Big Stromopoid Agate?

[Bev]

Southeastern Minnesota, Ordovician, Unknown Formation

Is this a big agate or a stromopoid (spelling?) or?

Thanks for looking! Any help would be appreciated.

[squalicorax]

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.

Edited May 23, 2014 by squalicorax

[4circle]

Seconded, hard to tell whether it's just chert, or if there's a biogenic origin to this structure.. The diagnostic structures in stromatoporoids are fine-scale and often lost on silicification.

[fossilized6s]

Nice find Bev!

It definitely has some fine lined layers, but i would lean more towards chert because of it's dull, milky coloring. But you could get away with passing it off as an agate to the unsuspecting!

[Bev]

Hmmmm, dumb question... :-)

What is an agate? My understanding is that an agate is created by minerals filling a gas pocket in lava in layers. Hence, in Minnesota we have a lot of red agates because of the iron ore here. Why would chert in layers not be an agate?

[Wrangellian]

I guess I am one of those unsuspecting who would have accepted it as an agate...

I guess you need to be able to determine the grain size, which is pretty small in chert and nonexistent? in agate (where is Painshill?)

It sure has the look of banded agate to me. Is it translucent? I don't think chert is supposed to be translucent either, at least aside from very thin slices, but I stand to be corrected.

Also, all the banded cherts I have seen have had more or less straight layers whereas the agates usually are curved (some straight but clearly limited to the space within a thunderegg 'nodule')

Edited May 23, 2014 by Wrangellian

[fossilized6s]

This is interesting:

http://geology.about.com/od/more_sedrocks/qt/About-Chert.htm

All of the agates that i have found have very fine banding. I think that is the only noticeable difference with this. But Bev, you have the rock in hand. Does it have very fine banding? Almost the width of a hair? I'll snap some pics of mine if you think that will help in any way.

[Sacha]

Painshill has given an excellent assessment of agate vs chert vs flint in previous posts, but since I've just come back from a Georgia chert trip I'll chime in a little. Chert can be very fine banded, or layered and has an amazing variety or variation in clarity. I've posted these pics in another thread but they apply here as well.

Bands, layers, masses are all common forms of chert in the coastal plain, but always associated with limestone. I'm not sure a foto is sufficiently diagnostic to ID the sample. Matrix would be a factor, micro structure would be a factor as would association.

I'ld love to cut it though and look inside!

[howard_l]

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.

[Mr_ed]

looks like onyx to me..Honey onyx and limestone are closely associated I think..used to find both in the same site.. have lots of agate.. Doesn't look right for agate.. But that is just my two bits worth

Cheers

Ed

Edited May 24, 2014 by Mr_ed

[Bev]

The lines are not hair width, most are rather thick and mosy all around the through the rock. It appears to have a milky translucent quality. It does not fluoresce under black light. The colors go from milky white to dishwater blond. It's a blond rock.

I can't actually say where it is from to be honest. The gal who owned the house before collected rocks - lots of quartz rocks and others. I collect rocks when I go places and have some here from Montana. She may have done the same.

I was watering the flowers and noticed what I thought was a strom and picked it up and noticed all the lines, like an agate. This does not look like any of the chert I find here. But it is BIG! I can hardly hold it up with one hand. I've never seen an agate this big even in shops up North.

[Mr_ed]

Onyx is formed of bands of chalcedony, it is cryptochriytalline, consisting of silica, quarts and moganite. it's bands are parallel to one another as opposed to the banding in agate which isn't always parallel to one another. ( From Wikipedia)

Cheers

Ed

[Wrangellian]

Onyx aside, does chert ever have that concentric sort of banding, as opposed to just flat parallel banding? I have only ever seen concentric banding in agates (and onyx, but the onyx I have seen and have examples of is also visibly crystalline as well as banded, but my experience is often incomplete!).

Edited May 24, 2014 by Wrangellian

[Bev]

I read the Wiki article on onyx. It is "flesh" colored...

[Mr_ed]

I haven't experienced much chert and none with parallel banding. That picture is not chert as far as I can tell ..actually I can tell by looking at it that it isn't chert. at least from my experience..but they are very closely associated..Chert is used to make arrowheads and is very brittle and breaks very smooth .. I don't think there is much doubt about whether that is chert or not.. Chert isn't even close to honey onyx in physical appearance..and I don't think it can be mistaken for agate either... Although they may well be similar in structure ..chert does not look like agate and visa versa from my experience. Chert and Jasper are sometimes hard to tell apart. I have found rocks that I was told were jasp-agate.. and they look like agate and jasper both in the same rock.. Chert that I have had anything to do with does not look like agate.. but maybe there is some that does...but I haven't seen any.

Edited May 24, 2014 by Mr_ed

[Mr_ed]

That rock is a bit flesh colored. They don't mean that the rock is like raw flesh.. I think they mean skin colored more than raw red.. Anyway I have found honey onyx that color and banded like that. So that may be honey onyx in my mind. It also looks like it has the right texture and sheen for onyx and lacks what would make it look like chert or agate. I think a hardness test would tell you if it is onyx because if my memory serves me right .. onyx is one number or more softer than agate on the hardness scale. I believe you can scratch onyx with a good knife but don't take that for gospel..but it is softer than agate.

