Geologic Tricksters: Pseudomorphs and Pseudofossils

In the world of geology, things may not always be what they seem....

If you've ever come across a rock that looks exactly like a prehistoric fern, or a crystal that appears in a familiar shape but seems like the "wrong" color, you may have encountered a geologic trickster known as a pseudomorph or pseudofossil! 

So let's pull back the magic curtain on these fun geologic phenomena.

PSEUDOMORPHS

What are Pseudomorphs?

The word pseudomorph literally translates from Greek to "false form". In mineralogy, this happens when a specimen displays the external crystal form of one mineral, but possesses the internal chemistry of another. 

Pro Tip: Collectors describe them using the term "after". For example, if Quartz replaced Fluorite, it is labeled "Quartz after Fluorite". 

How Do Pseudomorphs Form?

Pseudomorphs form from a substitution process where the outer shape of a mineral remains the same, but the original mineral is replaced by another. There are four primary types:

1.) Substitution (Infiltration) Pseudomorph

A substitution pseudomorph, sometimes called an infiltration pseudomorph, occurs when the original material is gradually removed while simultaneously being replaced by a new mineral. The shape remains unchanged, but the color, hardness, and density shift to the new mineral.

• Classic Example: Petrified Wood. As the organic wood cells decay, they are slowly replaced by Silica (SiO2 in the form of Quartz or Opal) to form Petrified Wood. The process is so precise that the original cellular structure and rings of the tree can still be seen in the fossilized wood!
  
  

2.) Alteration Pseudomorph

An alteration pseudomorph occurs when only a partial replacement occurs with the addition of a new material, or a partial removal of the original material.

When a mineral undergoes a chemical reaction, its composition can change into another with a similar composition. 

• Azurite to Malachite: These copper minerals have a very similar chemistry but different crystal shapes. When Azurite alters to Malachite, you'll see the sharp, bladed crystal shape of Azurite but the vibrant green color of Malachite.
  
  
• Chemical Change: Azurite Cu3(CO3)2(OH)2 to Malachite Cu2(CO3)(OH)2

 

3.) Epimorph (Incrustation) Pseudomorph

An epimorph pseudomorph or incrustation pseudomorph results from a process when one mineral is coated by another mineral, and the encased mineral then dissolves away. The mineral which covered or encased the other remains intact, creating the outline or shape of the original mineral underneath.

• Analogy: Like a plaster cast on a broken arm, once the arm heals and the cast is removed, the arm is "gone", but the shape of the limb remains the cast shell. 
  
  

 

4.) Paramorph Pseudomorph

A paramorph pseudomorph is one in which a mineral is changed on the molecular level only. The chemical composition stays the same, but the internal structure shifts.

• Example: Calcite to Aragonite. 

 

Now that we've looked at the pseudomorphs of the mineral world, let's look at some interesting occurrences in the fossil world: PSEUDOFOSSILS!

PSEUDOFOSSILS

What are Pseudofossils?

Pseudofossils are geologic formations that look like biological remains (plants or animals) but are actually just inorganic mineralization. They actually aren't fossils at all! 

Common Types of Pseudofossils

• Dendrites: These look exactly like fossilized moss or ferns. They form when Manganese or Iron oxides crystallize in branching patterns along fractures and bedding planes in rocks. While they look like ancient plants, these formations were never alive.
  
  
• Concretions: Often mistaken for fossilized dinosaur eggs or turtle shells, these "cannonballs" are simply rounded mineral masses. 

 

 

Conclusion

Geology is full of surprises. Whether it's a "false form" or a "fake fossil", these tricksters remind us to look closer at the chemistry beneath the surface. Grab one for your collection here!