Mudstone, Calcite : CaCO₃
Mack Brick Yard, Windsor, Hartford County, Connecticut, USA

How to use the mindat.org media viewer

Click/touch this help panel to close it.

Welcome to the mindat.org media viewer. Here is a quick guide to some of the options available to you. Different controls are available depending on the type of media being shown (photo, video, animation, 3d image)

Controls - all media types

Zoom in and out of media using your mousewheel or with a two-finger 'resize' action on a touch device.

Use the mouse or your finger to drag the image or the view area of the image around the screen.

< and > at the left and right hand side of the screen move forwards and backwards for the other images associated with the media you selected. Usually this is used for previous/next photo in a gallery, in an article or in search results. Keyboard shortcuts: use shift + the left and right arrow keys.

< and > in the bottom center are used for switching between the photos of the same specimen. Keyboard shortcuts: use the left and right arrow keys.

> in the bottom center, raises the information box giving details and further options for the media, < at the top of this box then hides it. Keyboard shortcuts: use the up and down arrow keys.

? opens this help window. Keyboard shortcuts: use the H key or the ? key.

Other keyboard shortcuts:

1	Fit image to screen
2	Fill screen with image
5	Display at full resolution
<	Make background darker
>	Make background lighter
space	Hide/dim titles and buttons

Scalebar

If the field of view (FOV) is specified for the photo, the scalebar appears in the left bottom corner of the viewer. The scalebar is draggable and resizeable. Drag the right edge to resize it. Double click will reset the scalebar to it's default size and position. If the scalebar is in default position, double click will make it circular.

Controls - Video

Video files have a standard set of video controls: - Reset to start, - Skip back, - Play, - Pause, - Skip forwards. Keyboard shortcuts: You can stop/start video play with the P key.

Controls - Animation (Spin Rotation)

Animation (usually 360 degree spin rotations) have their own controls: - enable spin mode. Note that while images are loading this option will not be available but will be automatically activated when the animation has loaded. Once active you can spin the image/change the animation by moving your mouse or finger on the image left/right or by pressing the [ or ] keys.

The button switches to move mode so that you can use your mouse/fingers to move the image around the screen as with other media types.

The button, or the P key will start playing the animation directly, you can interrupt this by using the mouse or finger on the image to regain manual movement control.

Controls - 3D Stereoscopic images

If a stereoscopic 3D image is opened in the viewer, the 3D button appears in the bottom right corner giving access to "3D settings" menu. The 3D images can be viewed in several ways:
- without any special equipment using cross-eyed or parallel-eyed method
- with stereoscope
- with anaglyph glasses.
- on a suitable 3D TV or monitor (passive 3D system)

For details about 3D refer to: Mindat manuals: Mindat Media Viewer: 3D

To enable/disable 3D stereo display of a compatible stereo pair image press the 3 key. If the left/right images are reversed on your display (this often happens in full-screen mode) press the 4 key to reverse them.

Controls - photo comparison mode

If a photo with activated comparison mode is opened in the viewer, the button appears in the bottom right corner giving access to "Comparison mode settings" menu.

Several layouts are supported: slider and side by-side comparison with up to 6 photos shown synchronously on the screen. On each of the compared photos a view selector is placed, e.g.: Longwave UV ▼. It shows the name of currently selected view and allows to select a view for each placeholder.

Summary of all keyboard shortcuts

1	Fit image to screen
2	Fill screen with image
3	Switch to 3D display of stereo pair
4	Switch left/right images in 3D mode
5	Display at full resolution
<, >	Make background darker/lighter
H or ?	Show/hide this help page
P	Play/Pause Video or Animation
[, ]	Backwards/forwards one frame (Animation only)
space	Hide/dim titles and buttons
up arrow	Show information box
down arrow	Hide information box
left arrow	Previous child photo
right arrow	Next child photo
shift + left arrow	Previous image on the page
shift + right arrow	Next image on the page

minID: 9NM-PUC

Mudstone, Calcite : CaCO₃

Locality: ⓘ Mack Brick Yard, Windsor, Hartford County, Connecticut, USA

Dimensions: 42 mm x 34 mm x 11 mm
Weight: 26 g

Very symmetrical mudstone concretion. Former Bentley collection.

