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Identity Helpsilicified mudstone?

15th Mar 2018 17:14 UTCDaniel Bennett

00438870016017857977238.jpg
this material comes from the McNamara formation part of the Missoula group of the super belt series. it barely will scratch glass and is easily scratched with a steel file. is it silicified mudstone or is it metamorphosed mudstone? or is it claystone? siltstone?. perhaps its similar to quartzite but with less quartz and sand. I think quartzite is harder. i like this stuff and its easy to work with. i have wondered what exactly it is for years. i think i got it now but i would love to hear any additional input regarding its formation or identity. maybe some basics about meta and semi meta mud, silt, or claystone and how it differs from quartzite. forgive me if i am asking too big a question? thanks in advance.


08386620015652152801861.jpg


a larger piece
03708080015652152817126.jpg

15th Mar 2018 18:35 UTCWayne Corwin

Have you tryed UV on it?

15th Mar 2018 18:57 UTCDaniel Bennett

no. i can check that later.

16th Mar 2018 15:26 UTCDonald B Peck Expert

The 1st and 2nd photos look a lot like gemmy antigorite. How hard is it? Compare: https://www.mindat.org/photo-225723.html

16th Mar 2018 17:47 UTCDaniel Bennett

09472530016017858016159.jpg
it almost wont scratch glass. but with effort it does. a file cuts right into it effortlessly. 5.5 ish hardness . the specific gravity is roughly 2.66. nothing unusual under a black light. below is another piece under natural light about an inch across. maybe its chert that is fully saturated with mud bringing the hardness down?

another notable feature is that it dries out over time. and can be shined up by rubbing it vigorously with a dry cloth. the colors at this occurrence range from green to grey.

09472300015652152811147.jpg

17th Mar 2018 16:36 UTCDonald B Peck Expert

Daniel,


The Sp. G. is in the ballpark for serpentine/antigorite, but your piece is too hard.at 5.5 or so. I don't have another good idea. We need someone else to jump in here.


Don

17th Mar 2018 18:04 UTCHolger Hartmaier 🌟

I've seen rhyolite that looks like this- it also has the white alteration rind that this specimen shows.

17th Mar 2018 19:51 UTCNorman King 🌟 Expert

That is peculiar-looking stuff. Translucency is what is getting me. Without that, I could suggest metasomatism of a sedimentary mudrock having remnants of the original terrigenous constituents. I have examples from Paleoproterozoic Huronian metasedimentary rocks from northern Ontario that likewise barely scratch glass, but they are dark-colored and opaque. They have the advantage of having certain similarities with rocks of the Belt Supergroup in Montana, however.


Here is an off-the-wall idea: metasomatism of bentonites altered beyond bentonite formation on the sea floor, perhaps during simple, but long-lasting, burial conditions of temperature and pressure. I can’t go into detail about what changes occurred and how. After all, this is just speculation. But there would be no opal-CT left–it should have converted to chert fairly early, having other constituents making for the unusual physical properties. My inspiration was this paper, available on the web (but not for the faint of heart!):


Christidis, G. E., Scott, L. P. W., and Marcopolous, T. (1995). Origin of the Bentonite Deposits of Eastern Milos, Aegean, Greece: Geological, Mineralogical and Geochemical Evidence: Clays and Clay Minerals: 43(1) 63-77. http://www.clays.org/journal/archive/volume%2043/43-1-63.pdf

17th Mar 2018 23:33 UTCBen Grguric Expert

Another possibility is a very fine-grained, compact muscovite (sericite) variety called damourite. You can cut it with a hacksaw and file it and it takes a decent polish. Often translucent like you've shown.

18th Mar 2018 22:19 UTCRoger Ericksen 🌟

I have found similar material at Tilly Foster that is associated with antigorite but harder. Have been guessing compact tremolite (nephrite?). The hardness fits.

19th Mar 2018 15:58 UTCDaniel Bennett

09859790016017858082246.jpg
thanks Don.i wont rule out antigorite. I do have a few more clues. I did my hardness tests again and found that while it will scratch glass with a lot of effort it can also be scratched by apatite and even fluorite. scratch tests on rock may not be exactly reliable. no matter how hard I try apatite will not scratch glass. I soaked it overnight in muriatic acid just for the heck of it. no response. on close inspection with a magnifying glass I see little pockets filled with undeniable mica crystals. not easy to capture in a picture. see below. thanks also for the other ideas. I will try and read that article about bentonite. I am pretty sure there is no igneous rock within a ten mile radius from the location. the geo map shows none.

