What's this magnetic rock? Is it possible it could be gunflint chert (a.k.a "stromatolitic chert")? Found along Lake Ontario.

[Rogue Embryo]

 

 

Edited March 12, 2022 by Rogue Embryo
clarification

[Harry Pristis]

Not likely a gunflint.

 

[Rogue Embryo]

I'm referring to "gunflint chert," (a.k.a. stromatolitic chert), which contains evidence of ancient stromatolites:

 

https://en.wikipedia.org/wiki/Gunflint_chert

 

From the website https://ucmp.berkeley.edu/precambrian/proterozoic.php:

 

"A sample of stromatolitic chert from the Bitter Springs Formation of central Australia, about 850 million years old. Note the typical fine banding patterns."

 

Edited March 12, 2022 by Rogue Embryo
clarification; added website and image

[hemipristis]

If it is magnetic, its not chert.  Chert is made of cryptocrystalline quartz (silicon and oxygen), not iron or chrome (which are required for magnetism)

[Rogue Embryo]

Magnetism is indeed the case with stromatolitic chert. Again, please refer to the Wikipedia article on "gunflint chert":

 

"The Gunflint chert (1.88 Ga) is a sequence of banded iron formation rocks that are exposed in the Gunflint Range of northern Minnesota and northwestern Ontario along the north shore of Lake Superior. The Gunflint Chert is of paleontological significance, as it contains evidence of microbial life from the Paleoproterozoic.^[2] The Gunflint Chert is composed of biogenic stromatolites."

 

https://en.wikipedia.org/wiki/Gunflint_chert#:~:text=The Gunflint chert (1.88 Ga,north shore of Lake Superior.https://en.wikipedia.org/wiki/Gunflint_chert#:~:text=The Gunflint chert (1.88 Ga,north shore of Lake Superior.

 

Also, please see this article, "Uncovering the Origin of Magnetism in Early Proterozoic Rocks From Lake Superior":

 

https://naturalhistory.si.edu/sites/default/files/media/file/2018-curry-poster.pdf

[JohnJ]

I see nothing about the rock you posted that suggests any kind of chert or flint.  It is missing key diagnostics, like satin or glassy texture and conchoidal fractures. It looks nothing like the flints you referenced.  However, it does have the characteristics of metamorphic rock.

[Lone Hunter]

So you were able to do this test for magnetic particles?                       Magnetic tests designed to diagnose the presence of magnetofossil chains (low temperature delta-delta test and FORC diagram fingerprinting) performed on a sample from the 1.88 Ga Gunflint chert yielded positive results. To further investigate the origin of this signal we conducted a detailed petrographic study. Thin sections were prepared for transmitted and reflected light microscopy and scanning electron microscopy. Chemical composition was determined using energy-dispersive X-ray spectroscopy and Raman spectroscopy. The chert is characterized by an abundance of euhedral siderite crystals. A magnetic field map collected with a quantum diamond microscope (QDM) showed that the magnetism of the sample is associated with the siderite crystals, with maximum field intensities matching crystal edges. Focused ion beam nanotomography of a siderite crystal edge revealed a thin (<100 nm) layer with variable thickness and an overall web-like morphology between the iron carbonate and the silica matrix. The magnetic evidence, coupled with the morphology of the layer, which includes strands the size of single domain particles, suggest that this layer is the source of the magnetofossil-like signature observed in the magnetic tests. 

[Rogue Embryo]

Obviously not. I used a strong magnet. I'm a beginner trying to learn. I'm not sure what your purpose was in quoting this text, which sounds very complicated to me, a non-scientist, and asking me if I was able to conduct such a specialized test.

[Harry Pristis]

5 minutes ago, Lone Hunter said:

So you were able to do this test for magnetic particles?                       Magnetic tests designed to diagnose the presence of magnetofossil chains  . . .   

Good work, Lone Hunter!  I was about to recommend these tests, particularly the focused ion beam nanotomography, but you beat me to it!  

