FMF - Friends of Minerals Forum, discussion and message board

[Forrestblyth]

A complimentary pair of minerals

[Matt_Zukowski]

[Forrestblyth]

[Michael Shaw]

When one mineral is changed into another but keeps the crystal form of the original mineral it is called a pseudomorph. This is an interesting mineralogical phenomenon and there are several different classifications - replacements, casts, epimorphs, etc.

one of the most commonly seen pseudomorphs is Goethite after pyrite. Pyrite has a cubic crystal form and Goethite is orthorhombic. When pyrite is replaced by goethite and retains the cubic crystal form of the pyrite it is known as Goethite pseudomorph after Pyrite. See image below.

The text in your photos is corrected to reflect two separate minerals and no pseudomorphism.

[Matt_Zukowski]

A pseudomorph (pseudo=false, morph=form) is a mineral that has replaced and taken on the form of another mineral. A true wulfenite after mimetite would be made of wulfenite (a tetragonal mineral) but have the outer hexagonal form of mimetite. There is much out there written about this so just google "mineral pseudomorph" for more info including pictures.

You could label this specimen as wulfenite on mimetite or wulfenite and mimetite.

In one sense, the wulfenite is after the mimetite in that the wulfenite is on the mimetite so it formed after the mimetite. But mineralogists reserve the word "after" in the construction "mineral x after mineral y" to mean a pseudomorph of x after y.

[Forrestblyth]

Hi Matt, and Michael.
Thank you for your most interesting explanation and photos.

Isn't nature just amazing.

I will educate myself as suggested and look forward to understanding the effect in more detail later. Just from what you said looking through my collection would the mineral posted below be remotely considered Siderite after fluorite.

Some of the siderite has formed a cube like shape of the fluorite instead of the blade like crystal of siderite

Am I sill off the mark understanding, Hope I don't sound thick lol..

Best regards Forrest

[Matt_Zukowski]

In your picture I see fluorite and what looks like it could be siderite, and in places it looks like the siderite is coating the fluorite. This is not what pseudomorphing is. With a pseudomorph, a mineral replaces a prior mineral and occupies the space the prior mineral once took. Perhaps what you have could be called an epimorph, but I would just call it a coating. Or at least that is what I can say from the picture you show which, actually, looks pretty blurry on my computer screen.

[Forrestblyth]

And how about this one.

Would this be considered a pseudomorph "cast

Limonite after fluorite ?

[Matt_Zukowski]

Again, the limonite is not replacing the fluorite, so not a pseudomorph.

[Forrestblyth]

Hopefully a clearer picture of the limonite taking the reverse shape of the fluorite

[Forrestblyth]

[alfredo]

"the limonite is not replacing the fluorite, so not a pseudomorph"

Some pseudomorph collectors would disagree with this statement and would consider these "moulds" to be one of the many different types of pseudomorphs, not all of which involve replacement. Although I suppose in some sense you could say the fluorite has indeed been replaced here... by nitrogen and oxygen ;))

Anyway, tomorrow I'm going to the Flagg Foundation's annual mineralogy symposium in Phoenix, and the topic this year happens to be pseudomorphs. I'll report back if anyone has an interesting opinion on moulds and casts.

[James Catmur]

A Goethite cast after Fluorite? Collected in 1983

[Forrestblyth]

[James Catmur]

Chemically the difference in composition may have allowed one to resist and the other not. That difference is what people use to prepare specimens