This Photo was Mindat.org Photo of the Day - 27th Dec 2020

This photo has been shown 835 times

Photo added:	5th Dec 2020
Dimensions:	4207x3456px (14.54 megapixels)
Camera:	CANON EOS 600D / Rebel T3i / Kiss X5

Data Identifiers

Mindat Photo ID:	1108114 📋 (quote this with any query about this photo)
Long-form Identifier:	mindat:1:4:1108114:1 📋
GUID:	c846b56b-c1ab-4ec3-8de9-4be43bc28632 📋
Specimen MinID	9NM-PUC (note: this is not unique to this photo, it is unique to the specimen)

View Harold Moritz's Photos View Mudstone Gallery Download Image

Discuss this Photo

PhotosTechnically a "clay concretion".

27th Dec 2020 19:21 UTCHarold Moritz 🌟 Expert

Thanks for using this image as a POTD. When I created the locality page, I tried to list the pieces as "clay concretion" or just "concretion" but mindat would only add "clay" to the list of rocks/minerals we could use. But these are technically "clay concretions", so it would be more accurate if such a rock/mineral could be added to the list. Concretion is in the mindat glossary.

Thanks.

27th Dec 2020 19:36 UTCPaul Brandes 🌟 Manager

It truly is a fascinating pattern that this concretion formed!

27th Dec 2020 20:11 UTCJeff Dunn

Great photo, and specimen. It reminds me of Meso American artwork.

28th Dec 2020 00:30 UTCDavid K. Joyce Expert

Hi Harold, It is an amazing shape! Just looking at it, the grain of it looks quite coarse for a clay mineral. Is it actually clay or is it calcite granules or some combination of clay and other mineral(s)?

There are quite a few clay minerals. To enter it into mindat properly, the actual type of clay should be determined and then it would be easy to enter it under the particular clay mineral. If it was a pyrite concretion, it would be entered under pyrite. If it was composed of calcite, it would be entered under calcite. Marcasite, marcasite, etc.

Thoughts? David KJ

28th Dec 2020 01:55 UTCJosiah Heyman 🌟

My wife was curious: what does it look like on the reverse side? Is it symmetrical?

28th Dec 2020 02:36 UTCHarold Moritz 🌟 Expert

Thanks, collecting these is not my primary interest, but I wanted to document the site and I like this one because of the symmetry but also because it looks like it is staring back at you (enhanced by the narrow view in the mindat home page).

The back side is not very interesting (kinda like the back of one's head compared to the front), so I didnt photograph it.

The granularity visible is not individual mineral grains, but just roughness from wear of the clay.

This is really a rock, so it should not go under a mineral name, but as noted in my initial post, the rock type is not currently an option in mindat, but needs to be. I believe these are cemented by calcium carbonate, but will look into it more. It is from varved glacial lake sediments, so it could be made from multiple clays.

28th Dec 2020 05:08 UTCKevin Conroy Manager

I've heard these called "clay babies".

28th Dec 2020 12:39 UTCHarold Moritz 🌟 Expert

"Clay dog" is another casual term.

28th Dec 2020 12:02 UTCPeter Nancarrow 🌟 Expert

Harold,

You don't say whether you own the piece, or who collected it, or whether the circumstances of its original collection are known, but pending any such information, and as the locality is a former brickyard, I have my doubts as to whether this is a natural shape?

A single mirror plane in a complex concretion would be an unusual feature anyway, but two at right angles, with the remarkably similar-sized concentric circles left right contrasting with the smooth N-S "Dog-bone" shape (also with very similarly-sized "knuckles") is one co-incidence too many for me to leave unquestioned, and make me think that this looks like a carving.

The other concretions illustrated on the locality page mostly appear to have a much smoother clay-like surface texture than this piece, which is distinctly more coarse-grained in appearance, and all the others are either simple discs or irregular shapes and aggregates - even those irregular ones which appear to have been selected for their "bear", "hippo", "duck", "penguin", etc lookalike shapes! (Which I can readily accept as natural!)

I am happy to be proved wrong on this, but what sort of natural process could anyone propose which might have formed a concretion with such striking two-fold symmetry?

28th Dec 2020 12:43 UTCHarold Moritz 🌟 Expert

It's possible, but as it was found in an old collection and the owner has passed, I don't know. This surface does look rougher than the back side, but the clay/silt varve layers in the deposit are only mms thick so it could just be natural variation. Where these occur there are so many that collectors tend to save only the most interesting ones, so you have collector bias to factor in.

