FMF - Friends of Minerals Forum, discussion and message board

[Robson Vieira]

Hi there.
Could this specimen be a garnet with a limonite covering layer? A pyrite?
I had suspected that cause the shape of the crystal, which is common in garnets with twelve losange faces, as dodecahedron pyrite has twelve pentagon faces. Is there a way to ensure this kind of stuff? I thought about the density but I'm not able to know the limonite layer, so it would be not conclusive, since pyrite is a little heavier than almandine. Have you a clue?
Because of the limonite covering I'm not able to share additional information other than the size and the dodecahedral shape.
Many thanks

[Bob Carnein]

The second photo looks to me like a combination cube and octahedron (that would have 14 faces), which would not be found in garnet. If that's the case, it's likely a pyrite crystal that has been replaced by iron oxides.

[Robson Vieira]

[Bob Morgan]

It's a simple rhombic dodecahedron. Garnets can deteriorate into rust. I'm not sure of the process but have collected softball sized ones from Little Pine Mine in North Carolina that crumble in your hand. Yours has a similar outside appearance but seems to be less deteriorated.

[alfredo]

Almandine garnets are often found in alluvial deposits, and so would seem to be immune from weathering, and yet there are nevertheless many locations on this planet where almandine has completely altered to limonite!

In lateritized soils in the Amazon basin of Bolivia there are limonite pseudomorphs after garnet that occur in contact with very lustrous unaltered staurolite crystals. Almandine and staurolite are chemically similar Fe-Al-orthosilicates, both having hardnesses slightly above that of quartz, both insoluble in most acids, and resistant to weathering, and yet in a laterite the garnet is desilicified and oxidized to dull goethite, and the adjoining staurolite looks fresh and unaltered. I am mystified.

[Robson Vieira]

[Robson Vieira]

[alfredo]

What I meant was that the fact that a mineral occurs at all in large quantities in alluvium means that that mineral is not very susceptible to chemical or physical weathering. In other words it is a tough mineral. Quartz, garnet, zircon, monazite, corundum, magnetite, gold, platinum - those are all tough minerals that survive weathering and transportation into alluvium. That is why I am surprised to see such a tough mineral as garnets being altered into limonite.

[Bob Carnein]

I agree with the foregoing. But I also think the second photo shows a flat (more or less) face perpendicular to what would be a 4-fold symmetry axis of the rhomb dodecahedron. This would be a cube (hexahedron), which would be very rare in garnet.

[Herwig]

The number of faces will tell us what combination of forms it is.
I assume we agree it is either a combination of an octahedron and a cube,
= 8 + 6 = 14 faces,
or a combination of a dodecahedron and a cube,
= 12 + 6 = 18 faces.

Of course, some of the cube faces might be missing, but the difference in number of faces will remain. For example, if only 1 cube face present, it will be
8 + 1 = 9 versus 12 + 1 = 13.

The cuboctahedron combo is very common for pyrite.
The cubododecahedron combo (in this case the combination of a cube and a rhombic dodecahedron) is impossible for pyrite (not enough symmetry).
Cheers, Herwig

[Bob Morgan]

It appears to me that there is a 4 sided pyramid at the top and another at the bottom and 4 faces around the middle to be a rhombic dodecahedron. I can't be sure from the photos, after all, it's a rough pseudomorph making discerning the faces as photo'd not so easy.

[Robson Vieira]

[Robson Vieira]

[Robson Vieira]

[Robson Vieira]

Hello People. I'm adding more pics of the crystal. The first pic is one that shows the cubic profile of that axis and the other three are at same axis but rotated 90°, showing the 4 faces of that axis.
The last pic shows the flat face we are discussing. In my humble opinion, it really is a face, unfortunately, cause I wish it was a garnet (lol). So, it seems to have 14 faces.
What do you all think about it? Should I label it as a limonite pseudomorph of pyrite or of garnet?
Thank you so much.