obsidian/flint shaped weirdly

[aliceh]

found this in covehithe, sounds glassy so assuming obsidian, but whys it look like this??

[Ludwigia]

I don't think that this is obsidian. Looks like simple good old flint to me.

[Tidgy's Dad]

Yup, flint / chert, I think. 

[Fossildude19]

Flint often takes odd forms due to weathering/sea wear/erosion.

[Mark Kmiecik]

Worn by waves and sand. That's why it has that "sandblasted" look.

[IsaacTheFossilMan]

3 hours ago, Fossildude19 said:

Flint often takes odd forms due to weathering/sea wear/erosion.

 

Interestingly, the jury is still out on this one. It's currently debated whether flint forms from entirely inorganic processes, or from prehistoric burrows that have been in-filled with silica. The latter would explain the complex nodular formations, and would also explain the presence of some fossils within - mainly echinoids, though others can be found. However, just purely from weathering, the flint is unlikely to form these complex shapes, as each specimen has a relatively constant distribution of density and thus Moh's hardness, meaning that it should typically tend to a sphaeroidal shape (if time stretches to infinity, assuming it isn't ever fully eroded!).

 

Although the eye of the scientific community seems to look on the burrow theory more kindly, I guess it's still for us to decide... I personally side with burrow theory!

 

[IsaacTheFossilMan]

Ah, almost forgot - your specimen is definitely flint.

Also, hang on, it seems the consensus is that chert forms within arthropod burrows. fluid escape structures or fractures. The complex nodular shape is NOT from weathering. @Fossildude19

 

Some people call it chert, others call it flint. Technically, flint is a variant of chert - all flint is chert, but not all chert is flint! Chert var. flint is allocated to chert specimen with a black, or nearly black, colour, whereas chert is for the lighter colours. Although, the actual definition isn't from the colouring, but the colouring is a diagnostic feature!

Note that the colours we are talking about is the colour inside the rock (chert is technically not a mineral!) - weathering can cause a coloured crust/rind, such as white, brown, etc. Yours has a yellow-grey crust.

 

Now, lets define chert vs flint.

 

 

Chert (including flint) is a fine, dense sedimentary rock, formed of silica dioxide (quartz), that is either microcrystalline (think vitreous, shiny lustre) or cryptocrystalline (think waxy, dull lustre) in nature. Flint is a type of chert - let's use the term "primary chert" for chert that is not flint, and "flint" for chert that... well, is flint.   Calcareous fossils can be found within both forms, becoming entirely silicified. In both forms, silica replaces carbonate from microscopic holoplanktonic protozoa "radiolarians" and microalgae "diatoms", forming radiolarite and diatomite, which eventually recrystallise into chert.

 

Primary chert is found as nodules which can form into layers of rock, if given enough time to coalesce. It forms in course-grained limestone that is poor with organic matter (not much life). This limestone has high iron content, which can oxidise into iron oxide and "dye" the chert various colours, such as red, yellow and brown. 

Flint also forms in limestone, but unlike primary chert, it forms in very fine-grained limestones rich in organic matter such as chalk or marl. This limestone has a reduced iron content, due to it being formed in primarily calm water such as the bottom of the ocean. The silica infiltrates and replaces the carbonate before it is oxidised, causing the flint to remain dark in colouration. 

 

Petrologists may make these exact differentiations, and, for the sake of accuracy and science, we should adhere to this! However, I often see people misappropriate the nomenclature of flint and chert.

 

I hope this helps!

Isaac

Edited July 6, 2022 by IsaacTheFossilMan

[JohnJ]

28 minutes ago, IsaacTheFossilMan said:

The complex nodular shape is NOT from weathering. @Fossildude19

 

@IsaacTheFossilMan you might cite your sources when making direct quotes.  

 

Further, in my experience with chert and flint, complex shapes can be formed due to weathering.  Spalling (micro and macro) from freeze/thaw, fire, erosive contact, or direct percussion can create very unusual forms with enough time.

[IsaacTheFossilMan]

1 minute ago, JohnJ said:

@IsaacTheFossilMan you might cite your sources when making direct quotes.  

 

Further, in my experience with chert and flint, complex shapes can be formed due to weathering.  Spalling (micro and macro) from freeze/thaw, fire, erosive contact, or direct percussion can create very unusual forms with enough time.

 

They weren't quotes, but I will find you some sources supporting this argument!

 

Boggs, Sam (2006). Principles of sedimentology and stratigraphy (4th ed.)

 

Blatt, Harvey; Tracy, Robert J. (1996). Petrology : igneous, sedimentary, and metamorphic (2nd ed.)

