic. It is called original or pseudomorphosed, while the mineral which replaces it is called pseudomorphic.

Pseudomorphism by alteration is that in which the pseudomorphic mineral still contains the elements of the original substance. Pseudomorphism by displacement is that in which this is not the case. In order to understand the difference which exists between these two kinds of pseudomorphisms, it suffices to mention as examples ironpyrites, which changes into limonite, still preserving its crystalline form; or fluor, which after being completely destroyed, is replaced by quartz. The name of paramorphism has been given to the kind of pseudomorphism which is produced without modification of chemical composition. Arragonite changed into calcite, and pyrite changed into marcasite are examples.

At first sight it seems that these metamorphoses of minerals must be very exceptional, but observation teaches us, on the contrary, that they are met with in a number of localities; they are, moreover, extremely varied. In fact, they include all the alterations to which minerals are subject in their structure and in their chemical composition. They include also, as a particular case, the decomposition of minerals; and kaolin, for example, results from a true pseudomorphism of felspar.