05341420015652152825124.jpg


here is another piece from the exact location I collected the other pieces to show the nature of rock it comes from. from the geo map it describes "bedded chert". but shouldn't that be harder that this stuff?
00407910015652152837130.jpg

20th Mar 2018 18:11 UTCGregg Little 🌟

Daniel;


You might check out bowenite, a variety of antigorite. It is in the ball park for hardness and specific gravity.

20th Mar 2018 19:32 UTCDaniel Bennett

Greg I agree that bowenite does resemble this stuff very much. is asbestos or antigorite, bowenite ever known to come from Precambrian sea beds?


Roger can your material scratch glass? or apatite? mine will scratch apatite fairly easily.


the definition of silification mentions replacing with fine grained quartz, chalcedony, or opal. the hardness of opal is 5.5-6.5. so maybe opal then. I also noticed a mineral listed under opal in mindat called opaline-chert. maybe that's what this is. bedded.


the hardness test is confusing. if i was just using those hardness picks that geologists use than i would probably have determined it to be a hardness of 3 or 4. a copper penny wont scratch it but fluorite will (with difficulty). so why does it scratch apatite and glass? and what should i consider its hardness to be?


thanks to all who've helped me to try and sort this out.

20th Mar 2018 23:47 UTCPaul Brandes 🌟 Manager

Something to think about. If it truly is some sort of mudstone or siltstone, there could be multiple constituents included in the specimen, giving you multiple hardness across the specimen which could lead to confusing results. And then throw in a little silica.....

21st Mar 2018 00:21 UTCRoger Ericksen 🌟

Yes, my material will scratch apatite but not microcline. Even though it looks similar to yours I don't they are related. Environments too different. Your thinking and Paul's seems to head in the right direction.

21st Mar 2018 02:39 UTCBen Grguric Expert

Guys a rocktype ID is fine but Daniel wants a mineral constituent ID.

The banded grey-green macroscopic textures suggests either a sedimentary layering or some sort of oscillatory banding (seen in skarns). If the first I'd guess these are psammitic (grey) and pelitic (green, fine-grained) layers. What was mud or clay looks now to be greenish muscovite (sericite or damourite variety) formed as a metamorphic product. Are the grey layers hard and siliceous?


The various serpentine options suggested require the rock to be either an altered ultramafic (e.g. a dunite or peridotite, pyroxenite) or a retrograde altered metamorphosed impure marble (forsterite-bearing which has altered to serpentine). The texture doesn't really resemble these.

Serpentine is decomposed in conc hydrochloric acid with the separation of silica. Daniel has tried this (not sure if it was conc or not) with no change. This would suggest muscovite rather than a serpentine.


Scratch tests are not reliable if the sample is fine-grained like yours and may contain more than one mineral phase as Paul has suggested. The bulk of the green stuff could be sericite with a few microscopic quartz grains here and there. These will scratch things the sericitic matrix won't.


Daniel if you are determined to get an ID on the green material send a small fragment to Attards Minerals for X-ray diffraction analysis (https://attminerals.com/mineral_identification/). It will cost you $50. My guess is it will come back as muscovite 2M1.

21st Mar 2018 03:11 UTCGregg Little 🌟

Sedimentary rock was mentioned a couple of times but in the translucent and earlier pieces I can't see any evidence of bedding or other sedimentary structures and as well, there is a flaky to almost sub-conchoidal fracture which indicates a more uniform and almost microscopic granular or crystalline texture to it. The photo # 3.jpeg might have the appearance of bedding but the structure doesn't seem to go all the way through the specimen.


you asked, "bowenite ever known to come from Precambrian sea beds?" Bowenite like antigorite is serpentinized mafic igneous rock which is common in sea floor extrusive sequences.


To reiterate what Paul has said, as the sample is probably rock then the poly-minerallic nature would give varying hardness test results.

