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Identity HelpMystery Beach Pebble

30th Dec 2019 00:25 UTCBernadette G

03549780015776579337524.jpg
I found this tiny pebble at a Lake Michigan Beach in Illinois. Like most mineral rocks I find, chances are it originates from the Lake Superior area and was brought here by glacial drift.

Last summer when I picked it up, I just thought it might be patterned porcelain, so I dropped it in with my other man made rocks: the slag, tumbled terrazzo, brick "balls" and porcelain bits, etc.  But now that I'm looking at it closer, i see there is a vug and the "pattern" goes all around the little pebble, so it must be a natural rock.
 
Since my last ID inquiry, I have - thanks to my daughter - acquired a LED lighted hand lense, streak plates and a $7.50 phone clip-on microscope that lets me quickly see and photograph small details. The photos aren't great quality, but still better than I expected.  I'll post a close-up of the vug and inclusions as a reply next.

The rock left a reddish brown streak just once.  I can't replicate this though, no streaks now, no matter which side I try with. And I can't tell if it's the red or black parts that caused it. They're intermixed all around the rock. 

The rock isn't attracted to magnets.  

Is it possible to tell what minerals they are and how this little pebble originated?
Thanks so much!

30th Dec 2019 00:27 UTCBernadette G

08199340015776656148815.jpg
Vug close-up

30th Dec 2019 00:40 UTCBernadette G

06385520015776664087280.jpg
details on inclusions

30th Dec 2019 04:46 UTCPaul Brandes Manager

Bernadette,
Can you possibly take a steel needle and attempt to scratch each of the colours (brown, black, and white) to see if it leaves a mark? I know the specimen is small, but it might narrow down the possibilities a bit.

30th Dec 2019 05:27 UTCBernadette G

Hi Paul,
I did as you instructed. Tricky business. That thing is so small. But I did manage with the help of a lightly padded vise which helped to hold the slippery pebble in place. 
So the steel needle left steel on all colors. The steel streak looked silvery on the red and black areas and blackish on white. All streaks came off when I rubbed the rock with a wet finger. 

30th Dec 2019 05:48 UTCPaul Brandes Manager

So it seems the materials are harder than 6-6.5, the typical hardness of a steel needle.
I don't recognise this as coming from anywhere further north. I wonder if this isn't something that was carried in as ballast in a ship and dumped in Lake Michigan?

Btw, much of the material you're finding is likely not coming from west/central Lake Superior, but points further east in Ontario as that is the direction the ice came from.

30th Dec 2019 10:16 UTCBob Harman

As PAUL says, if natural, certainly nothing even remotely local anywhere near to Illinois.  Also doesn't look like anything the glaciers left so maybe in a ship's ballast.

But it is so pleasingly colorful, it really looks more man made. I think, if it was truly  natural, certainly we would see more collected examples of something like that.
CHEERS......BOB

31st Dec 2019 07:45 UTCBernadette G

09944190015777755798100.jpg
Paul:
Btw, much of the material you're finding is likely not coming from west/central Lake Superior, but points further east in Ontario as that is the direction the ice came from.

Yes, I can see that the granites come from the canadian shield and the basalts and rhyolites could come from the northeast as there were ancient extinct volcanoes in Ontario as well.

But do you think it possible that one of my favorite beach rocks, which I found near the Wisconsin border, a Liesegang sandstone with perfect tiny "target" circles (see photo above), could have originated in Ontario?  I do know that there are visible Liesegang outrops by Lake Superior, so I've always assumed that to be the source.

And how about the Jacobsville and the Picture sandstone rocks that are quite plentiful at the beaches here? From Ontario too?

And what about the agates I find? I don't think there is a source for these in Ontario, or is there?  


Bob:
if natural, certainly nothing even remotely local anywhere near to Illinois. Also doesn't look like anything the glaciers left so maybe in a ship's ballast.