 

The flints from Portsdown Hill

 

And a few more:

 

Abdel‐Wahab, A., & El‐Younsy, A. R. M. (1999). Origin of spheroidal chert nodules, Drunka Formation (lower Eocene), Egypt. Sedimentology, 46(4), 733-755.
 

Gao, G., & Land, L. S. (1991). Nodular chert from the Arbuckle Group, Slick Hills, SW Oklahoma: a combined field, petrographic and isotopic study. Sedimentology, 38(5), 857-870.Knauth, L. P. (1979). A model for the origin of chert in limestone. Geology, 7(6), 274-277.
 

Maliva, R. G., & Siever, R. (1989). Nodular chert formation in carbonate rocks. The Journal of Geology, 97(4), 421-433.

 

And a museum link Identification – flint, fossil sponge | (swmuseumsoc.org.uk)

 

I could continue to list some, but lots can be found by searching "Why does flint form nodular shapes?" or some variant thereof ("Why does chert form nodular shapes?", "Why does flint form complex shapes?", etc).

 

Isaac

 

[JohnJ]

1 hour ago, IsaacTheFossilMan said:

They weren't quotes, but I will find you some sources supporting this argument!

 

My error.  They seemed formatted as such. 

 

I live on the edge of the Balcones Escarpment in Central Texas.  We have chert and a local flint.  I am familiar with the science of its formation.  

 

I was contesting your assertion that weathering could NOT shape complex nodules.  

[abyssunder]

What about silex? 

If the specimen was found in UK, is flint (usually grey to dark in color). If it was found in th US, is chert (usually yellow in color). 

[IsaacTheFossilMan]

2 minutes ago, JohnJ said:

My error.  They seemed formatted as such. 

 

That is quite alright. It appears to be my error, my friend - it was me who formatted them, after all... 

 

4 minutes ago, JohnJ said:

I was contesting your assertion that weathering could NOT shape complex nodules.  

 

Indeed, I shouldn't have implicitly asserted that, - thank you for reminding me to cite some sources!

My assertion was also poorly worded - it would be better to say "the complex nodular shapes exhibited by most chert specimen is not from weathering". - weathering indeed can shape complex nodules, however I am not aware of this taking place with flint - note the Mohs value of flint/chert is the same as quartz (obviously!) and it is rated at 7.0.

 

Quoting from 5.3 The Products of Weathering and Erosion – Physical Geology – 2nd Edition (opentextbc.ca).

Quote

Figure 5.3.1 Products of weathering and erosion formed under different conditions.

 

...it’s worth considering here why the sand-sized sediments shown in Figure 5.3.1 are so strongly dominated by the mineral quartz, even though quartz makes up less than 20% of Earth’s crust. The explanation is that quartz is highly resistant to the types of weathering that occur at Earth’s surface. It is not affected by weak acids or the presence of oxygen. This makes it unique among the minerals that are common in igneous rocks. Quartz is also very hard, and doesn’t have cleavage, so it is resistant to mechanical erosion.

 

 

Chert is very difficult to erode, taking a long time. To form the complex nodular shapes exhibited by most specimen, considerable time must be taken lying with one face upwards, to allow that side to erode, and then flipping around to different faces for different durations. As radiolarite and diatomite are crystallised into "bedded chert" upon being buried thousands of metres under the surface - as exhibited in the Monterey Formation of California, I find this process unlikely to happen. Would you be able to go into further detail explaining how weathering could form such pronounced knobs on chert formations?

 

I love this place for encouraging us to be better scientists! Thank you, my good sir

[IsaacTheFossilMan]

18 minutes ago, abyssunder said:

What about silex? 

 

From Wikipedia:Silex: 

Quote

In modern contexts the word [silex] refers to a finely ground, nearly pure form of silica or silicate.

Definitely not silex! I am aware that Europeans (especially the French) sometimes refer to silex as the umbrella term for hard stones occurring in limestone, much like others often refer to them as "chert/flint", but as we are talking about the nitty gritty details here, we shouldn't use localised umbrella terms.

 

18 minutes ago, abyssunder said:

If the specimen was found in UK, is flint (usually grey to dark in color). If it was found in th US, is chert (usually yellow in color). 

Could you elaborate on the assertion that UK chert is flint, and US chert is primary chert? 

Chert var. flint is known from all over the world, an example of American flint is the so-called "Ohio flint", which is in fact, the state's gemstone. I've seen posts and articles on forums stating that "true flint can only belong to European flint", but I am yet to see a substantial, comprehensive scientific argument detailing this perspective. Most generally, the terminology "true flint" and "american flint" refer to its ability and affinity with flintknapping, and how the flint itself can be shaped and formed.

As of now, I haven't seen a petrological reasoning behind this reservation; it just seems to be to do with modern flintknapping. For example, the Allegheny and Pottsville Groups (undivided) of Harrison County, America has very-fine grained sandstone and limestones in which chert var. flint forms. Source

 

As I am coming from a petrological background on geologic processes, I hope you can understand my scepticism.  