21st Mar 2018 05:06 UTCBen Grguric Expert

Greg, the only seafloor extrusive that is magnesian enough to generate massive bowenite (after hydrous alteration) would be a komatiite. There aren't any komatiites in the Missoula Group. There are abundant bedded cherts and micaceous rocks however.

23rd Mar 2018 17:21 UTCDaniel Bennett

Ben asked "Are the grey layers hard and siliceous?" not quite I guess. I believe about 5.5. like all the mudstone from that area. translucent and not. they are all about that hard.

I like the compact muscovite idea.since there seems to be a few little pockets or crevices containing reflective mica looking crystals.(like fish scales). but isn't 5.5 a little too hard for that material?


this is from the chert description page on mindat.


Bedded cherts may form by compaction and recrystallization of silica-rich biogenic sediments made of opaline tests of single-cell organisms (diatoms, radiolaria) or remains of silicious sponges, both in marine and in lake environments. During diagenesis, the silica in the sediments undergoes a transformation from opal-A through opal-CT to microcrystalline quartz in the mature chert (Oldershaw 1968; Calvert 1971; Lancelot 1973; Hein et al 1981; Pisciotto 1981; Riech 1981; Levitan 1983; Jones et al 1986; Compton 1991).


this is also from that page at the bottom



Siliceous precursor rocks of comparable texture that are primarily made of opal-A or opal-CT are sometimes also called chert, but usually with the addition of the dominant silica phase, for example, "opaline chert" (e.g. Weaver et al 1972; Pisciotto 1981, Calvert 1983; Cady et al 1996).


so maybe I should call it "immature" bedded chert. or maybe bedded opaline chert. or semi silicified mudstone. semi-chertified mudstone.

I am starting to doubt it to be simply silicified mudstone because it really isn't quite hard enough I don't think.


I like the "single-cell organism and silicious sponge remains idea because actually there is one last geologic clue I left out. right below where I found this material (about 10 feet or so) is a bed containing fully hardened "chert nodules" with odd shapes resembling some sort of stromatolite or sponge.

I once asked about those here on mindat identity help titled "stromatolites?".



anyway if anybody has any ideas for an official title for this translucent greenish rock I am interested to hear it. thanks.

23rd Mar 2018 19:26 UTCNorman King 🌟 Expert

00787700014950989724249.jpg
That (photo 3.JPG) is bedded chert. The Belt Supergroup has a lot of it.


I have found similar rock in the Gunflint Chert of the Huronian Supergroup in northern Ontario (north shores of Lake Superior and Lake Huron). Portions of it may be impure, resulting in hardness of less than 7. However, I didn't test any of these for hardness. Rocks do not have a physical property known as hardness.



04117970014977339529550.jpg
Small-scale stromatolites in Gunflint Formation chert, Kakabeka Falls

Kakabeka Falls Provincial Park, Oliver Township, Thunder Bay District, Ontario, Canada


02321650014977337917157.jpg
Close-up of Gunflint chert, north shore of Lake Superior

Schreiber Beach, Killraine Township, Thunder Bay District, Ontario, Canada

26th Mar 2018 19:19 UTCDaniel Bennett

01357180016017858141320.jpg
thank you Norman. that would make sense if that is bedded chert considering the geologic map text for the McNamara formation describes thinly bedded chert and chert rip-up clasts. thanks for the pictures too. and the willingness to engage this topic.

I had ruled it out as being chert because of its softness. my rock and mineral book (the Audubon society field guide to rocks and minerals) describes mudstone as being easily scratched with a knife. also under chert it says "hardness: 7 ; hardness is distinctive." the mindat description says 6.5-7 and composed mainly of microcrystalline quartz.


I am having a hard time feeling any closure on this one. it seems like either mindat and other guide books are wrong or geologists have misidentified rocks as chert enough that its accepted as chert now. eitherway I would like to propose in a humble way that mindat change the official hardness of chert to 5.5-7(actually it may be 4-7) and/or note at the bottom that "some chert" is softer and questionable as to if it is chert at all.


is the hardness of chert really irrelevant in the identification of it?


