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Limonitized garnet?
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Robson Vieira




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PostPosted: Jan 22, 2025 07:52    Post subject: Limonitized garnet?  

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



20250122_091423.jpg
 Mineral: Almandine?
 Locality:
Teófilo Otoni, Mucuri Valley, Minas Gerais, Brazil
 Dimensions: 4cm x 4cm
 Description:
 Viewed:  2076 Time(s)

20250122_091423.jpg



20250122_091432.jpg
 Mineral: Almandine?
 Locality:
Teófilo Otoni, Mucuri Valley, Minas Gerais, Brazil
 Dimensions: 4cm x 4cm
 Description:
 Viewed:  2076 Time(s)

20250122_091432.jpg



20250122_091542.jpg
 Mineral: Almandine?
 Locality:
Teófilo Otoni, Mucuri Valley, Minas Gerais, Brazil
 Dimensions: 4cm x 4cm
 Description:
 Viewed:  2075 Time(s)

20250122_091542.jpg


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Bob Carnein




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PostPosted: Jan 22, 2025 10:53    Post subject: Re: Limonitized garnet?  

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 If that's the case, it's likely a pyrite crystal that has been replaced by iron oxides.
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Robson Vieira




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PostPosted: Jan 22, 2025 15:11    Post subject: Re: Limonitized garnet?  

Bob Carnein wrote:
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 If that's the case, it's likely a pyrite crystal that has been replaced by iron oxides.

Really, but the other faces have not this patern, just losanges.
Otherwise, your statement is significant.
Thank you
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Bob Morgan




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PostPosted: Jan 22, 2025 15:39    Post subject: Re: Limonitized garnet?  

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.
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alfredo
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PostPosted: Jan 22, 2025 18:45    Post subject: Re: Limonitized garnet?  

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.
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Robson Vieira




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PostPosted: Jan 23, 2025 12:46    Post subject: Re: Limonitized garnet?  

Bob Morgan wrote:
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.

Thank you Bob. The gereneral apearance seems to me a garnet crystal and yes, it has protected faces wich makes me too happy.
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Robson Vieira




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PostPosted: Jan 23, 2025 12:50    Post subject: Re: Limonitized garnet?  

alfredo wrote:
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.

Alfredo, according to your comment the garnets are mostly found in alluvial deposits and for that they are safe for weathering. I would like to understand better that point. Could you please explain better why a crystal in alluvial deposit is safe for weathering? It seems you stated about environmental conditions of preserve/deteriorate specimens and this subject call my attention at most.
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alfredo
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PostPosted: Jan 23, 2025 15:59    Post subject: Re: Limonitized garnet?  

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.
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Bob Carnein




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PostPosted: Jan 23, 2025 16:19    Post subject: Re: Limonitized garnet?  

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.
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Herwig




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PostPosted: Jan 23, 2025 21:39    Post subject: Re: Limonitized garnet?  

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
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Bob Morgan




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PostPosted: Jan 23, 2025 22:31    Post subject: Re: Limonitized garnet?  

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.
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Robson Vieira




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PostPosted: Jan 24, 2025 04:07    Post subject: Re: Limonitized garnet?  

alfredo wrote:
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.

Ok friend. Now i have got you. Nice explanation, clear and precise. Thank you
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Robson Vieira




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PostPosted: Jan 24, 2025 04:11    Post subject: Re: Limonitized garnet?  

Bob Carnein wrote:
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.

Hi Bob. You make me feel anxious to solve the question. I'm going to analyze further the crystal in order to figure out its precise shape. I'm going to bring more info. See you.
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Robson Vieira




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PostPosted: Jan 24, 2025 04:15    Post subject: Re: Limonitized garnet?  

Herwig wrote:
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


Hello my friend. You have instigated me to research a little further. I'm going to count precisely the number of faces and bring here to our discussion. See ya.
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Robson Vieira




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PostPosted: Jan 24, 2025 06:06    Post subject: Re: Limonitized garnet?  

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.



20250124_073443.jpg
 Description:
'Front' reference side. Notice the cube formed.
 Viewed:  1044 Time(s)

20250124_073443.jpg



20250124_073455.jpg
 Description:
90° rotated.
 Viewed:  1028 Time(s)

20250124_073455.jpg



20250124_073957.jpg
 Description:
180° rotated
 Viewed:  1032 Time(s)

20250124_073957.jpg



20250124_074028.jpg
 Description:
270° rotated
 Viewed:  1030 Time(s)

20250124_074028.jpg



20250124_074433.jpg
 Description:
Apparent square face
 Viewed:  1038 Time(s)

20250124_074433.jpg


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