Jump to content
Bev

Big Stromopoid Agate?

Recommended Posts

Bev

Do you have rhodochrosites there Bev. I still don't think it is agate.

Ed

LOL you hit another new one for me. I looked it up on Wiki, searched the google images, checked MN, and went to the mineral data base. I don't know... B)

That said, 99.9% of all Minnesotan's are fossil and mineral ignorant (me too). That said, anything with lines is an agate to us. :P Even our science teachers go... "Lines? Agate."

It is not red or pink, but I did find images of brownish ones. We have a fair amount of iron ore near Spring Valley. Yes, we have some hot water caves.

What should I try scratching it with to tell the difference between onyx and rhodochrosite? Rhodochrosite is 3 to 4 on the hardness scale.

Share this post


Link to post
Share on other sites
Mr_ed

A knife should scratch it if it is 3-4 because most knives are around 5 . Onyx is 6-7 so a knife would not scratch onyx .. although I believe there may be some softer types.

cheers

Ed

Edited by Mr_ed

Share this post


Link to post
Share on other sites
Mr_ed

Bev.. if you have a sharp piece of agate around.. try scratching it with that...

Ed

Share this post


Link to post
Share on other sites
Tethys

One more link to Equatorial Minnesota for a good discussion on the Platteville and dolomite. Maybe if i link to his blog enough, the owner will join the forum and share his knowledge with us. :)

Share this post


Link to post
Share on other sites
Wrangellian

Do you have rhodochrosites there Bev. I still don't think it is agate.

Ed

Isn't rhodochrosite bright pink to red? I guess we still need to see the results of a scratch test..

BTW When you say onyx, do you mean travertine? I think the Honey Onyx from Golden area is travertine, which is much softer than agate.

I would characterize it as chert, ...

Would you call it a nodule, then, or part of one?

Share this post


Link to post
Share on other sites
Bev

It scratches with a glass like crystal. Now what? :)

Share this post


Link to post
Share on other sites
John K

Agate is found in both Igneous and sedimentary environments but both are formed essentially the same way. The Igneous and sedimentary rocks just act as hosts providing porosity for the agate to form. In Igneous, gas bubbles form in lava and after the rock has long cooled and been buried in an area where groundwater can enter the pockets in the lava and slowly form agate. In Sedimentary rocks fossils or gypsum nodules that may also might have been fossils originally are dissolved away long after the host rock (typically Limestone) has been formed and buried again where groundwater can enter the voids to form the agate. In Kentucky the well known Kentucky agate actually forms in a shaley siltstone in the transition zone between the Nada and Cowbell Members of the Borden Formation. Typically all fossil fabric is destroyed in the geodization process.

Chert on the other hand forms mostly in Limestones and can perfectly preserve fossils in great detail.

For most people the safest way to discuss this type of replacement would be to refer to these materials as silicified which would cover any fossil replaced by quartz.

+1

it's a piece of banded crystalline quartz, formed in a sedimentary void (of which there are many in the strata you've found this in, Bev.) It looks like it came from a rather large one that allowed a thicker than normal deposition of layers of crystalline silica. It differs from Lake Superior agates, as already mentioned, in that those were formed far to the north and transported via glacial drift.

I've found and cut many of these. though both are composed of quartz, these tend to be a degree softer than LSAs, due to their crystalline structure, and they tend to orange-peel easily. But they do polish nicely if you get a good one.

Edited by John K

Share this post


Link to post
Share on other sites
Bev

So it is a really big agate this is also a stromopoid?

Share this post


Link to post
Share on other sites
Mr_ed

Hi Eric.. yes I am referring to honey onyx when I say it is softer than agate. I used to find it at Clinton.

Cheers

Ed

Edited by Mr_ed

Share this post


Link to post
Share on other sites
Mr_ed

Thank you John K. I think we all learned something here.

edit.. I think what I used to think was honey onyx was actually Rhodochrosites.

Regards

Ed

Edited by Mr_ed

Share this post


Link to post
Share on other sites
Bev

Well, I'm still confused. :blink:

What is it?

Share this post


Link to post
Share on other sites
Herb

Generally chert is opaque and agate is translucent. Chert is cryptocrystaline quartz.

Share this post


Link to post
Share on other sites
Bev

Okay, dumb me. This is translucent to me. What is it?

Share this post


Link to post
Share on other sites
Mr_ed

If it can be scratched with agate or a crystal is likely as John K said.. a piece of banded crystalline quartz. That would mean that it is not agate. I have never had anything to do with or any knowledge of stromopoids, but I gather that rock is not known to be a stromopoid, it is crystalline quarts that has filled a larger than normal void in the sedimentary rock. Crystalline quartz is a little softer than Agate and not generally as clear (more milky or cloudy in the translucent bands) and the general appearance of an agate compared to your rock the same size and color would be like looking through fog at the one (crystalline quartz) and not looking through fog at the other (agate). In other words the bands and such are not as well defined as in agate generally. Honey onyx is also cloudy like crystalline quartz, and even though it is banded and translucent ... it does not look like agate other than in a very general way.

Hope that helps.