McNamara mudstone exposure

06299000015652152832386.jpg

01421950015652152847403.jpg

26th Mar 2018 19:20 UTCDaniel Bennett

01139330016017858168833.jpg
this rock came from the above exposure.

its opaque



"chert" conglomerate also from McNamara formation
07720050015652152841149.jpg



last picture showing a scratch at the lower left easily applied with a knife. are we sure this is chert?
03527090015652152856019.jpg

26th Mar 2018 20:44 UTCReiner Mielke Expert

I think you would need a thin section to determine what it is.

26th Mar 2018 21:39 UTCWayne Corwin

Daniel


Often with "ROCKS", they can very, some gets "cooked" better than other layers or parts.

So it might 'Almost' be on it's way to being chert,,, just needs a bit more cooking.

26th Mar 2018 22:46 UTCPaul Brandes 🌟 Manager

I'm thinking thin sections would be quite interesting.......

26th Mar 2018 23:07 UTCDaniel Bennett

Wayne great idea. I will put it in my wood stove for a few weeks and see if it turns in to chert ; )


I'd like to take back what I said about geologists misidentifying chert. I of coarse haven't looked all over the McNamara formation to say this is the closest thing to chert beds that are present.


Paul would it be interesting enough to you for me to send you some so you could slice it up and check it out? i would love to know more about it out of curiosity. i don't really want to make a monetary investment in it however. I'd pay the shipping.

27th Mar 2018 05:11 UTCNorman King 🌟 Expert

I looked up "chert" in Mindat. The page is titled "Chert: Chert mineral information and data. - Mindat.org." That is the first mistake. Chert is a rock, not a mineral. After a few photos on that page, chert is described as having a hardness of 6-1/2 to 7. That is the second mistake. As I pointed out earlier, rocks do not have hardness. They may have toughness or induration (how easy it is to bust them up), but not hardness in the same way that minerals do.


Some discussions "out there" of chert are realistic enough to state that what one means by "chert" may have to do with whether it occurs in nodules or as bedded material, and whether it is Precambrian in age or much younger, and whether it directly precipitated in an ocean, accumulated as tiny siliceous skeletons of marine creatures, or formed by replacement of something else. The Precambrian ones tend to occur in beds with shale and "dirty" sandstones. Dirty shales and sandstones grade into materials composed of at least 50% quartz, which is the point after which they qualify as cherts. Actually, the siliceous material in cherts tend to clump together, as if "wanting" to be fairly pure, thus making transitional material less common than one might expect. (I don't know of studies that document what I just said, but maybe there are some; maybe not.) There must be enough quartz in a silica-rich sediment for chert to become cemented. When it does, however, there still can be a lot of clay and loose sand in it. So when you try to do a "hardness" test, you are really just knocking apart the grains of quartz. It won't scratch glass or even a knife blade, and you can easily scratch it--really just gouging out finely divided, softer material. That will be a problem until your sample is fully silicified all the way through. However it may still contain a lot of softer materials, and therefore you may not be able to find a piece that will give you a reading of 7 or even 6-1/2 on the hardness scale. The bedded cherts I pictured here are likely to be relatively impure.


What to do about this situation is a good question. In igneous geology we used to have just granite, diorite, and gabbro, or so the elementary geology textbooks suggest. In fact, there are dozens of intermediaries in that simple scheme with names you have never heard of that practicing geologists use all the time. After all these years, though, it seems that chert is still chert. There are diatomite, porcellanite, and novaculite, however, and there are jasper and flint. So we're making some headway, but we still have a long way to go with siliceous sedimentary rocks. In sedimentary rocks, however, variations in mineral composition are not very important for understanding why a particular sediment was deposited here or there, and how. I'm sure that a very elaborate scheme could be developed that allows us to put cherts into dozens of cubby holes in a new classification chart, and I would not say we shouldn't try to do that. But the people who make a living dealing with sedimentary geology haven't felt that need yet.

27th Mar 2018 07:23 UTCAlfredo Petrov Manager

I went to the Chert page on Mindat, and it is quite clearly classified as a rock type, not a "mineral": https://www.mindat.org/min-994.html

27th Mar 2018 10:54 UTCUwe Kolitsch Manager

> The page is titled "Chert: Chert mineral information and data. - Mindat.org."

Where does it say that - can't find it.