Here's a bit from wikipedia (italics added) https://en.wikipedia.org/wiki/Concretion

Disc concretions composed of calcium carbonate are often found eroding out of exposures of interlaminated silt and clay, varved, proglacial lake deposits. For example, great numbers of strikingly symmetrical concretions have been found eroding out of outcrops of Quaternary proglacial lake sediments along and in the gravels of the Connecticut River and its tributaries in Massachuset and Vermont. Depending the specific source of these concretions, they vary in an infinite variety of forms that include disc-shapes; crescent-shapes; watch-shapes; cylindrical or club-shapes; botryoidal masses; and animal-like forms. They can vary in length from 2 in (5.1 cm) to over 22 in (56 cm) and often exhibit concentric grooves on their surfaces. In the Connecticut River Valley, these concretions are often called "claystones" because the concretions are harder than the clay enclosing them. In local brickyards, they were called "clay-dogs" either because of their animal-like forms or the concretions were nuisances in molding bricks.

28th Dec 2020 16:36 UTCEd Clopton 🌟 Expert

Here's a remarkably similar concretion that also came from an old, unattributed collection via a dealer. Front and back are very similar. It has a masking-tape label on the back, badly faded but reading "Clay Concretion / Logan [. . . .] N.J." The dealer also had a few accompanying concretions that were simple disks without the rectangular section in the middle.

I can imagine a process of deposition that nucleates at a particle of chemically distinct material and spreads radially from that point but is confined between less porous (or less chemically hospitable) layers, resulting in a circular disk rather than a sphere. If the process also nucleates at a third point between the other two and proceeds outward, it would collide with the other two in the E-W direction but would proceed in the N-S direction, producing the bowtie shape in Harold's specimen if the process proceeds far beyond the size of the end disks, or the rectangular shape in my specimen if it stops sooner. Seems plausible to me.

28th Dec 2020 20:28 UTCTerry Burtzlaff

Hi Harold, I have collected similar concretions from the varved clays of the Schoharie valley in New York, at one time a glacial lake. Each concretion nucleates around a small pebble, and grows outward, mostly as simple discs. In some instances the discs intersect, forming groups such as yours.

28th Dec 2020 20:58 UTCHarold Moritz 🌟 Expert

These other pieces are nice and we need to get them on mindat under the rock type "claystone" or "clay concretion". I was hoping management would chime in on this thread and add that rock type to mindat so these aren't just listed under "clay".

28th Dec 2020 21:58 UTCRalph S Bottrill 🌟 Manager

I must admit to being somewhat sceptical about this. The shape is almost too perfectly symmetrical for starters, but we have to admit that nature can be remarkable. But we need to know more about its composition also. It looks quite sandy in texture? When concretions form in clay they are usually due to carbonates, or less commonly silica, sulphates or pyrite, growing in the matrix, sometimes cementing grains and sometimes pushing them away. So they may contain a lot of clay, silt or sand inclusions, often dominating over the matrix, but are usually named for the cementing material. So I really doubt it’s just clay, but we need an analysis if possible?

29th Dec 2020 00:05 UTCHarold Moritz 🌟 Expert

These were described in the 19th century, very well-known in northern (glaciated) areas. As mentioned above, they are calcium carbonate-cemented clay/silt concretions. Not sandy, form in varved silt/clay glacial lake deposit. If you break off a piece and crush it, it turns to fine powder. Where some streams cut into the deposits, thousands can end up as a gravel bed lag deposit and they have been coming out of numerous brickyard pits since the early 19th century.

A good summary, with chemical analyses and many images, including nicely symmetrical ones, is:

Sheldon, J.M.A., 1900. Concretions from the Champlain clays of the Connecticut Valley. University Press, Boston. pp.74.

29th Dec 2020 00:47 UTCRalph S Bottrill 🌟 Manager

Thanks Harold, good work, so these should probably be labelled calcite rather than just clay. Though I have seen some composed of aragonite, and some similar ones in the same clay pit composed of baryte, so an XRD would be good.

29th Dec 2020 12:59 UTCHarold Moritz 🌟 Expert

Well, these are rocks, and since mindat now has rocks to choose from when posting localities and photos I was just hoping mindat could add concretions, of various kinds, to the official pick list. "Clay", even in the sedimentary sense, is not accurate, but was all I could choose from or that mindat would "accept" when trying to add "clay concretion" or "concretion" to the locality via the usual Edit - Add Rocks/Minerals form. Concretions are in the mindat glossary.