Ed

Edited by Mr_ed

Share this post


Link to post
Share on other sites
Precambrian Man

Hello Bev... 100% agate is what i believe you have there. This fossil thing is recently new to me since i have spent the last 15 years collecting agate. I have seen and i have a wide range of shapes, colors and sizes. If you look at upper left corner of your photo [photo that shows concentric banding] that is what is called a flow channel. The silicified ground water has to enter the cavity at some point. As the cavity fills,the concentric bands form,layering one atop the other and that flow channel acts as the entry point till the cavity fills completly. Unless of course a void remains, in which the term used is Geode. Now keep in mind cavities come in all shapes and sizes. By no means are they all sphericle in nature. I have agates resembling the shape of yours! Also, from what i see in your photos is that this agate has pieces broken off of it. So, it was likely a bit different in shape when originally formed [talking about the sides or edges], Photo you have of the underside? where it is knarly [area that is not smooth] i also have agates with same texture. I think that comes from part of cavity which was very rough and did not allow solution to completly form over.Or may have something to do with mineralization/precipitation. I call that that area rough quartzy. Keep in mind agate is quartz. Coloring,well,that is very diverse,certainly localized material is distinctive. but you do not have to travell far for that to change. Now, i am not saying that what you have could not be a "replacement",meaning,inorganic/minerals replacing organic/creature. Similar to the clams,belemnites etc. in australia being opalized. I think just about anything organic can be replaced by minerals, plants,animals etc. I dont know how common agatized creatures of what your calling it are? but i do know of agatized dino bone,agatized wood. What ever you got,it is very nice and i hope you solve the puzzle one day thankyou, wally

Share this post


Link to post
Share on other sites
Bev

Squalicorax is super knowledgeable and this is what he said about the fossil question,

"Hard to say if this is a silicified stromatoporoid or banded chert.

Due to the curvature and mound like shape i'd say this is a stromatoporoid."

So now a couple of other pics...

post-9628-0-96119500-1401286569_thumb.jpg

post-9628-0-26458100-1401286603_thumb.jpg

The one on the top has previously been identified as the agatized internal mold of a large brachiopod.

I believe the two lower ones on the left are also stromatoporoids. And at least one appears to be agate or???

Thoughts?

Share this post


Link to post
Share on other sites
Mr_ed

Well Bev .. that is a nice rock and since there are a number of rocks that are visually different .. such as chert and jasper and agate and quartz and several others that have the same basic composition of elements, then I guess if you want to call it agate you can and nobody will argue with you. If it was mine I would not call it agate no would I call any of the other ones in the picture agate. In my eye they are quartz because that is what I have become accustomed to calling them. To me they don't have the distinctive sharp changes from banding to translucence and such..

That is a nice piece of quartz in my mind and I haven't seen or read anything that will make me think differently. That doesn't mean it is not agate and I wouldn't call anyone wrong that wants to call it that. But to me .. it is quartz from what I can see of it. I guess maybe I call it quartz because it is not something that I would throw in my agate bin.. Not much logic to that but..The one on the bottom left is in the same category and the other two do not look like agate to me either, but I can't see them well enough.

I have no knowledge of stromatoporids.

Share this post


Link to post
Share on other sites
Tethys

I agree with squalicorax. Our chert doesn't form as nodules within the sedimentary rocks. Entire geological formations have been silicified by the process of dolomitization. I will quote Justin Tweet from my link on the previous page.

For paleontology, the alteration of limestone to dolomite often dissolves or destroys fossils, because many fossils are also made of calcium carbonate. The process may also create natural molds and casts of the original fossils.

It may not be technically correct, but for me if it formed in sedimentary rock it is chert. Calling it quartz would only make it more confusing since druzy quartz is also a common component of our geology.

Share this post


Link to post
Share on other sites
Mr_ed

That being the case.. I would guess that rock wasn't formed in sedimentary rock.

Share this post


Link to post
Share on other sites
Precambrian Man

A few photos of stromatoporoids, i can see where you draw the comparisons. The last photo was labeled a stromatolite, looks to be a slice or a cross section of one?

post-15180-0-83336000-1401306336_thumb.jpg

post-15180-0-83752700-1401306355_thumb.jpg

post-15180-0-16149300-1401306382_thumb.jpg

post-15180-0-87331200-1401306564_thumb.jpg

post-15180-0-51908800-1401306581_thumb.jpg

Share this post


Link to post
Share on other sites
Wrangellian

A few photos of stromatoporoids, i can see where you draw the comparisons. The last photo was labeled a stromatolite, looks to be a slice or a cross section of one?

Yes the last one looks like a slice of stromatolite.

I would have guessed the other thing that you show 3 views of looks like a coral, but I would want to see the other side to see the internal 'cup' part. Where did you get the name stromatoporoid for it, and is this from Arkona or somewhere over there?

Edited by Wrangellian

Share this post


Link to post
Share on other sites
Mr_ed

It is from Gotland.. wherever that is.. off sandbain.worldpress.com

Ed

Share this post


Link to post
Share on other sites
Bev

Given what Precambrian Man showed for stroms, here is another odd rock...

post-9628-0-69868100-1401327793_thumb.jpg

post-9628-0-62966900-1401327814_thumb.jpg

Share this post


Link to post
Share on other sites
Bev

And these are what I consider quartz here...

post-9628-0-18420100-1401327868_thumb.jpg

post-9628-0-98198300-1401327875_thumb.jpg

post-9628-0-65283600-1401327884_thumb.jpg

We have a lot of quartz. And I collect Quartz Drusy rocks.

Share this post


Link to post
Share on other sites
Mr_ed

The first one looks like sandstone.

The others for the most part are quartz. I am not sure about the two on the bottom left of the bottom picture, but you are right on calling them quartz.

Your big banded rock has something that those don't have that is for sure.

Have you read the thread under rocks and minerals .. Rare Oregon Jasper and Agates.

That thread will give you a bit of insight into how a rock can look like one thing and be another..

It also shows fairly clearly that banded agate has very defined lines. They are not foggy or fuzzy or milky..

Cheers

Ed

Share this post


Link to post
Share on other sites

Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!

Register a new account

Sign in

Already have an account? Sign in here.

Sign In Now


  • Recently Browsing   0 members

    No registered users viewing this page.

×