27th Mar 2018 12:57 UTCNorman King 🌟 Expert

Glad you guys asked. I googled "chert mindat" and got these:


https://www.mindat.org/min-994.html (first return)


https://www.mindat.org/min-49384.html (second)


https://www.mindat.org/min-49385.html (third)


https://www.mindat.org/min-49383.html (fourth)



In the classification sections it is indeed referred to as a rock, but the "chert" page is a mineral page, and it is said to have the hardness values I listed. However, the text does indeed indicate that chert is a rock.


My history here at Mindat includes arguing "chert" and "jasper", and maybe some other terms, quite a bit. Most of what I said about how practicing sedimentary geologists (as was I before retiring) actually use those terms, including in academic circles and current peer-reviewed literature and textbooks, was not given much credence.


Oh, I should add to my earlier long post that I am not talking about a clastic rock that is rich in quartz, but a chemical rock. The clastic quartz-rich rock would be sandstone or siltstone. And, as you know, there are also metamorphic rocks consisting primarily of quartz.



* * * *

For what it's worth: Per my first line, I search for things within Mindat by googling them. I find it easier to find what is useful to me by using Google. I noticed Reiner Mielke has issues also (https://www.mindat.org/forum.php?read,7,433763,433832#msg-433832; March 25, 2018 12:37PM). I am concerned about Mindat becoming in-grown, in that you have a group of old-timers at Mindat who do not seem to be listening.

29th Mar 2018 00:16 UTCDaniel Bennett

well said Norman. I appreciate your willingness and bravery to contradict all the sources I have seen regarding the hardness of chert. I feel inclined to believe you. I've heard no arguments here.(nobody is agreeing either).I wonder why that is.

two questions. if this material turned out to be only 49% quartz could it be considered "silicified mudstone"? whats the difference between chert and silicified mudstone?

29th Mar 2018 01:29 UTCAlfredo Petrov Manager

The problem with expressing Mohs "hardness" of heterogenous substances like rocks is that you get different results from X scratching Y, and Y scratching X. I personally would express the hardness of chert as 7, because it is dominantly composed of quartz, and so a sharp edge of chert will always scratch feldspar (and of course softer things like a glass bottle). But you might get results other than 7 if you are trying to scratch a piece of impure chert with something else - Then you are just disaggregating the quartz grains, not scratching them.

29th Mar 2018 03:54 UTCNorman King 🌟 Expert

02556010015733143227525.jpg
Daniel,


Another good question. Chert does refer to a chemical sedimentary rock. It tends to occur in blebs in clastic rocks. That’s what I was referring to when I said: ‘Actually, the siliceous material in cherts tend to clump together, as if "wanting" to be fairly pure, thus making transitional material less common than one might expect.’ The answer to your question is that a rock consisting of 49% silica is most likely to have regions of much purer silica (chert) within a background matrix where there is little continuous chert. The rock would have a gross nodular texture. That probably happens because silica in solution tends to crystallize more readily where it contacts silica that has already crystallized. As a result the blebs become larger. See outcrop photo of Armuchee Chert below--Devonian chert near Rome, Georgia.





Other outcrop photos we (both) have displayed are probably largely argillites. Argillite is a mudrock that has been altered to a rather tough (we often carelessly say “hard”) rock that has become recrystallized, with clays and iron and aluminum oxides forming micas, chlorite, serpentine, feldspar, epidote, and other metamorphic minerals. Chert layers and metaquartzites are often interbedded with the argillite. At least one of your outcrop photos looks very shaly to me--not likely an argillite yet (needs more burial time).


Some cherts I have dealt with may be the product of metasomatism of silica-rich mudrocks. The mudrocks were replaced under the influence of hot interstitial fluids that replaced the original material in small batches, and foliation did not develop. Here is a thin section of chert from Lookout Pass, a locality I have enjoyed studying for several years that resulted from metasomatism of mudrocks. Interbedded limestone and dolostones were not affected as much, and apparently the fine-grained silicates supplied much of the silica for production of the chert.





This shows how complex cherts may be--multiply faulted, different generations of quartz crystallization including later vugs now filled by coarser quartz crystals and fibrous chalcedony, and darker layers that were originally especially clay-rich. This degree of complexity, plus the variety of conditions under which chert may form, makes it difficult to make broad generalizations about chert.

