In the course of mineral collecting, you may have come across these mysterious rocks known as pseudomorphs. With a name like that - it's clear that these rocks hold some secrets, and there is more going on than meets the eye. So what is a pseudomorph?
The term pseudomorph means “false form” - but how can a rock be false? What's actually happening here, is that one mineral has taken on the form of another - a mineralogical wolf in sheep's clothing. Some of the popular pseudomorphs you might have seen in the collector market include rocks in the copper carbonate family - such as malachite or chrysocolla pseudomorph of azurite. You might also see a mineral such as goethite pseudomorph of pyrite - retaining the distinct cubic shape of the pyrite, while appearing dramatically different in luster and coloration. Perhaps the most well known type of pseudomorph is petrified wood - in which silica infiltrates the porous wood and eventually replaces the wood entirely. Another popular pseudomorph in the metaphysical collecting realm is known as a “prophecy stone” - these unusual looking minerals are composed of Limonite and Hematite, which have replaced Pyrite and Marcasite.
Hematite ps. Marcasite "Prophecy Stone"
In 1907, James Dwight Dana laid out a basic classification scheme for pseudomorphs that is still very useful to understand the different ways these fascinating minerals form. Dana observed that pseudomorphs may form through three main mechanisms: substitution, deposition, or alteration.
Substitution: When a substitution pseudomorph forms, the original mineral is replaced by a different mineral.
Deposition: Deposition pseudomorphs form when a mineral is deposited on another mineral, known as incrustation, or when it fills voids left in another mineral, known as infiltration.
Alteration: Alteration pseudomorphs form when the original mineral changes it's internal structure or composition.
In substitution pseudomorphs, you have the second “pseudomorph” mineral replacing the first. This is happening on a molecular level - and since the first mineral has already formed its entire external crystalline shape, the second mineral will then replace the original one, but keep it's original shape. In substitution, the replacement mineral is not just replacing the outermost layer - it is replacing that host mineral throughout the entire body of the mineral - and as a result, the properties of the original mineral change. Some of those properties that “morph” include the texture, color, luster, and cleavage characteristics of the original mineral. So a mineral that was originally vitreous, can appear waxy or granular. One that would have originally broken cleanly along a cleavage plane, may now fracture instead.
In deposition pseudomorphs, the secondary mineral may form over the surface of another mineral, in a process known as incrustation. It is very common to see quartz incrustation on another mineral host rock like fluorite. This may also occur, followed by the dissolution of the host, which creates a cast of the original mineral, retaining the original mineral's shape as a “void” under the new mineral. These are referred to as epimorphs. In the other form, the secondary mineral infills voids in the original material, such as when a hot solution containing a new mineral flows into these voids, and cools and crystallizes.
In alteration pseudomorphs, This may include the loss of a molecular element, the gain of an element or elements, or a change in the internal lattice structure of the mineral, without undergoing a chemical change. Examples of this include the loss of oxygen molecules oxygen from an oxide mineral, such as when cuprite (Cu2O) pseudomorphs into copper (Cu). The last form, in which the chemical structure remains the same, but the crystalline structure changes, is known as a paramorph or allomorph. An example of this is when aragonite changes to calcite. Both of these minerals are still forms of calcium carbonate (CaCO3), but the calcite is hexagonal, while the original aragonite is orthorhombic. Two distinct minerals with teh same chemical composition are known as polymorphs.