Pseudomorph

It can occur due to the loss of water or through the action of atmospheric agents, such as oxidation, hydration, or carbonation.

Pyrite crystals transformed into limonite, a compact mixture of iron oxides where goethite generally predominates, are common.

For example, the cubic form of silver sulfide, argentite, does not actually exist below 173°C, and all are pseudomorphized into the monoclinic mineral acanthite.

When the structure is very different, as in the aragonite and calcite, the unstable mineral can remains indefinitely in a metastable form, although transformation can occur under some conditions.

Examples of quartz epimorph after calcite are found in the La Viesca Mine, Siero (Asturias), Spain.

The name refers to the similarity in appearance between the cephalopod that released it and the cloud itself, in this context meaning literally "false body".