29th Mar 2018 04:29 UTCGregg Little 🌟

Daniel;


Regarding to the two questions.


"if this material turned out to be only 49% quartz could it be considered "silicified mudstone"? whats the difference between chert and silicified mudstone?"


Sedimentary rocks are usually determined by their major constituent(s) and its genesis. In your example (49% silica and 51% clay) would be a silicified mudstone. If the percentages were reversed then I would tend to call it chert with a further notation of "siliceous replacement of mudstone".


Further to the discussion of rocks verses minerals, chert is like limestone a mono-minerallic rock, or at the same time both a single mineral and a deposited one mineral rock.

29th Mar 2018 15:52 UTCNorman King 🌟 Expert

Chert is often not a monomineralic rock. Or, one might say it is "impure." Most rocks do not have quite the mineralic composition they are "supposed" to have. That's what causes hardness test problems and classification problems.

29th Mar 2018 16:18 UTCLarry Maltby Expert

09593120016017858163098.jpg
Norm,


I thought that you may be interested in this deposit of nodular chert at a remote location on the shore of Lake Michigan near Norwood. The nodules are large and are interspersed with veins of chert in a distinct zone in the limestone.


Also, there was a thread some time ago when you described in detail he difference between chert and flint. If you find time could you give us a link to that post? I would like to copy the information into a word document for my files.


Thanks, Larry,



P.S. It appears that the nodules pushed the limestone away during formation.

29th Mar 2018 18:29 UTCGregg Little 🌟

Greetings Norman;


You are quite right in that monomineralic rocks are quite rare/unusual. A striking example might be some of the limestone on the North American continent which technically are not "pure calcite" but really, little is except for possibly in a lab.


As stated previously by others, rock naming is less concerned with the absolute mineralogy and usually references the dominant mineral in the rock. This mineral can be derived from many sources (mechanically, chemically, biologically, etc.). A sandstone is still a sandstone even though it may only be 50% sand. What is more important is the genesis of the rock and the name we hang on it. So with chert the concern is not the purity of the dominant mineral but the environment that causes the silica deposition.


At the beginning of this thread we were shown a small piece of rock, out of context to the surrounding geology and expected to guess the definitive mineralogy. That's a tough road for any geologist.

29th Mar 2018 22:28 UTCPaul Brandes 🌟 Manager

Larry Maltby Wrote:

-------------------------------------------------------

> I thought that you may be interested in this

> deposit of nodular chert at a remote location on

> the shore of Lake Michigan near Norwood.

> limestone away during formation.


Larry,

Would these happen to be in Traverse Group limestones?


Edit: I guess I should have paid more attention to your photo name...... Petoskey Formation, which is Traverse Group! ;-)

30th Mar 2018 19:55 UTCDaniel Bennett

05432770016017858171521.jpg
nice info everyone.


Alfredo Petrov wrote "and so a sharp edge of chert will always scratch feldspar" Alfredo you seem highly knowledgeable about rocks and minerals but your comment doesn't fit for this material in question. this seems to be a kind of softer chert. below shows a sharp piece of "chert" that I scratched back and forth like a saw on a piece of feldspar.

09517280015652152854227.jpg

04862630015652152862717.jpg

30th Mar 2018 19:56 UTCDaniel Bennett

00834780016017858181998.jpg


if did seem to clean to spot but no scratch
01264970015652152878359.jpg

06523050015652152876147.jpg



I hear what you guys are saying about the grains being knocked off rather than being scratched. this material will leave a scratch on glass. not feldspar. Norman did mention that sometimes this kind of stuff wont even scratch a knife blade. to me it seems completely uniform in texture, color, and hardness.(no spots seem sandier than others). I think I am understanding this stuff to be chert now because nobody has taken the time yet to reclassify it as some kind of soft intermediate chert. it doesn't matter that it wont scratch feldspar. its still chert.


correct me if I'm wrong.

30th Mar 2018 20:05 UTCDaniel Bennett

Greg Little wrote "At the beginning of this thread we were shown a small piece of rock, out of context to the surrounding geology and expected to guess the definitive mineralogy. That's a tough road for any geologist."

sorry about that Greg. I have a way of doing that sort of thing. I did try to state the facts about where it was from without dubiously describing more geologic details about it.

