November 3, 2011
Mystery Rock #4
Posted by Evelyn Mervine
Time for another mystery rock! This one was sent to me by Jess back in July. Jess, I’m sorry this took me so long to post. I blame my thesis… and wedding busyness… and also the fact that this rock actually has me stumped. I’m afraid that I can’t provide too much more insight into what this rock might be, other than that I think it’s sedimentary. Not very helpful, I know. I’m hoping that other geologists in the geoblogosphere can provide some more useful insight.
Here’s Jess’s original message to me:
Hello Evelyn!
I would SO appreciate your expertise in figuring out what kind of rock I have. My husband and I just recently took a trip to Traverse City, MI, and I found this rock about a mile from the shore in Lake Michigan. We took it to a few rock shops, and one owner said he had only seen a couple in the past, and that is was a jasper-hematite “blend”. He also said that one man brought one in, wanting to find out if he could tumble it, and because it is so hard, the shop owner told him it would take about 9 months to tumble. The man cut his rock in half with a wet-saw, which reportedly took about 6 hrs. to cut all the way through. The pattern on the outside is apparently the same throughout the rock. I’m barely able to scratch it with a roofer’s nail, and despite thorough cleanings it still smells faintly of sulfur (?). If there are any other details you need, please let me know! I would LOVE to know what this rock is, if you could help me out. Rock-hounding is an absolute passion of mine… determining what it is I’ve found is half the fun for me, but this time I’m stumped! Thank you either way:)
-Jess
Well, I’m stumped, too, Jess! Hopefully some other geologists can help us out.
Here are several more pictures of the mystery rock:
Any ideas, anyone?
*******************
More Mystery Rocks:
jasper/hematite blend is probably essentially right – looks like fracture-filling and Leisgang banding FeOx cements. Not a true breccia because it appears to have been cemented one fracture at a time – I don’t see any evidence for clast rotation. But the pattern of sharp angular edges with successive rounding of cement rings toward the center of rock clasts is consistent with diffusion of the cementing fluids from discrete fractures into the wallrock. cool.
It reminds me of a Petoskey stone, but probably only because they are so abundant in the area. http://en.wikipedia.org/wiki/Petoskey_stone
I would just add that this sort of rock is not part of the local bedrock near Traverse City, MI, but most probably is a glacial erratic carried down from the Upper Peninsula in the area near Marquette, MI or possibly further north in Canada. No doubt it’s a keeper, though, and deserves a spot of honor among your collection of Petoskey stones. 🙂
I’ve seen hydrothermal breccias that look much like that. Second Ron’s observation, too: this is not local to Traverse City area. The clasts are softer- or at least, more susceptible to weathering and erosion- than the vein fillings as well. This suggests that the veins and clasts are not simply differently colored versions of the same material, but different substances. The veins are likely some version of chalcedony/quartz, while the clasts may not actually be nameable; there isn’t a good nomenclature for metasomatic rocks.
I’d say it’s a lump of concretionary ironstone: mostly clay with a carbonate component, probably including siderite (iron carbonate; brown) and iron hydroxides (goethite, “limonite”; brown). The black stuff probably is or includes microcrystalline pyrite or marcasite which is suffering from “pyrite disease” (chemical weathering to iron oxides and sulphuric acid in the presence of water), hence the sulphur smell. This specimen wouldn’t look out of place in many Cretaceous formations where I live, in the Western Canada Sedimentary Basin, though I wouldn’t hazard a guess as to its provenance in Michigan.
Petoskey Stone is fossil coral, which this clearly isn’t. The polygonal lines on this specimen are due to jointing and/or diagenetic, concretionary growth boundaries.
I would have to agree with Howard. My first thought was ironstone concretion… For the same reasons, too… I grew up in the Western Canadian Sedimentary basin too and concretions are pretty common in the sedimentary rocks there. =)
My sedimentary petrology final exam is older than your birth certificate, but I’m inclined to agree with Howard. Some of the clasts look like material different from the matrix and the clasts themselves. Could the sulphur smell come from some hydrated iron sulphate mineral?
I would like to submit images of a some very similar MYSTERY rocks that I found in Utah. Because I’m new in this blog I right now do not how to. Can somebody give me advice, please?