I'm more inclined to think it's man made than I believe that it came to my beach as part of ballast. That would be ok with me, as I'm not above collecting man made materials as well. I think some of it quite interesting.  :-) 
And yes, I agree, that little "pebble"  IS pleasingly colorful, at least when photographed and blown up. 

Ballast I've seen at  South Milwaukee and south Chicago beaches which are both close to commercial harbors, looks out of place at a beach. The rocks tend to still be sharp and rough, not smooth and round like proper beach rocks, and where it is found, there is a lot of it. 
But the Chicago Northshore is residential, the small harbors serve leisure boats only. I've not seen any ballast at the beaches around here.  There are some easily accessed beaches where vintage construction materials can be found: from tumbled bricks in every shade of yellow, orange and red to lovely terrazzos and even tumbled asphalt (quite beautiful, actually),  but no ballast. 

I'll probably never know what and how this little pebble became what it is. If it'll stay a little mystery rock, that's all good too.


1st Jan 2020 14:06 UTCLarry Maltby Expert

03861540015778874633651.jpg
Bernadette,

The above photo is of an Ahumada Meteorite at the Smithsonian Institution. I show this only because of the similar appearance to your pebble. It has been said over and over on this site that meteorites can only be identified by professionals. I see that you are a researcher otherwise you would not have known about Liesegang Rings. If you do a google search on the key words “Meteorite, Ahumada” you will see a lot of information on this. It is very unlikely that your pebble is a Meteorite. Anything in the stone that appears to be metallic would be a clue.

I do think that there are Liesegang Rings on your second specimen.

1st Jan 2020 15:56 UTCLarry Maltby Expert

Another thought,

If you wanted to differentiate between natural and manmade an EDS analysis of the white stuff would tell a lot for a little expense.

2nd Jan 2020 08:23 UTCBernadette G

Larry Maltby Expert  ✉️

The above photo is of an Ahumada Meteorite at the Smithsonian Institution. I show this only because of the similar appearance to your pebble. It has been said over and over on this site that meteorites can only be identified by professionals. I see that you are a researcher otherwise you would not have known about Liesegang Rings. If you do a google search on the key words “Meteorite, Ahumada” you will see a lot of information on this. It is very unlikely that your pebble is a Meteorite. Anything in the stone that appears to be metallic would be a clue.
I do think that there are Liesegang Rings on your second specimen.
 Larry, 
Comparisons to a meteorite? Haha, I wish...
Alas, similar appearance of rocks in photos signifies nothing much, does it? That meteor weighs almost a kilo, so is quite a large chunk of rock, whereas my pebble is tiny and its weight doesn't even reach one single gram. So, placed next to each other, they actually wouldn't look similar at all. Also, nothing on my pebble looks like it's metallic to me. The close-up photo I posted above is deceiving, as the LED of the clip-on miniature microscope creates that artificial sheen on the colorful parts. That  shine isn't visible under regular lights. 


I see that you are a researcher otherwise you would not have known about Liesegang Rings.

Thank you. But you give me credit where not much credit is due. I had initially googled descriptions of the Liesegang rock, but not knowing what terminology to use, Google's results were rubbish.  Image searches of the rock also brought up nothing helpful, so  I simply asked my daughter - who, lucky me,  is a geologist.  She explained to me the complicated processes by which these rings get created and directed me to a USGS site, which accepts questions about rocks and minerals, etc.  And yep, a group of helpful geologists there confirmed the patterns to be Liesegang circles. 
And here I am on mindat, where my "research" consists of posing questions to another group of friendly experts. 

Thanks Larry. You made my day, comparing my tiny little pebble to a  meteor at the Smithsonian.  

1st Jan 2020 17:10 UTCPaul Brandes Manager

I'm not sure how close you are to either Chicago or Milwaukee, but each has a natural history museum that you may want to take this specimen to and have them look at it. The thing about ballast is that it can be a multitude of things. In addition, ballast can easily have a tumbled smooth appearance depending on how long it's been in the water and how far it has traveled from where it was dumped. One certainty about rocks in the Great Lakes is that they don't tend to stay in one place very long.....

