FMF - Friends of Minerals Forum, discussion and message board

[Bob Morgan]

The square outline with 90 degree angles is made by 4 dodecahedral faces around the middle of the crystal as you look down on it from the top. In the first photo there are still 4 faces forming a pyramid toward the camera. The ridges that separate them are discernable. The hidden side has the other 4 faces making a pyramid to complete the twelve of the dodecahedron.
If you rotate the crystal around one of its three sided pyramids you will get the square cross section again.

As much as I love pyrite and have collected limonite pseudo's this is not one of them. It would be an awefully skewed development to have large cube faces around the sides and then have large octahedral pyramids on both ends.

[Bob Carnein]

OK, these certainly look like a simple rhombic dodecahedron.

[Robson Vieira]

[Robson Vieira]

[Roger Warin]

Hello,
I believe in a pseudomorphosis of a garnet.
The texture of this specimen is not that of a crystal.
I think we often forget that water is an extracting agent of SiO2.
Obviously the water must be in a "critical" state (high temperatures and pressures).
SiO2 is then soluble in water to give various silicic and polysilicic acids. One or more iron oxides remain in the mixture.
These words are only a suggestion.
Roger.

[Herwig]

Thus:
a garnet crystal, having the (classic) shape of a rhombic dodecahedron, has been replaced by a mixture of iron oxides & hydroxides, thereby retaining the shape of the original crystal. Since we don't know the actual composition of the thing that replaced the garnet, but we assume it to contain oxides and also probably hydroxides of iron, it's best to call that material "limonite", especially since it looks "earthy".
more info on limonite here: https://www.mindat.org/min-2402.html

in short:
limonite pseudomorph after garnet

Cheers, Herwig

[Robson Vieira]

[Robson Vieira]

[Robson Vieira]

I'm very grateful for the help you all provided to me, especially in terms of clearance your points over the topic that allowed me to identify and better yet, understand related questions about environment, mineral formation, pseudomorphism, wearing conditions and so on.
Thank you all

[Roger Warin]

Isn't IMA, limonite, a rock and not a mineral species?

[alfredo]

"Limonite" is a field term for iron oxides which have not been analyzed and for which the species is not known, although 90% of the time it is either goethite or a mixture of hematite and goethite. So, its usage is similar to field terms like "psilomelane" (unanalyzed hard manganese oxides) or "wad" (unanalyzed soft manganese oxides).

[Roger Warin]

Thank you, Alfredo,
My comment concerns the IMA, whose scientific rigor, obviously necessary, sometimes bewilders the amateur. Scientists obviously have a background.
It seems to me that there is a difference in philosophy between mineralogists and gemologists, who sometimes seem more didactic. I'm not talking about “meteorists”, for whom olivine does exist.
My comment concerns not the scientific articles, but the contents of Fleischer's Glossary. This catalog is widely used by amateurs as a reference. It even encourages amateurs to collect systematically, since it includes a checkbox.
However, the Glossary does not mention olivine. I think it's even banned from publication... except when it talks about the olivine group.
What's the olivine group? Need we remind you that it's one of the main components of the Earth's mantle?
This problem is of course due to the existence of solid solutions. This definition (which I'm not criticizing) is bizarre, since there is no solvent.
My wish is that the Glossary will insert a few sheets to remind us of the historical existence of certain ubiquitous solid solutions, whereas limit poles are almost an illusion, it seems to me. There are even cases where the second limit does not exist.
I'm probably a utopian.

[alfredo]

Roger, I don't think you can insist that Fleischer's glossary should include more than what it states to be its intent: a list of species. So anything that is not a "species" won't be included. If you want a list of species + groups + isomorphous series + historical names, then you yourself should publish "Warin's Glossary of Species, Series, Groups, and Historical Names". (And yes, I will buy it ;))

[Herwig]

[Roger Warin]

Please help me.
After 70 years of collecting, I no longer know how to write a label for this automorphic mineral, whose face indices I have noted.
I think I've reached a good level of mineralogical knowledge, even though I'm still an amateur.
At the moment, I'm inclined to use the old name “peridot”, which might escape controversy.
What would a museum curator do?