Cheers

Ed

[Mr_ed]

There is an agate found in Mexico called Laguna Agate that they say can be banded and shades of white and tan ..so if that rock meets the hardness test for agate it could have came from Mexico I guess.. but that is a long shot. I have never seen agate with bands like those.. Agate nearly always has bands that border two distinct parts of the rock they adorn..like a border.around a picture almost ... or a border between two colors or between clear and clouded.. etc.. most are very fine lines..lacey looking..

cheers

Ed

[4circle]

Onyx aside, does chert ever have that concentric sort of banding, as opposed to just flat parallel banding? I have only ever seen concentric banding in agates (and onyx, but the onyx I have seen and have examples of is also visibly crystalline as well as banded, but my experience is often incomplete!).

Yes, chert can have concentric banding, especially when it forms in nodules (similar to the concentric layers in any sedimentary concretion)

[4circle]

I would emphasise the post above - the best descriptive terms are those which describe the gross mineralogy (in this case silica), rather than 'onyx', 'agate' etc, which are often used in the vernacular inconsistently with their true definition. People often ascribe them on characteristics such as how well a stone will polish up, rather than how it formed or it's microstructure.

Here's a quick lowdown on the siliceous rocks:

CHERT - a bona-fide sedimentary rock, formed as a primary deposit from siliceous organisms (radiolaria etc); or by diagenetic concentration of redissolved silica from calcareous sediments (often nodules in this case). Fine-grained.

FLINT - nodules of chert which form in chalk. Bona-fide chert.

CHALCEDONY - a large grouping or siliceous rocks (technically united by their crystal structure) which are generally shiny and banded. No visibly distinct grains. Can form by a variety of methods from both sedimentary and volcanic origins and include:

AGATE - vernacular term for chalcedony with banding and pale colours

ONXY - vernacular term for chalcedony with particularly dark banding

JASPER - a vernacular term for red chalcedony

The colours of all of the above are highly variable (any colour of rock or rainbow) as are their bandings and 'layering'. There are many other vernacular terms to describe particular varieties of chalcedony. These are solely vernacular and none are recognised classification terms.

OPAL - (this one is mineralogically distinct) - a silica polymorph which hasn't quite turned to chert or chalcedony. Characteristically iridescent. Volcanic or sedimentary (diagenetic) in origin.

So, a sedimentary rock or fossil which has become silicified should best be described as 'silicified' (rather than 'agatised' or 'cherted'). However, the prevalence of terms such as "Turitella Agate" (technically a silicifed gastropod limestone or a gastropod chert) are likely to continue confusing people

[Mr_ed]

Thank you 4circle.. for the run down and how these rocks are associated or dis-associated.. Most of us won't remember but it is nice that you explain the association for those that are interested.

Regards

Ed

[Mr_ed]

So there you go Bev.. It is whatever you want it to be.. agate or onyx or chalcedony.. but not likely chert.

That rock would be an interesting one to color.

Cheers

Ed

[Wrangellian]

In any case, sticking to the vernacular, I am betting Bev's rock is agate (but I won't bet much). There is a 'Lake Superior agate', I wonder if this was found in the LS agate territory?

[Bev]

In any case, sticking to the vernacular, I am betting Bev's rock is agate (but I won't bet much). There is a 'Lake Superior agate', I wonder if this was found in the LS agate territory?

Because of the glacial drift we have many agates here. The Lake Superior Agates are usually reddish in color from the iron ore of the iron range. But this would be a mega huge agate. It is the same color as the agaztized Silurian brachiopod I found last year that is very large for an agate too. A retired high school science teacher I know is going to stop by this week and take a look at it.

[Mr_ed]

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

Ed

[Tethys]

It's a very interesting piece. I am curious to see what your teacher friend thinks. Its definitely not lake superior agate, which is red to brown glassy, usually transparent, and frequently has opaque white banding.

I would characterize it as chert, which is common in the dolomitized Ordovician and Devonian limestones of MN. It's honey color looks like the Devonian variety, the Ordovician ones tend to be in dirty looking black, .brown, buff, jasper red or rusty colors due to high iron content.

It appears to have a partial, segmented spiral shape of a section of large cephalopod or gastropod shell. Such shell fragments formed the hardgrounds for many different encrusting reef builder organisms. Both stromatolites and stromatoporoids were major components which overgrew and filled voids within the reefs. In the intervening eons those reefs have been pyritized, karsted, dolomitized, and silicified. It is common for nothing to remain of the original shells but ghost patterns on the surface of the chert cast. There are apparently echinoderms fragments in some of the strata near the UofM that are preserved in this manner.

Here is a huge pdf (253 pages) on the sedimentology of the Platteville formation in MN* that has lots of technical detail on the variability and markers within the different layers.

*According to this report, the St Peter sandstone and Platteville are now middle Ordovician rather than late Ordovician. Can anyone point me to information on this change?