31st Dec 2020 09:26 UTCLidja Kuresevic

I've found that book available on line. It has some very interesting illustrations at the last part of it.

https://babel.hathitrust.org/cgi/pt?id=uc2.ark:/13960/t53f4nf18&view=2up&seq=1

31st Dec 2020 12:49 UTCHarold Moritz 🌟 Expert

I perused it via Google Scholar.

2nd Jan 2021 22:45 UTCJolyon Ralph Founder

The reason for these being so perfect is in no small part due to collection bias. Those that are most symmetrical are the ones that are most desirable and are therefor the ones that are most publicised.

28th Dec 2020 22:11 UTCJohannes Swarts

I can attest to the perfect symmetry of clay concretions.

When I was around 13 years old, I was taken to a location near Putney, VT to dig for concretions. We found one that looked like a perfect dumbbell - two flattened spheres connected by a narrow neck. It was probably an inch and a half across. Another one we named "the flying saucer". It was also a flattened sphere (ellipsoid?) with a perfect protruding rounded rim running all the way around the "fat" part of the ellipsoid. Interesting thing about the rim was that it was a different color than the ellipsoid.

I wish I still had these specimens...

I've been meaning to go back there - maybe next spring.

Hans

29th Dec 2020 12:52 UTCHarold Moritz 🌟 Expert

I have seen plenty of pieces from Putney, VT but have not been there. Personally I have only collected these a couple of times because once you go to a good place there are so many you can't even give them away! I have a few other interesting ones to post.

Note to all readers that the term "Connecticut" in these documents generally alludes to the Connecticut River valley and local tributaries because the river follows the basin that the primary glacial lake (Lake Hitchcock) these came from occupied. The clay/silt sediments extend from a bit south of Hartford all the way to nearly the border with Canada, with the widest parts in Connecticut and Massachusetts.

29th Dec 2020 04:00 UTCKevin Conroy Manager

Please note that another paper on the Connecticut concretions reported an analysis of mainly silica. See page 456 of: https://www.journals.uchicago.edu/doi/pdfplus/10.1086/621531

29th Dec 2020 04:02 UTCKeith Compton 🌟 Manager

Kevin - link doesn't work

29th Dec 2020 04:50 UTCDon Windeler

Try this one.

https://www.journals.uchicago.edu/doi/pdfplus/10.1086/621531

Another victim of mindat baking in extra spaces and punctuation into pasted URLs... there was a space at the end.

29th Dec 2020 04:56 UTCKeith Compton 🌟 Manager

Thanks Don

That worked

29th Dec 2020 15:46 UTCKevin Conroy Manager

Don Windeler ? ✉️

Another victim of mindat baking in extra spaces and punctuation into pasted URLs... there was a space at the end.

Thanks Don, I forgot to check this before I logged off last night.

30th Dec 2020 04:28 UTCRalph S Bottrill 🌟 Manager

Unfortunately it’s hard to know what to make of that reference. He gives chemical analyses of clays and concretions from a different location to that described, and the second one does not total correctly (>100%). Despite him saying the concretions effervesce in acid he says they contain more lime and less CO2 than the clay, which makes no sense. His argument about mud balls forming in sediments isn’t really the same as concretionary growth post-deposition. I am not convinced that clays alone would grow into concretions in sediments but am happy to be proven wrong. I think we still need to get a mineralogical analysis of these objects; I would be surprised if they don’t contain a fair amount of carbonate.

As to their classification as a rock type, good question! We have concretions in the glossary but not as a rock type, it’s in the same category as nodule, geode, pisolith, stalactite, etc: more of a type of mineral growth forming part of a rock rather than a true rock type itself. If we knew what these were composed mostly of, we could label them accordingly, eg siltstone, mudstone etc. If that sounds unsatisfactory, we should think also about terms like pebble, clast, vein, band, xenolith, etc.; sometimes we can describe the texture without much idea of the rock type or mineralogy and that’s fine. This photo would be wonderful to illustrate the glossary entry!

30th Dec 2020 05:26 UTCKevin Conroy Manager

Yes, and it reads in the style of much of the scientific text from around 1900: research mixed with some opinion. I'm not saying this is a bad thing. It's just the way it was, and they didn't have the technology that we take for granted.

I've also mulled over how to handle these. I don't think that many (any?) folks would pay to get them analyzed. Finding research that did so was VERY challenging, and like my previous link is usually outdated.