30th Mar 2018 21:49 UTCNorman King 🌟 Expert

Daniel,


Thank you for that “demonstration.” Yes, show us something we haven’t seen before!


For a variety of reasons, some scientific and others sociological, that is the best posting I’ve seen on Mindat in a long time! In this case, genius isn’t getting the highest score on the Mensa test or knowing what is in the scientific literature, it’s showing us knowledgeable people something we haven’t seen before. It involves comprehension, conviction, tenacity, and demonstration.


Do you have any more of that clearly uniform stuff? It does need a thin section, and it does need an XRD. I might be able to arrange for both, with interpretations of the data (but I’m not sure what the time frame would be). I can contact you by PM (private message) to make arrangements (note the “Send a PM” at the bottom of the message form).

30th Mar 2018 21:56 UTCNorman King 🌟 Expert

Larry,


Here is the chert discussion from way back in 2010-11. I don't remember what all happened, but by the end of this I decided that I might enjoy doing other things more than this.


https://www.mindat.org/forum.php?read,7,207246,207985


BTW, thanks for the photo at Norwood. Those are bodacious chert nodules! Really impressive.

31st Mar 2018 04:44 UTCAlfredo Petrov Manager

Norman, with regard to, "In the classification sections it is indeed referred to as a rock, but the "chert" page is a mineral page," - Mindat does not have separate cataloguing programs for minerals, rocks, petroleum, etc, so they are all filed as "minerals". Here is the page url for Basalt, for example: https://www.mindat.org/min-48492.html - This does not mean that Mindat is claiming that these are "minerals"; one has to go to the respective page itself to find out the substance's status as a mineral or not.


Daniel, Your photos are great, and we should encourage all testing to be documented this way! I would recommend using a smoother feldspar cleavage (or polished feldspar), as hardness tests can be difficult on rough surfaces.

31st Mar 2018 16:53 UTCDaniel Bennett

Norman I do have more of it. that sounds like an incredible offer. thank you. I sent a PM.

31st Mar 2018 18:58 UTCNorman King 🌟 Expert

The hardness of glass and chert is difficult to research. You probably cannot find what you’re looking for by doing a search on “hardness of glass” or “hardness of chert.” You just get seemingly endless entries about Moh’s hardness scale. And it doesn’t help to search on other hardness scales–there are quite few used in industry. It matters what industry you are talking about. Temperature history is well-known to be important for durability of glass. Like, how many times has the glass been heated to temperatures approaching those of melting? It may not be as important for chert/chalcedony, especially for relatively pure chert (i.e., relatively pure quartz), because quartz is quartz no matter how hot it once got.


Impurities are also discussed mainly as generalities. Impurities in chert may be present in the form of discrete grains of a foreign material. That is probably the main reason for loss of transparency or translucency in impure chert. However, impurities are also present on the atomic or molecular level, and those may be essentially invisible, except perhaps for their effects on color absorption, magnetic properties, and perhaps some other issues.


I have seen a figure of 5-15% non-silica material typically in cherts, but that was a gross generalization not documented by rigorous data. Other minerals may also have considerable impurities, but 5-15% is probably on the high side for most. For minerals, the impurities may be present as scattered intergrowths of tiny domains or discrete crystals of different minerals that are individually much purer than the 85-95% figure (i.e, allowing for 5-15% impurities, but this not really the kind of impurity we are dealing with here). But don’t forget about solid solution in minerals, e.g. sodium plagioclase to calcium plagioclase. The impurities (or variations) in solid solution mineral series’ are at specific sites in crystal lattices, rather than at unspecified sites as in chert.


Glass is not a mineral, it is an amorphous solid. The glass in Moh’s hardness scale is silica glass. Metals can also solidify into glasses (amorphous solids). Metallic glasses are mainly various kind of metal alloys that can be cooled fast enough to form solids lacking crystal structure, just like silica glass. “Liquidmetal” and “Vitreloy” are commercial names of a series of amorphous metal alloys. “Liquidmorphium” is one example that includes zirconium, copper, aluminum, nickel and silver. (Those may be trademarked names). Such alloys have lots of high-tech uses.


