pseudomorphosis

Pseudomorphosis refers to a mineral that has been replaced by another mineral while retaining the external shape of the original mineral. This process, known as mineral alteration or substitution, occurs when a mineral is unstable under certain environmental conditions, such as changes in temperature, pressure, or chemical composition of its surroundings. The original mineral is dissolved, and the ions released are then reprecipitated as a new mineral that takes on the crystal form of the original.

Common examples of pseudomorphs include limonite after pyrite, quartz after calcite, and malachite after azurite. These pseudomorphs are often of interest to mineral collectors because they preserve the characteristic shapes of rarer or less stable minerals, providing a tangible representation of geological history. The external form is a direct imprint of the original mineral's crystal lattice.

The identification of a pseudomorph often relies on recognizing the original crystal shape combined with the properties of the replacing mineral. For instance, a cube-shaped mineral with the hardness and streak of limonite suggests it is a pseudomorph after pyrite, which crystallizes in cubes. Pseudomorphosis is a common phenomenon in mineralogy and geology, offering insights into the chemical and physical processes that have affected mineral formation and alteration over geological time.