30th Dec 2020 13:44 UTCHarold Moritz 🌟 Expert

The main purpose of posting the references is for historical/geological/geographical context and to show the diversity of natural forms - these are widespread and have been known for a long time. They form in unconsolidated varved silt/clay - they are a lithified portion of it, simply the matrix very locally cemented by calcite (or something) into silt/claystone.
I just tried adding "claystone" to the locality page under "Add more rocks/minerals" and mindat accepted it, so why not just go with that?

30th Dec 2020 19:38 UTCRalph S Bottrill 🌟 Manager

If it’s a lithified mixture of clay and silt its a mudstone, probably a sandy mudstone from the look of the first photo. It would still be good to see how it reacts to acid.

31st Dec 2020 01:18 UTCHarold Moritz 🌟 Expert

I'll test it. But these are not sandy, occur in well-documented, very thick, varved silt/clay formations used in brick-making where sand is a detriment. Surface roughness you see in the pic is just wear, I could easily smooth it out with a toothbrush.

31st Dec 2020 13:27 UTCHarold Moritz 🌟 Expert

Fizzed right up with one drop of HCl on the back.

31st Dec 2020 22:32 UTCRalph S Bottrill 🌟 Manager

Good one thanks Harold, I would make the title of all these calcite and mudstone. Some sand calcite crystals, sand roses etc have more sediment than crystal, but it’s the cementing mineral that gives them the interesting forms.

30th Dec 2020 13:46 UTCHarold Moritz 🌟 Expert

Here are some on display at the Field Museum in Chicago.

2nd Jan 2021 22:31 UTCRalph S Bottrill 🌟 Manager

Thanks Harold for updating the photos. I have updated the locality page, let’s know of any amendments needed?

2nd Jan 2021 23:11 UTCJolyon Ralph Founder

https://www.mindat.org/min-55282.html

5th Jan 2021 20:42 UTCRalph S Bottrill 🌟 Manager

Hi Jolyon

As they are lithified they should probably be classified as varieties of Marlstone not Marl, though the latter does tend to be used informally for rocks also.

5th Jan 2021 23:44 UTCHarold Moritz 🌟 Expert

This classification does not seem correct. The sediment is siliciclastic, not carbonate. Only the cement is carbonate. The silt and clay is an essential component. It's a siliciclastic mudstone with a carbonate cement.

6th Jan 2021 20:42 UTCRalph S Bottrill 🌟 Manager

A marl can be anything from a carbonate-bearing siliceous mud to a siliceous lime-mud. I don’t think there is an agreed formal lower limit for the carbonate content, maybe 25-35%. The carbonate in the concretions was probably deposited as a sediment and during diagenesis moved and recrystallised into concretionary structures. Marlstones (lithofied marls) would have similar composition and internal texture to concretions, though the name is usually applied to rocks in thicker beds. But the actual rock type is much the same whether it’s found as a small pebble or a formation 100’s of metres thick.

To get the carbonate content of the concretion, grind one up and drop in acid, weighing before and after. It the carbonate content is <25% you can call it a limey mudstone.

6th Jan 2021 21:07 UTCRalph S Bottrill 🌟 Manager

Just looking at our definitions. We define lime-mudstone on Mindat as “at least 75% mud grade (< 32μm) calcium carbonate ”, but describe marlstone as a subtype equivalent to lithified marl, which has two definitions: “65% to 35% carbonate ” or “calcium carbonate may range from 90% to less than 30% ”. That should suitably confuse everyone! Looking at Wikipedia they classify marl/marlstone as 65-35% carbonate, below that it’s argillaceous marl (15-25%) or calcareous mud (5-15%). That makes good sense but then they describe the marl mined in New Jersey for agriculture as having <14% carbonate! I suspect few others on Mindat care but I work work on describing these rocks and sediments more clearly.

6th Jan 2021 22:25 UTCFrank K. Mazdab 🌟 Manager

Ralph, I care, and let me say I appreciate the efforts you're making to improve our rocks.

I ought to be joining you for this, but one day I looked at the metamorphic rock page (https://www.mindat.org/min-51421.html ), featuring as header images a photo of igneous lherzolite and two photos of moldavite (somewhat borderline as a metamorphic rock, per se, and certainly an esoteric choice even if one accepts that... are there really no nice explicitly metamorphic rock photos on mindat?), and then I looked further down at the rock list, which is largely just a list of every igneous and sedimentary rock one can image with an added "meta-" prefix, and I shook my head and said "nope... not today"...

EDIT: to whoever replaced the above-mentioned photos with photos of gneiss and eclogite, thank you!