[Rogue Embryo]

Thanks, @JohnJ. I'm very intrigued by the stromatolite chert, which I had never heard of. You're right, my rock doesn't look like chert, and I'm sure you're correct about it being metamorphic.

[Lone Hunter]

A metamorphic rock should not be magnetic on any scale you could detect.  Are you sure it's magnetic?  Does the magnet stick to every part of it?

[Rogue Embryo]

I have several gneisses that are magnetic.

 

The sample rock in question is magnetic, mainly on its dark side.

[Lone Hunter]

The dark part could be magnetite but doesn't look like it to me,  that's the only thing that could be magnetic. 

[FranzBernhard]

Quartzite with very sparsely, very finely disseminated magnetite?

The dark layer has a stronger magnetite enrichment, but its far from pure magnetite. Maybe I am seeing a few, tiny crystals of magnetite on that layer?

 

1 hour ago, Rogue Embryo said:

I used a strong magnet.

Neodymium magnet?
Try other rocks and their magnetic properties with your magnet and compare.

 

1 hour ago, Lone Hunter said:

A metamorphic rock should not be magnetic on any scale you could detect.

Metamorphic rocks containing 0.X to X.X Vol.% magnetite are not especially rare, but also not extremely common, though.

Franz Bernhard

[Lone Hunter]

Yes I should have worded that better, I wasn't thinking of ones with magnetite inclusions, just the plain ole metamorphic rocks. .   How disseminated can magnetite be and still attract a household magnet?

[hemipristis]

4 hours ago, Rogue Embryo said:

Magnetism is indeed the case with stromatolitic chert. Again, please refer to the Wikipedia article on "gunflint chert":

 

"The Gunflint chert (1.88 Ga) is a sequence of banded iron formation rocks that are exposed in the Gunflint Range of northern Minnesota and northwestern Ontario along the north shore of Lake Superior. The Gunflint Chert is of paleontological significance, as it contains evidence of microbial life from the Paleoproterozoic.^[2] The Gunflint Chert is composed of biogenic stromatolites."

 

https://en.wikipedia.org/wiki/Gunflint_chert#:~:text=The Gunflint chert (1.88 Ga,north shore of Lake Superior.https://en.wikipedia.org/wiki/Gunflint_chert#:~:text=The Gunflint chert (1.88 Ga,north shore of Lake Superior.

 

Also, please see this article, "Uncovering the Origin of Magnetism in Early Proterozoic Rocks From Lake Superior":

 

https://naturalhistory.si.edu/sites/default/files/media/file/2018-curry-poster.pdf

Ah, I see my problem.  I assumed "gunflint chert" was a lithologic description, but it's not.  The Gunflint Chert is a formational designation of associated rocks, which includes the banded iron formations from that area.  While I knew of the famed formations, I didn't know their name.  My bad. The "Gunflint Chert" rocks consist of interbedded chert with iron oxides, the latter of which would be magnetic.

 

A few things:

• I don't know where you found the specimen in relation to the GC exposures. You're in the right general area.
• I've not seen one up close, but the images I found online have a passing resemblance, so it could be.
• That said, the fact that it is laminated and magnetic does not, in and of itself, make it GC.  Location would help, or someone more familiar with the material could provide more information.

Interesting piece, if so!

[Mahnmut]

Hi Rogue Embryo,

there is often some confusion about magnetism in minerals.

Most iron compunds/minerals are not noticeably ferromagnetic (the kind of magnetism that is attracted by a magnet), Not even all kinds of steel are, most ironstone concretions are not.

But magnetite, surprisingly ^^, is.

While it would have been hard to notice disseminated magnetite in a rock with a "normal household magnet" from the eighties(the kind I played with as a youngster), with the strong rare earth magnets we have today (the kind that can kill a kid if swallowed) it can  be noticeable in much smaller concentrations.