Also Bernadette, you mentioned earlier that "...what about the agates I find? I don't think there is a source for these in Ontario, or is there?" Actually, Mamainse Point and Michipicoten Island north-northwest of Sault Ste. Marie are well documented localities for beautiful agates. I've also heard of agates being found on Caribou Island and Ile Parisienne in Whitefish Bay, so it is very possible your agates are coming from these places based upon ice flow direction. 

I believe your liesegang rings are exactly that. However, I have seen compositional banding in a sandstone appear much like this as well. It would help to examine this in person and look at it from multiple angles.

3rd Jan 2020 04:52 UTCBernadette G

Paul, 
Good idea, to take it to one of the natural history museums.
I think I just might do that. I haven't been to the MPM in Milwaukee. Maybe time to change that. 

As to the ballast, I still doubt it, but of course anything is possible. It's just that everytime I find something unusual in Lake Michigan, it's suggested that the piece is ballast. But the likelihood for me to keep finding ballast among the gazillion other beach stones is really very, very small. I don't think there is anywhere near as much ballast lying around as people seem to think. 

Actually, Mamainse Point and Michipicoten Island north-northwest of Sault Ste. Marie are well documented localities for beautiful agates. I've also heard of agates being found on Caribou Island and Ile Parisienne in Whitefish Bay, so it is very possible your agates are coming from these places based upon ice flow direction.

 Oh I do agree. All these places are in the Lake Superior area. When you mentioned "points further east" in Ontario, I thought you meant much further east. 

I believe your liesegang rings are exactly that. However, I have seen compositional banding in a sandstone appear much like this as well. It would help to examine this in person and look at it from multiple angles.

I'm not quite sure what you are saying in regards to compositional banding in sandstone looking similar. I would expect it would.   
I'd be happy to take photos of my little "bulls-eye" rock from various angles and post them here or on a new thread. Let me know if you're interested.

Thanks for helping.



1st Jan 2020 17:25 UTCMatt Courville

Hi Bernadette,  my vote would be that the brown mineral is garnet.  You can see that some sections are stand alone chunks while others have a thin band around the blackish mineral which may be some type of pyroxene.  I would take it that the white material is a variety of quartz after it was geologically altered into a conglomerate  form and then polished down by water action.

1st Jan 2020 19:04 UTCGeorg Graf

Hi Bernadette, my guess: A fine graind breccia of the brown and black material, likely from a vein. That means, by tectonic processes a crack was formed in the earth crust. From the walls of the crack little pieces of the brown and black wall rock were fallen in the crack. Later the nuggets are cemented by hydrothermally precipitated, transparent and white Chalcedony. (Just a thought.) Georg

1st Jan 2020 19:11 UTCGeorg Graf

A further look on your "00:40 UTC" photo leads me to this speculation: The material from the wall rock fallen in the crack originally was black. By hydrothermal fluids/processes it was altered to the brown substance. Then the brown/black pieces were cemented by the white material, maybe Chalcedony. Gg

3rd Jan 2020 05:37 UTCBernadette G

Matt and Georg, 

Oh my!
From man-made material, to ballast from who-knows-where,  to a :"meteor look-alike", to quartz with garnets and pyroxene inclusions, to chalcedony which cemented the black and brown substances together. Love both of those last suggestions. 

I have to admit, my head is spinning just a bit... so many possibilities of what this tiny pebble could be and where and how it might have formed.  

I have nowhere near enough knowledge to judge the likelihood of any of the guesses/theories put forward (other than the ballast, which I think is very unlikely)
I love both your theories, but as Bob and Paul pointed out above: nothing like this is known to occur anywhere near Illinois, the Lake Superior region included. Still, who knows?

Thank you so much for contributing your knowledge and valuable time. I really do appreciate it.

17th Jan 2020 04:41 UTCGregg Little

Bernadette;
It certainly is an unusual rock that I would have to see in person to make any definitive determinations but having said that, I am leaning towards the sedimentary rock camp as well (sandstone?).

