✅ SOLVED Please help identify this Great Lakes rock.

[john17wi]

Hello group.

I found this rock on a beach in Bayfield, WI along Lake Superior. I broke off a small corner of it with a pliers to make sure it wasn't metal, because it looked a little like it by the color. It broke easily. It almost looks like the raised, kidney bean shaped lump is another rock that fused with the flat portion. The entire rock sparkles just a little bit in the sunlight. Thanks in advance for any help you can offer.

 

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[Dougie Webb]

Welcome to TreasureNet!

Neat-looking rock/thing. Did you try to see if a magnet will stick to it and/or if it will attract a magnet if you lay it on a table?

 

[john17wi]

Welcome to TreasureNet!

Neat-looking rock/thing. Did you try to see if a magnet will stick to it and/or if it will attract a magnet if you lay it on a table?

Thanks for the welcome Dougie!

I hadn't thought about that, but did just try it. No magnetization at all. Here are couple more macro photos showing the surface better, as well the corner where I broke off a piece so you could see the inside.

I don't think it's a piece of pottery.

 

[Dougie Webb]

The close-ups help, thanks. Are there foundries in the area? It looks like it could be some sort of slag, especially with the glass-like particles. And iron slag can be non-magnetic, IIRC. One thing makes me doubt my assessment is that slag tends to have "bubbles."

How heavy is it? Do you have a scale that weighs in grams? Do you know anyone with a metal detector?

 

[Red-Coat]

It look to me like hematitic sandstone with a larger pebble or a concretion embedded in the stratification. Hematite itself is not magnetic. It only has that property when magnetite is present as a secondary mineral. Try a streak test on the unglazed back of a porcelain tile. Hematite will streak red to reddish-brown.

 

[blauer]

Limonite!

 

[Dougie Webb]

I think redcoat is right on it. Go to this page - Hematite - and search for sandstone. There's a cross-section there that closely resembles yours.

Do the scratch test too. If you don't have a spare porcelain tile, just use the inside of your toilet tank cover. Or better yet, ask your neighbor to use their bathroom, and do it there, but be sure to leave a secret message like "we are watching you."

 

[How Lucky]

Looks like weathered biotite mica.

 

[john17wi]

I think redcoat is right on it. Go to this page - Hematite - and search for sandstone. There's a cross-section there that closely resembles yours.

Do the scratch test too. If you don't have a spare porcelain tile, just use the inside of your toilet tank cover. Or better yet, ask your neighbor to use their bathroom, and do it there, but be sure to leave a secret message like "we are watching you."

I'm thinking redcoat is right as well. Attached is a picture of the scratch test. Looks like it's streaking as you guys said it would. I did weigh it as well, and it's 21 grams.


If you think the weight, and scratch test strengthen your belief that it's hematite, I'll mark it as solved. Thanks guys!

 

[blauer]

BZZZZz. It's Limonite not hematite.

 

[Dougie Webb]

BZZZZz. It's Limonite not hematite.

But that streak looks reddish rather than brownish. Or rather, to me it does.

 

[Dougie Webb]

I did weigh it as well, and it's 21 grams.

Does your scale weigh in tenths or hundredths? If so, you can easily calculate the specific gravity, which should differentiate it from limonite. It's 3.6-4.0 for limonite and 4.9-5.3 for hematite. Of course, that doesn't take into account whatever that kidney bean looking thing is. Its density could be completely different and throw the whole thing off.

If I were you...I'd get a big hammer and put it under a towel on the garage floor and bust it open to see what the kidney bean is.

 

[Red-Coat]

I'm thinking redcoat is right as well. Attached is a picture of the scratch test. Looks like it's streaking as you guys said it would. I did weigh it as well, and it's 21 grams.


If you think the weight, and scratch test strengthen your belief that it's hematite, I'll mark it as solved. Thanks guys!

BZZZZz. It's Limonite not hematite.

But that streak looks reddish rather than brownish. Or rather, to me it does.

Does your scale weigh in tenths or hundredths? If so, you can easily calculate the specific gravity, which should differentiate it from limonite. It's 3.6-4.0 for limonite and 4.9-5.3 for hematite. Of course, that doesn't take into account whatever that kidney bean looking thing is. Its density could be completely different and throw the whole thing off.

If I were you...I'd get a big hammer and put it under a towel on the garage floor and bust it open to see what the kidney bean is.

It's not unusual for iron minerals to be mixtures and the streak result will of course reflect that. If you're seeing reddishness then there will at least be some hematite and it may or may not be the dominant mineral.

Densities are only an indication unless you have a relatively pure mineral deposit. Hematite is significantly more dense than limonite but a rock specimen may not always exhibit that conclusively. What you have looks to me to be a granular sedimentary deposit with an iron mineral cement which may or may not be a mixture. Hematite is a very common cementing mineral for sandstones and they typically have the colour and appearance of your specimen.

 

[Dougie Webb]

It's not unusual for iron minerals to be mixtures and the streak result will of course reflect that. If you're seeing reddishness then there will at least be some hematite and it may or may not be the dominant mineral.

Densities are only an indication unless you have a relatively pure mineral deposit. Hematite is significantly more dense than limonite but a rock specimen may not always exhibit that conclusively. What you have looks to me to be a granular sedimentary deposit with an iron mineral cement which may or may not be a mixture. Hematite is a very common cementing mineral for sandstones and they typically have the colour and appearance of your specimen.

What he said.