Iron nodules

[Kato]

Hi, I'm having a brain lapse and need help identifying the form of iron I'm seeing. Some of the nodules I found seem to have a cubic structure much like standard pyrite but others are perhaps suggestive of hematite in its many crystalline forms? Also, some of the crystals are a glassy reddish color and slightly gemmy in appearance while others are completely black. Apologies for the shots. They were taken outside in a shady area but some of them show a yellow reflectance on the top facing reflective faces. Only the 3rd shot of the black nodules actually has yellowish and red staining.

 

These were found coincident to a formation known as the Sly Gap formation which is a Devonian formation here in southern New Mexico. I believe I recall reading that that formation had nodules like this. The nodules range in diameter from 10mm to 30mm. 

 

 

 

 

 

 

Edited March 14 by Kato
entered incorrect formation

[Mark Kmiecik]

Pyrite spheres. Beautiful specimens.

 

The red crystals may be garnet, although garnet crystals are not cubic.

Edited March 14 by Mark Kmiecik

[Kato]

They are non-magnetic and I've yet to do a streak test...

 

Also, I initially entered the incorrect name of the formation. The correct name is the Sly Gap formation of the Devonian here in southern New Mexico

Edited March 14 by Kato

[Tidgy's Dad]

Pyrite is not magnetic.

I agree that these are lovely pyrite spheres.

[ynot]

garnet would not be found in that environment.

[Kato]

I once knew the name of the form of iron crystal that appeared as a crystalline form that was dark red in coloration...but I can't seem to pull it up from the depths of my memories or maybe I'm just dreaming! 

 

Here's an excerpt from a paper of this formation

 

The "black" shale of the Sly Gap formation is noncalcareous, and the precise color is grayish or brownish black. Rounded iron sulfide concretions up to an inch in diameter are common in these shale beds.

Edited March 14 by Kato

[Ludwigia]

These are definitely rounded pyrite (iron sulfide) concretions. They make for very pretty colors when they begin to oxidize.

[Kato]

Thanks to everyone for your help.