Some of the textures that are persuading me are the relatively uniform grain sizes (about 0.25 to 1.0 mm = medium to well sorted) and consistent angularity of the grains (sub angular to sub rounded) both in the red and black grains. Small range in grain size and no sharp angles on the grains or fragments argue against it being a breccia.

Zooming in on areas of the white material, patterns of cementation or secondary void fillers can be seen in a botryoidal or radial pattern. Possibly this could be seen in the vug.

Finally, the dark grey material appears to form cores or nuclei which the red mineralization precipitates around, or coats. This has a strong resemblance to clastic carbonate rock where algal and bacterial activity causes mineral precipitation that coats fragmented fossil material. A depositional analogue might be oolitic ironstone, but not specifically that texture. There are not multiple layers in your sample as opposed to those seen in ooids.

19th Jan 2020 18:04 UTCBernadette G

08414900015794495868887.jpg
Hi Gregg,

Thank you for your interest in my little mystery pebble and reviving my ID help thread. 

Reading various discussions here, I've come to realize just how difficult it is to identify a rocks from a photograph.  I can totally understand why geologists would prefer to handle a specimen, doing various tests, maybe an EDS analysis and taking thin sections, etc. 
So I really appreciate you taking the time to look and think and share your thoughts.
It's amazing really! 

Let's see if I understand you correctly. (Please know, If I don't, it's entirely my ignorance in regards to basic geology, processes and terminology that's to blame.) 
So let's see:
I am leaning towards the sedimentary rock camp as well (sandstone?).
So the reddish/brown and black / dark grey fragments are sand grains that together make up the sandstone.  

Some of the textures that are persuading me are the relatively uniform grain sizes (about 0.25 to 1.0 mm = medium to well sorted) and consistent angularity of the grains (sub angular to sub rounded) both in the red and black grains. Small range in grain size and no sharp angles on the grains or fragments argue against it being a breccia.

Yes, googling breccia, I see it's composed of freshly broken,  >2mm, angular fragments, not look well sorted, meaning of various sizes, while the grains in my pebble  are slightly rounded off  (tumbled for some time?) and are fairly uniform in size. 

Zooming in on areas of the white material, patterns of cementation or secondary void fillers can be seen in a botryoidal or radial pattern. Possibly this could be seen in the vug.

With "botryoidal or radial pattern", do you mean the faint white disk shapes I circled in blue?  I have to admit, I never saw these until now.  I am familiar with 3-D botryoidal calcite in my tabulate coral fossils, they look like tiny snowballs. So these botryoidals here never grew three dimensional? They didn't start out as "snowballs" and then were subsequently smoothened/flattened by being beach tumbled?


Finally, the dark grey material appears to form cores or nuclei which the red mineralization precipitates around, or coats. This has a strong resemblance to clastic carbonate rock where algal and bacterial activity causes mineral precipitation that coats fragmented fossil material. A depositional analogue might be oolitic ironstone, but not specifically that texture. There are not multiple layers in your sample as opposed to those seen in ooids.


Many of my Lake Michigan found rocks and fossils are stained either reddish brown, red and even pink. The light red stains by the vug are then likely to be coated fossil material.
Locally, this is explained as iron/rust stains, very common in the Great Lakes.  


My only issue is the "carbonate" rock. That would mean the rock would be either limestone or dolomite. Thing is, no part of the rock can be cut by my steel knife. In fact, the knife leaves a metallic streak on all parts of the rock and the streaks rub off.
Also, the white material has the look and "feel" of glossy, opaque white quartz and the sound it makes when dropped is different from the sound made by small limestone pebbles. 

I do know that quartz stains (I have lots of samples of reddish quartz veins in my rock finds, so the red stains wouldn't exclude quartz, no? 
Is it possible that instead of carbonate rock it could be quartz that's cementing the sandstone together? 

21st Jan 2020 23:23 UTCGregg Little

Hi Bernadette; Thanks for the inquisitive reply and I'll try expand on your points and questions.

