POPULAR PSEUDOMORPH MINERALS
Rock&Gem Magazine|December 2020
More Than They Seem
BOB JONES

The term pseudomorph, abbreviated ps or pseudo, means “false form.” This two-part article will describe a wide variety of minerals and fossils that have changed from their original form to something else and how that happens. Every good collection should have examples of common pseudomorphs.

The keyword, when discussing “pseudos,” is change. Minerals crystallize and exhibit a particular shape or crystal form. Such things as solution Ph, temperature, pressure, the richness of the solution, even location all play a role in crystallization. Some minerals, once formed, are unstable, so changes also occur as conditions change.

One typical example of a pseudomorph is iron rusting to iron oxide. The original shape remains but not the properties. Iron sulfide pyrite can also change, altering to hydrous iron oxide goethite. When pyrite changes to goethite, it retains the original crystal form of a cube or octahedron but is no longer brassy pyrite but dull dark brown goethite. Goethite after pyrite is a common form of a pseudomorph process called replacement.

UNDERSTANDING REPLACEMENTS

Replacements are substances in which the original entity is slowly replaced molecule by-molecule with a new substance while preserving the original form. Other forms of pseudomorphs include casts, paramorphs, and epimorphs. Plus, pseudomorph change scan happen in fossils as well as minerals.

A common example of a replacement pseudomorph in fossils found all over the world is petrified wood. This replacement pseudomorph forms when silica-rich water slowly invades and replaces wood that is quickly buried to avoid rotting. The wood is best preserved or petrified when buried in silica-rich volcanic ash. Arizona has the world’s finest example of this process in Petrified Forest National Park. The huge colorful stone logs present in this park are replacement pseudos as their wood cells are perfectly preserved in their original structure. These solutions carry trace amounts of iron oxide, manganese oxide, and even colorful uranium compounds, which create Arizona’s lovely gem-quality petrified wood.

The specific petrified wood deposit in Arizona formed eons ago when giant trees were felled in some distant place then carried and deposited by fast-flowing waters losing their limbs in the process. Once in place, they were quickly buried by nearby volcanism and the silica-rich volcanic ash, given time, slowly replaced the wood cells with silica minerals. Now Arizona’s petrified wood is the state’s official gem.

Fossilized wood is not the only replacement type of fossil. We find perfectly preserved stone or pyrite ammonites and shells in many limestone deposits, perfect images of the original hard parts. Dinosaur bones we unearth, reassemble and display are replacement fossils. The most unusual dinosaur bone I’ve seen is a massive leg bone, which had been cut in half to reveal the original soft tissue replaced by lovely banded agate.

IDENTIFYING CASTS FROM PARAMORPHS

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