head scratcher...probably shouldnt be but..

[fuss]

I was a rural area in Dodge Co WI stuck at a RR track waiting for a very long train to pass and saw a large pile of black "trap" rock, (what they use to spread along the sides of the tracks). So I hoped out and grabbed a couple to see what they were using in this area
(figured basalt at initial look). At home under the loupe im now thinking black chert/jasper with quartz and Potassium feldspar phenocrysts, but I have not heard of chert having phenocrysts like basalt does, but im fairly certain it is a chert/jasper/chalcedony material. I know this is not that interesting of a rock to most but im pretty interested in getting the ID for it.

The hardness is around 7-7.5, no streak, no reaction to LW or SW UV, but here is the weird one a couple of the pieces are attracted to a magnet slightly and a couple others are not...

Thx for any help. the third image shows a fresh break.

 

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[ToddsPoint]

I don't know what your sample is. I do know that about every RR track bed in TX has flint/chert of some sort for the ballast. I even found black chert ballast (no phenocrysts) on tracks here in IL. I think they use it as it will outlast limestone by many years. Gary

 

[fuss]

no doubt this thing is durable, they use this rock, dolostone, basalt, granite and who knows what else up here.

 

[stdenis_jd]

I think it's a tillite, which would make the k-feldspars "clasts" instead of phenocrysts. It just looks sedimentary, then metamorphic as opposed to primary igneous. I think I see some small magnetite nugs in there too

So being likely a tillite, it's black sand (quartz + basalt) with other larger quartz and k-spar clasts. At least that's my opinion

 

[fuss]

Identified see link and photos in the link. Its some variation of Rhyolite , The quarry in the link was not far from where I found mine.

https://www.mindat.org/loc-262885.html