Your first question about sandstone is basically correct.  Sandstone is only a term for the size of the grains (termed framework) which for sandstone ranges from 0.625 mm to 2.0 mm regardless of the grain's composition.  In your sample the grains seem to present two stages of formation; a darker core of the original detrital material then a later stage during transport, and prior to deposition, of coatings or over-growths of the reddish material.

Yes, googling breccia, I see it's composed of freshly broken, >2mm, angular fragments, not look well sorted, meaning of various sizes, while the grains in my pebble are slightly rounded off (tumbled for some time?) and are fairly uniform in size.
The darker cores appear to have been tumbled (transported) for some time but the reddish layer appears to be a stage of minimal transport allowing coatings to grow completely around the dark cores without abrading off.

With "botryoidal or radial pattern", do you mean the faint white disk shapes I circled in blue? I have to admit, I never saw these until now. I am familiar with 3-D botryoidal calcite in my tabulate coral fossils, they look like tiny snowballs. So these botryoidals here never grew three dimensional? They didn't start out as "snowballs" and then were subsequently smoothened/flattened by being beach tumbled?
Yes, that is texture seen in the mineral cementation (void filler) around the grains. They did grow into the 3-dimensional "snowballs" in the void spaces between the coated grains once the grains settled.  When the coated grains settled there is at least 35 to 40% of the sediment that is open space (pore space), plenty of space for the snowballs to nucleate and grow on the surfaces of the framework (sand) grains.  To get the feeling of space in recently deposited sediment just think of golf balls, or marbles, in a jar and how much water that can be held in the spaces between the "grains".

The white mineralization in the pictures filled the void space after the grains settled.  These snowballs were not flattened or smoothed; what you are looking at is cross-sections through the snowballs on the surface of your beach pebble.

Many of my Lake Michigan found rocks and fossils are stained either reddish brown, red and even pink. The light red stains by the vug are then likely to be coated fossil material.
Locally, this is explained as iron/rust stains, very common in the Great Lakes.
I'm not sure of the exact nature of the lighter red stained fragments (three on the lower side of the vug) but I would suggest they have the same origin as the coatings on the dark grains. I wouldn't call these fossil fragments as I don't see any evidence of fossils in the rest of the pebble pictured, only the dark cored grains with a lighter reddish brown coating, possibly precipitated by bacterial action.

In my previous post I only mentioned fossils in reference to the coating of the detrital grains in your sample. In limestone bioclastic deposits, like in reef environments, bacterial and algal precipitated calcareous coatings of fossils fragments is very common.

My only issue is the "carbonate" rock. That would mean the rock would be either limestone or dolomite. Thing is, no part of the rock can be cut by my steel knife. In fact, the knife leaves a metallic streak on all parts of the rock and the streaks rub off.
Also, the white material has the look and "feel" of glossy, opaque white quartz and the sound it makes when dropped is different from the sound made by small limestone pebbles.
According to your hardness test, I agree completely with your ID that the rock is all quartz.  Silicification is a common alteration in sedimentary rock and often, as seen in your sample, silica will faithfully replicate the original sedimentary structures and minerals down to the minutest details. Even your snowballs could have originally been precipitated as calcite but compositionally its all quartz now.

26th Jan 2020 23:27 UTCBernadette G

Hi Gregg,   

Thanks so much for your generosity in patiently answering my many questions and setting right my misconceptions. Your thoughtful comments are very helpful.

I have to say, how much more valuable it is for me, to follow your reasoned explanations versus receiving a quick answer in the form of an ID. Sure, a name allows me to slap a label on a rock, but I wouldn't be any the wiser of the whys and hows. 

I've learned enough on this thread, to realize just how little I do know and that it's time for me to brush up on what I'd learned too many years ago in my HS geology class. I don't remember the course even touching on mineralogy, so I will - somewhat belatedly - educate myself on at least the basics. Should be fun!  

