Probably not a fossil, but can you ID the mineral?

[winnph]

I took my 8 year old on a  concretion hunting expedition to the Lincoln Creek formation over the weekend, and we didn't find anything too great, mostly they contained this reddish brown crystalline mineral in various unidentifiable shapes.

 

My son says he thinks this one is a fish, and I told him I didn't see it, but he said I should "ask the internet" because I "don't know everything," so I'm asking away!

 

Any thoughts on what the mineral is that's inside these? It doesn't appear to be iron-based since it weathers pale, not rusty, as shown by the second photo. I forgot to include something for scale, but the rectangular faces of the broken concretion (left side of photo) measure about 5cm x 4cm. 

[winnph]

Second photo showing the weathered face (this is a different concretion with the same mineral as the one above)

[ynot]

I would go with a concretion and the central part fits an iron rich section.

In a concretion the "concreting" mineral(s) fill the microscopic voids in the matrix, so when the iron erodes out it can leave the other minerals- making for lighter colors.

[Mark Kmiecik]

Definitely not fossil. I can see the part that looks like a fish fin and tail, but there should be underlying structure and there just isn't any. Also the specimen would have very regularly spaced intervals between those underlying structures. Also missing. 

 

P.S. -- Yeah, dad, what would you know? You're only 3 or 4 times as old as he is, right?

[hemipristis]

The rock appears to be limestone, making the crystalline, fracture-infill most likely calcite (the primary mineral that comprises limestone), which is brownish due to iron (and probably manganese) staining.

 

One surefire way to test this idea is to take a piece and add some vinegar to the mineral. If it fizzes, it's calcite.

[winnph]

20 minutes ago, hemipristis said:

One surefire way to test this idea is to take a piece and add some vinegar to the mineral. If it fizzes, it's calcite.

 

And it fizzes! I clipped a laser pointer lens to my phone camera lens and managed to get this almost-focused photo of the crystal structure, too. I told him maybe next time he'll find a fish...