We don’t know what went into the making of any particular silica glass. Some contain more silica than others, and, conversely, less impurities. So, in the issue of this thread, we have two variables–purity of the chert and purity of the silica glass being tested against each other. We have no “control” (i.e., no knowledge) of those two variables. Thus, we shouldn’t get too concerned about a piece of chert not scratching a piece of glass. It might be true, however, that the maximum Moh’s hardness of chert really does match that of quartz.

31st Mar 2018 22:42 UTCLarry Maltby Expert

Norm,


Thanks for the link. I tried to find it by searching your posts but I see I did not go back in time far enough.

17th Aug 2018 17:33 UTCDaniel Bennett

03775660016017858193390.jpg
here are three more similar pieces. the smaller two cant seem to be scratched with a file. the bigger piece can be scratched but with light pressure the steel from the file will rub off on to the rock. I tried to lightly rub the felsdpar with the file but could not get any steel to rub off. it did scratch it. so my final verdict is that all this material is actually harder than feldspar. perhaps they vary only in porosity.





so hardness test on rocks may not be completely useless but special care and understanding is certainly required.




17th Aug 2018 17:34 UTCDaniel Bennett

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one last specimen to show for pondering. its seems to be a conglomerate of sorts with rounded mudstone pebbles and hollow cavities. some partially and some completely filled with pure chert . the large crystal cavity/geode and chert nodule seemed to have influenced the surrounding pebbles during formation suggesting a fluid like state in the rocks and mud at that time. I cant help but think that the pure chert penetrated the soft mud balls creating a sort of chertified mudstone coglomerate. (those rounded pebbles in the conglomerate may have become rounded from a liquid state rather than the normal river rounded or glacier rounded process ).


so anyway thanks again very much to Norman King and the rest of you for the input, insight, and ideas .


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11th Jun 2019 07:02 UTCEch Noch

08229290017055249138395.jpg
Here is my contribution to this discussion, I pulled these from a softer zone of serpentine along a fault. There are small zones of this stuff scattered over a short distance along the fault. I found this discussion when searching for “silicified serpentine” on google. Your stuff looks very similar to mine. Does yours sing when clinked together?

03292110015652152899172.png

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11th Jun 2019 07:10 UTCEch Noch

08728000016017858345335.jpg
Under 10x magnification.

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11th Jun 2019 20:08 UTCDaniel Bennett

not sure about the singing. I will listen for that when I get a chance. the cherty mudstone is unique because it can scratch a bottle and be scratched by a fluorite crystal. a file scratch's it with pressure but leaves a steel mark and no scratch with light pressure. can you say that about your material?

12th Jun 2019 00:58 UTCEch Noch

The different layers very significantly in quality and hardness, some are impervious to light while others are translucent. Some are hard and have a “clink” sound when tapped and other have a flat “clack” sound. The similarities to shale and slate are unmistakable aside from the translucency. Some have sulfides and others have a shimmery look with microscopic mica flakes. Overall, everything you have said about your material applies to my material at different places and parts of the same vein even.

12th Jun 2019 01:24 UTCEch Noch

The different layers very significantly in quality and hardness, some are impervious to light while others are translucent. Some are hard and have a “clink” sound when tapped and other have a flat “clack” sound. The similarities to shale and slate are unmistakable aside from the translucency. Some have sulfides and others have a shimmery look with microscopic mica flakes. Overall, everything you have said about your material applies to my material at different places and parts of the same vein even.

12th Jun 2019 03:02 UTCDoug Daniels

"the cherty mudstone is unique because it can scratch a bottle and be scratched by a fluorite crystal"


Somethings not right here. If the material can scratch a bottle, it has a hardness of about 5.5 to 6 (or more). If a fluorite crystal can scratch it, it has a hardness of 4 or less. The hardness can't be both.

12th Jun 2019 16:47 UTCDaniel Bennett

interesting Jobe it does sound like you have a similar thing. cool. do your geology maps show fransican chert around where you got this from?


Doug , I know what you mean. think of a hard packed dirt road. if you rubbed a bottle on it the bottle would be scratched. and yet you could write your name in the road using a fluorite crystal.
 
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