Thanks again Gregg! Hoping to see your comments on some of my ID help threads that I'll be posting soon. :-)
 

19th Jan 2020 19:39 UTCKevin Conroy Manager

Does it fluoresce?

19th Jan 2020 22:27 UTCBernadette G

Hi Kevin,

I have no idea. 
I'd have to acquire a UV light to check that. 
So if the white material would fluoresce, it would indicate calcite. 
Maybe it's time to acquire such a light... I'll report back once I've exposed the pebble to blacklight.




 





19th Jan 2020 22:59 UTCKevin Conroy Manager

It could also possibly be chalcedony as Georg suggested.  A few drops of muriatic acid will tell you if it's a carbonate.   The only way that you're really going to know what this is is by having it tested. 

27th Jan 2020 13:33 UTCHarold Moritz Expert

Bernadette, that is an interesting sample, I'm not from the area so I cant offer much insight (I'm on the fence between an artificial material or some kind of chalcedony) but I wanted to thank you for posting very good photos and including pertinent information - you can see how it will generate useful responses. You may not think of yourself as a "researcher" but the definition is basically someone who poses questions and looks for answers, so you are one!

28th Jan 2020 02:57 UTCBernadette G

Harold, 
Thanks for your kind remarks, you're too kind! 

27th Jan 2020 15:48 UTCD Mike Reinke

Bernadette, having walked those beaches around Kenosha, Wi./Zion, I’ll. the most,  my inclination is to say it is man-made. I found some stuff a little larger, a gray breccia  in a white matrix, that I  thought was pretty enough to wire wrap, but it was suggested to me by even a ‘non-mineral friend’ that handled it that it was man-made. I kind of didn’t want to believe him, but that’s me! I lean very heavily toward what Bob said much much earlier in the thread that if it’s from a natural environment, there would be more of it. It would’ve been classified, defined, somewhere; and likely originate under similar conditions in more than one place on earth. Plus, You should see at least a little more of it washing up on the beach. 
  When I think of the piece I found, and yours, I think of something along the lines of a terrazzo flooring type of material. Not that I’m saying it is or was flooring, just that it has that sort of look to it.
  I budget most of my funds toward collecting my own stuff, but a little of the budget is spent on identifying what I can’t figure out, but I do tend to save that for the more aesthetic pieces.
  You are enthusiastic, you have an attentive eye, and you enjoy learning. You can find satisfaction in all of that whether you know what the piece is, or not. Keep looking, and you may find more interesting stuff that you would rather have spent you’re analyzing money on if your budget is like mine. Just a thought...

28th Jan 2020 08:30 UTCBernadette G

02742210015801810196817.jpg
Hi Mike, 

I started out thinking this thing was porcelain, later on, I thought it might be white opaque glass that(while molten) was dropped into a pile of sand and then discarded.  But I'm pretty convinced now that it's a natural rock. 

Bob wrote this upthread:
But it is so pleasingly colorful, it really looks more man made. I think, if it was truly natural, certainly we would see more collected examples of something like that.
But look at the photo of the pebble along with some tumbled Terrazzo for comparison. It only looks "pleasingly colorful" when photographed as a close-up and then blown up at least x30. In its natural size (1 cm x 0.8 cm)  its pattern is way too tiny to be noticed, it just looks like a muddy-brown unremarkable little pebble. 

Sure, it attracted my attention, but that doesn't mean much.  Because I do pick up odd things nobody else looks at or even notices, but  which for various reasons, I find interesting.
 
See, for example, if I was into jewelry making, I WOULD have wire wrapped that man-made material you found, as I don't think anything wrong with with wearing a bit of concrete jewelry, or dangling tumbled asphalt earrings.  ;-)  

I have no intention to have this pebble tested, but one of these days,  I will follow Paul's advice and when I know I'll be in the vicinity I'll take it to either the Field Museum or the MPM in Milwaukee. 
 As I said in my response to Gregg, I've learned more about how to think about rocks and what questions to ask, by reading people's comments/theories, than if I had just received a definitive ID. 

Thanks Mike! 
 
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