I could not find in the text you provided if the samples from the gunflint chert are macroscopically ferromagnetic (noticeable with a magnet like your sample) or if they reveal their magnetic properties only under the complicated measurements mentioned above.

Although the gunflint chert formation does not have to be chert as I learned here, I agree with the fellow forum members that this rock does not look like a promising candidate for fossils (being metamorphic).

Best Regards,

J

[Rogue Embryo]

I did not say that I was using a "household magnet." I'm using neodymium magnets.

 

With all due respect to the scholarly minds gathered here, I'm not yet convinced that my rock is metamorphic. This thread contains several misunderstandings and misdirection, and I don't see general agreement among the contributing members about this rock. I'm leaving the door open for the possibility that the rock may in some way bear traces of ancient stromatolites. To close that door, to my mind, would be premature. Please understand that I don't have any stake in the rock being stromatolitic. However, I would like to hear the opinions of geologists / paleontologists who are more familiar with these types of rocks.

 

Hemipristis, thank you for the clarification. I found the rock in Toronto, along Lake Ontario. Lots of igneous and metamorphic rocks end up in the lake, far from their origins, for various reasons. I will just say that I have never seen a metamorphic rock (i.e., the usual gneisses from Ontario) that looks quite like this one.

[Ludwigia]

13 minutes ago, Rogue Embryo said:

However, I would like to hear the opinions of geologists / paleontologists who are more familiar with these types of rocks.

 

As I'm sure you'll agree, the scholarly minds here are trying to do their best based on their knowledge, but I don't think that it's possible for them to come to a conclusive judgement in this case just based on a few photos and your information. So I would suggest that you take your sample to the ROM where you can ask a resident geologist for his or her expert opinion.

[Rogue Embryo]

Thank you, Ludwigia. I was responding to the person who said, "I agree with the fellow forum members that this rock does not look like a promising candidate . . . "

 

I agree with you about taking the rock to ROM. Excellent idea.

[Mahnmut]

Hi again,

I was speaking in your favor as I meant to say that others talking about household magnets may not have considered the strength of the magnets you where indeed using,

supporting your argument that you can detect disseminated magnetite without hightech.

On the other hand I can not clearly tell from the pdf poster if that kind of easily measured magnetism is even present in the stromatolithic rocks we are talking about. If it is your main reason to think that stromatolithe may be what you have, I do not think it is very strong evidence.

Speaking for myself, I am not even close to being an expert on that formation, just a guy with a broad interest, and some knowledge I hope, on mineralogy and paleontology.

I cannot dismiss the possibility that you have a piece of stromatholite, but it does not look like it in my eyes.

There are a lot of forum members with much more specialized knowledge, chances are good that someone with knowledge of the area comes along and can help you, but not all the members read the forum on a daily basis.

Curious to hear what comes of it,

Best Regards,

J

 

[Rogue Embryo]

Thank you, Mahnmut. I'm curious as well. I do appreciate everyone's contributions to this discussion. I find it fascinating, and it is helping me to gather evidence for the pros and cons on the identity of the rock.

[Kane]

Apart from the ROM, you could probably bring it to the UoT's earth sci department. Someone there would likely recognize it on sight as these glacial erratics are fairly commonly distributed throughout much of the southern Ontario region. I know I've encountered several of this particular type out here in the west of the province under its thick blanket of till. 

[Rogue Embryo]

Good idea also.

 

This is what really puzzles me. The rock is flat, about the size of my cell phone only thicker, and the layers of the rock align with that flat shape (sorry, I didn't photograph the rock from a sideways perspective). However, the dark "layer" in the first photo seems to have its own lineation, in a diagonal direction -- in the first photo, from upper right to lower left. If it's gneiss, you'd normally see all the lines (separation of minerals) perpendicular to the direction of the pressure on the rock.

[Kane]

There are a number of geological processes that can explain the differential striation, such as slickensides.