Pseudomorphs

March 2001

pyrite after marcasite (a pseudomorph from Mineral City)

At the March 2001 meeting the topic was pseudomorphs and Ronnie Van Dommelen gave the club a short lecture, along with pictures and samples, about these tricky minerals. Here are some of his notes and pictures from his collection:

April Fool's Day is coming up, so seems a bit fitting to talk about those foolish minerals. No, not fool's gold, but rather pseudomorphs - minerals in disguise. The term pseudomorph means 'false form' and is used to describe crystals in which an original mineral has been replaced by a second mineral while retaining the outwardly crystal form of the original mineral. So the obvious question would be "How does this happen?". Well, it turns out pseudomorphs are classified into three categories.

The first is when the starting and ending minerals have the same chemistry, but there is a change in the crystal structure. Usually the first mineral forms under high pressure or temperature and is unstable at normal temperatures and pressures. As the temperature and pressure fall, the atoms rearrange themselves within the crystal to form the new mineral. An example of this is calcite after arragonite. Both are CaCO₃, but the crystal structures are different. These pseudomorphs are called paramorphs to distinguish them from the other two types.

The second type of pseudomorph requires the starting mineral to gain or lose an ingredient. Examples include copper (Cu) after cuprite (CuO₂) due to the loss of oxygen, malachite (Cu₂CO₃(OH)₂) after cuprite (CuO₂) due to the addition of carbon dioxide and water. It is also possible for the starting mineral to lose one ingredient and gain another. The important part is that at least one ingredient of the starting mineral remains in the pseudomorph.

The final pseudomorph process is when the starting mineral and the final mineral have nothing in common. Consider the photograph below of a specimen from the Boylston pit. The mineral is dolomite but the overall crystal shape is not that of dolomite; it looks more like calcite. The dolomite coated what was once a calcite crystal, and then the calcite dissolved away, leaving only the hollow shell of dolomite. This is called an epimorph. If, given more time, the shell were to fill with another mineral, and the dolomite broke away, we would have a pseudomorph after calcite. And what if the mineral that filled the shell was calcite? Well, we would have a calcite pseudo after calcite!

Halfway to a pseudomorph: dolomite cast of calcite from Boylston.

© 2001, Nova Scotia Mineral and Gem Society

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