Pseudomorphism - Minerals in Disguise

Pseudomorphism happens when one mineral has the exterior shape and form of another. The word is derived from the greek words Psudo and Morph meaning False and Form. It happens when a mineral is altered in such a way that whilst its internal structure and chemical composition is changed its external form is preserved. There are three basic mechanisms:

Substitution - In this type the chemical components of a mineral are simultaneously removed and replaced by other chemical components during the alteration. An example is the replacement of wood fibers by quartz to form fossil wood. This has the outward appearance of the original wood, but is composed of quartz. Another example is the alteration of fluorite, which forms cubic crystals, and is replaced by quartz during alteration. The resulting quartz crystals look like the cubic fluorite, but are different chemically.

Encrustation - In this type of change the alteration begins by the formation of a thin crust of a new mineral on the surface of one already in place. Subsequently the the preexisting mineral is gradually removed, leaving the crust behind. In this case the thin crust of the new mineral forms a cast of the form of the original mineral. Sometimes the cavity becomes completely filled by the new material, forming a mineral similar to that created by substitution.

Alteration - In this final type of change the transformation is only partially completed. Consequently the space originally occupied by the pre-exisiting mineral now contains a mixture of the old and the new. Examples include serpentine pseudomorphed after olivine or pyroxene, anhydrite pseudomorphed after gypsum, limonite after pyrite, and anglesite after galena.

Polymorphism - Diamons aren't for ever

Polymorphism is a widely used mineralogical term which is often misunderstood and frequently confused with other terms including isomorphism and pseudomorphism. The word is derived from two Greek words meaning "many forms". The term refers to the ability of a given chemical entity to adopt different internal structures and external forms, in response to different conditions of temperature and/or pressure. For example, Diamond and Graphite are both forms of the element Carbon, but each has very different properties and appearance. Both are apparently stable under normal conditions and are thus found free in the environment. In contrast, other polymorphs are unstable and therefore rarely found naturally, except when artificially made by applying different amounts of heat and pressure. Any polymorphs produced under such conditions return to the original state when the heat and/or pressure return to normal.

Examination of the way different structural polymorphs behave and the conditions under which they form, have enabled mineralogists to identify three distinct sets of changes which they have called polymorphic transformations:

Reconstructive Transformations - involve the extensive rearrangement of the crystal lattice which commonly requires the breaking of chemical bonds and reassembling the atoms into new spatial arrangements. Because of the high energies needed the rate at which this type of transformation occurs may be very slow. Any unstable polymorphs may therefore exist for long periods of time before they revert back to any original state. This is often referred to as metastability. For example, diamond is a metastable polymorph of carbon formed at extremes of temperature and pressure deep within the Earth's crust. The chemical bonds holding the carbon atoms in graphite are broken and rearranged into the three dimensional lattice of diamond. This is very difficult to break down, even as heat and pressure drop, so although diamond does revert back to graphite, it does so exceedingly slowly, taking billions of years. To all intents and purposes the change is irreversible.

Displacive Transformations - involve only small adjustments to the crystal structure. Generally no bonds are broken and the only thing to change are the angles between the atoms in the lattice. Because there is so little change, displacive transformations involve almost no change in energy and the transformations are usually instantaneous and readily reversible. Thus, no unstable polymorphs will occur. An example of this is quartz, which normally adopts the beta-quartz structure below 580 degrees C. As soon as the quartz is heated above this temperature it easily transforms into a different form known as alpha-quartz, which is also stable but only above this temperature. The change is perfectly reversible and therefore alpha-quartz is never seen in rocks in the upper crust where the heat is low.

Order-Disorder Transformations - involve varying amounts of order or disorder in a crystal structure. As temperature increases, the degree of randomness in a lattice increases, so that the higher temperature forms of minerals are more disordered than the lower temperature forms. Because the state of ordeliness changes gradually with rising temperature, there are no definite temperature boundaries at which individual transformations occur. An example of a transformation of this type involves is the substance KAlSi3O8. At high temperature the only stable form is Sanidine. At lower temperature the structure changes to Orthoclase, and at even lower temperature the structure becomes that of the highly ordered structure of Microcline. There is no definite temperature at which Sanidine changes to Orthoclase or Orthoclase changes to Microcline, since the structure changes gradually as temperature decreases. If the cooling is very rapid, then unstable polymorphs can "freeze out" and continue to exist even at a lower temperature.

Polymorphism is widespread in the mineral world. Here are more common examples:

Carbon C - has two polymorphs. At high pressure it has an isometric lattice structure commonly known as Diamond. As pressure drops, Diamond very slowly undergoes a reconstructive transformation to the hexagonal structure of Graphite. Because this change involves a complete rearrangement of atoms it takes place extremely slowly. For this reason Graphite is the "natural" form of Carbon, and Diamond is only metastable because of the slowness of the change.

Aluminium Silicate Al2SiO5 - has three polymorphs. The high pressure form is Kyanite, the high temperature form is Sillimanite, and the low temperature, low pressure form is Andalusite. Transitions between all three are reconstructive and thus all can exist in rocks. Transformation rates are faster, however, deep within the crust where it is much hotter.

Calcium Carbonate CaCO3 - has two polymorphs. The high pressure form is Aragonite and the low pressure form is Calcite. The transformation between the two polymorphs is reconstructive, so metastable polymorphs can exist, and both are found in rocks.

Silicon Oxide SiO2 - has six polymorphs. With decreasing temperature at low pressure, Cristobalite undergoes a reconstructive transformation to Tridymite. Lowering the temperature further causes the Tridymite to undergo a reconstructive transformation to a-quartz. Additional cooling results in the a-quartz undergoing a displacive transformation to b-quartz. Cristobalite and Tridymite can exist metastably at the low temperatures near the Earth's surface, and thus are found in rocks. But because a-quartz transforms to b-quartz when as it cools it is never found in rocks. With increasing pressure, at low temperature b-quartz undergoes a displacive transformation to Coesite, which in turn undergoes a reconstructive transformation to Stishovite at even higher pressures. Both are metastable and thus occur in rocks.

Potassium Aluminium Silicate KAlSi3O8 - this material has three polymorphs that undergo order-disorder transformations with falling temperature. The hot polymorph is Sanidine. It is only found in igneous rocks that have cooled rapidly so that a higher state of order is not achieved. With slower cooling, Sanidine eventually undergoes a change to Orthoclase, and the latter eventually transforms to Microcline with further slow cooling.