Edited July 6, 2022 by IsaacTheFossilMan

[abyssunder]

1 minute ago, IsaacTheFossilMan said:

 

From Wikipedia:Silex: 

Definitely not silex! I am aware that Europeans (especially the French) sometimes refer to silex as the umbrella term for hard stones occurring in limestone, much like others often refer to them as "chert/flint". 

 

Could you elaborate on the assertion that UK chert is flint, and US chert is primary chert? 

Chert var. flint is known from all over the world, an example of American flint is the so-called "Ohio flint", which is in fact, the state's gemstone. I've seen posts and articles on forums stating that "true flint can only belong to European flint", but I am yet to see a substantial, comprehensive scientific argument detailing this perspective. Most generally, the terminology "true flint" and "american flint" refer to its ability and affinity with flintknapping, and how the flint itself can be shaped and formed.

As of now, I haven't seen a petrological reasoning behind this reservation; it just seems to be to do with modern flintknapping. For example, the Allegheny and Pottsville Groups (undivided) of Harrison County, America has very-fine grained sandstone and limestones in which chert var. flint forms. Source

 

As I am coming from a petrological background on geologic processes, I hope you can understand my scepticism.

 

They are generally accepted terms. All of them are silex type material.

I usually use the term  "silex" for the undefined/ unknown silica-rich material/rock.

[IsaacTheFossilMan]

6 minutes ago, abyssunder said:

They are generally accepted terms. All of them are silex type material.

I usually use the term  "silex" for the undefined/ unknown silica-rich material/rock.

 

Gotcha, I've not come across that one before!

 

Alice's specimen is most definitely chert, though - which can refer to both chert var. flint, and any other form (Onyx, Agate, Chalcedony, Jasper, Radiolarite, Novaculite, Porcelanite, etc).

 

Though, America can definitely yield flint, not just chert.

Edited July 6, 2022 by IsaacTheFossilMan

[JohnJ]

50 minutes ago, IsaacTheFossilMan said:

Would you be able to go into further detail explaining how weathering could form such pronounced knobs on chert formations?

Weirdly shaped nodules have been eroding from their source locations for millennia.  Falling to strike other stones, crashing into other stones in flash flood events, spalling in naturally occurring fires, and fracturing in freeze/thaw cycles can alter the 'weird' into the 'bizarre'.  Additional effects come from the differential hardness of impurities and the erosive environments of a beach or river.  These are general comments not specific to the OP's specimen. 

[abyssunder]

One would say, if the location of the find is Covehithe, it's from UK, so it could be called "flint".

Maybe Tarquin ( our specialist from UK can give more light on this).

[IsaacTheFossilMan]

14 minutes ago, JohnJ said:

Weirdly shaped nodules have been eroding from their source locations for millennia.  Falling to strike other stones, crashing into other stones in flash flood events, spalling in naturally occurring fires, and fracturing in freeze/thaw cycles can alter the 'weird' into the 'bizarre'.  Additional effects come from the differential hardness of impurities and the erosive environments of a beach or river.  These are general comments not specific to the OP's specimen. 

Ah, I was caught up thinking that you were referring to OPs specimen... Duh!

Flint however, doesn't fall prey to these, as its crystallisation occurs when hidden from the world, deep underground.

[IsaacTheFossilMan]

13 minutes ago, abyssunder said:

One would say, if the location of the find is Covehithe, it's from UK, so it could be called "flint".

Maybe Tarquin ( our specialist from UK can give more light on this).

It is my understanding that flint, a variant of chert, is named not by where it was collected from, but under the conditions with which it formed, leading to different chemical properties. 

[JohnJ]

1 hour ago, JohnJ said:

Weirdly shaped nodules have been eroding from their source locations for millennia.  Falling to strike other stones, crashing into other stones in flash flood events, spalling in naturally occurring fires, and fracturing in freeze/thaw cycles can alter the 'weird' into the 'bizarre'.  Additional effects come from the differential hardness of impurities and the erosive environments of a beach or river.  These are general comments not specific to the OP's specimen. 

52 minutes ago, IsaacTheFossilMan said:

Flint however, doesn't fall prey to these, as its crystallisation occurs when hidden from the world, deep underground.

What?!  Of course it does.    What I described above happens to chert or flint after it erodes from the source rock...well after it was formed.

[IsaacTheFossilMan]

8 hours ago, JohnJ said:

What?!  Of course it does.    What I described above happens to chert or flint after it erodes from the source rock...well after it was formed.

My stupidity knows no bounds! Of course it does

[Fossildude19]

@IsaacTheFossilMan

 

I stand by my original statement.