Something that I'm frequently asked concerning mineral cards is "This piece says Blah after Blah. What does that mean?". This generally denotes that a specimen is a pseudomorph. Here is the short answer: A pseudomorph is a mineral that has the shape of one but the composition of another. Usually through some form of chemical change. The first "blah" is the current mineral and the second is the original on the card. The reality though is a bit more complicated. There are a wide variety of different types of pseudomorphs.
The classic pseudomorph would be the "substitution" or "infiltration" type. Where the original shape is completely preserved but the mineral has undergone a complete chemical change to another type. What you see above and in the subsequent photos are another psuedomorph called a "perimorph" or "incrustation" type. Perimorphs form as one mineral coats another, creating a crust. The crust stays intact while the former mineral dissolves away leaving a mold-like structure behind. In this case these are "Pyrite after Galena". Meaning the pyrite coated the galena and for whatever reason, the galena is long gone. Here is another example:
Getting more complicated you have "paramorphs" (also known as allomorphs) which only have a delineation based on molecular structure. They keep the form of the original, but have altered in molecular structure to another type. Common examples would be aragonite turning into calcite (they are both Calcium Carbonates they just have different molecular structures) or brookite turning into rutile. Pseudmorphs that are the result of the addition or loss of an ingredient are also classified as paramorphs. For example cuprite loses an oxygen molecule to form copper, or water is removed from gypsum to form anhydrite.
Lastly are the "epimorphs" which form as the result of environmental conditions. They are generally "casts" left behind by another structure. Technically you could consider a geode as an epimorph. Since they are generally either the result of trapped gasses from volcanic activity or nodules that dissolved away to leave a cavity for crystals to line over time.
If you've read this far and want a nifty pseudmorph to add to your collection you can find them here!
If you have a general geology question you'd like to see in another blog post e-mail me at email@example.com or find me on facebook/instagram @mooneyfinemineral.