7th Jan 2021 01:59 UTCHarold Moritz 🌟 Expert

Ralph:

I think it's great that you are working on defining these rocks for mindat. In the case of concretions of various kinds they may not fit into the normal sediment/rock hierarchy in that they can differ significantly from their hosts.

I can't speak to all types of concretions, but in the case of the "clay-dogs", their post-glacial lake hosts formed over metamorphic terrane with very little carbonate rock and my general knowledge is that the slit/clays deposited therein are siliclastic. Most general references and detailed mapping just refer to the material as "silt and clay" without giving the chemical composition of them, but they tend imply that they are not carbonate rich (I don't think one can make bricks from carbonate-rich mud). But the sediment source was ground-up schists, gneisses and granites, basically. I don't have recent documents handy, but I will check for more recent data with local glacial geologists I know at the state and USGS. (this is the first time I've tried to find this level of detail on these concretions, so I'm learning as I go along here!). But in the meantime, I again skimmed through:

Sheldon, J.M.A., 1900. Concretions from the Champlain clays of the Connecticut Valley. University Press, Boston. pp.74.

https://babel.hathitrust.org/cgi/pt?id=uc2.ark:/13960/t53f4nf18&view=2up&seq=4

In it analyses of concretions show about 50% calcium carbonate cement and most of the rest alumino-silicates clay silt. Given that porosity of such fine-grained unconsolidated sediment is about 50%, then there should not be much calcium carbonate in the uncemented sediment. Indeed on page 34 [of the document] are two analyses of uncemented clay/silt. They show only about 2 to 4% calcium carbonate, so it is not even a "calcareous mud". The summary on page 37 says "the essential difference between the concretion and the surrounding clay, as shown by analysis, is the small percentage of calcium carbonate in the latter as compared with that of the former." Keep in mind that there has not been any diagenesis here, really, the deposits are still at the surface and only up to 15K years old.

So, as far as I can find out so far, the unconsolidated matrix is not a marl, but the concretion is a calcium carbonate cemented mudstone. But I don't know if that makes it a "marlstone" if the original sediment was not a marl. Perhaps a separate hierarchy for concretions be developed on mindat? (I can't think of geology text that addresses them!)

7th Jan 2021 19:32 UTCHarold Moritz 🌟 Expert

Here are 5 analyses of unconsolidated clay. They show very little lime.

From Clay and Clay Industry of Connecticut, 1905.

https://www.biodiversitylibrary.org/item/140196#page/3/mode/1up

8th Jan 2021 23:42 UTCRalph S Bottrill 🌟 Manager

Hi Harold

Thanks for the extra info, sorry I’ve been pretty busy the last couple days but will get back on this soon.

One of the interesting things about concretions is that their composition tends to be fairly much unrelated to their matrix, eg you commonly get carbonate concretions in siliceous sediments containing little carbonate and chert concretions in carbonate sediments containing little silica. The other point we need to remember is that petrologists try to make at least the broader lithology names descriptive rather than genetic, eg a chert may be sedimentary, diagenetic or hydrothermal, and hosted by limestones, volcanoes or argillaceous sediments but it’s still chert if it has the right mineral constitution and texture.

9th Jan 2021 02:11 UTCHarold Moritz 🌟 Expert

I think that's why these have been called "lime-clay" concretions, which is descriptive rather than genetic. As is, the hierarchy here https://www.mindat.org/min-55282.html
requires an essential "marl" sediment. But these form directly within the "mud" level two steps higher. As you mention above, too, you can have compositionally very different concretions within a variety of hosts, so does it really make sense to put them in a hierarchy that flows through the specific hosts to get to them? Concretions may best be put only directly below the "sedimentary rock and sediment" level because of the variety of hosts (but obviously they are not in metamorphic or igneous rx). Then separate subdivisions of concretions could be developed.

10th Jan 2021 07:50 UTCRalph S Bottrill 🌟 Manager

One problem is we don't know what the include material is, or if the host sediment is clay: its probably just mud, with a lot of silt. I did a modal mineralogy based on analysis 3, and get about:

Quartz 8.8

Illite 40.0

Albite 23.0

K feldspar 4.0

Rutile 0.8

Kaolinite 5.0

Calcite 3.5

Chlorite 12.5

Montmorillonite 5.0

You can play with the numbers a lot, but with similar results.

This means there is only about 50% clay minerals unless you include chlorites; most of the quartz and feldspar will be in the silt fraction. So the included material should be called mud, not clay (the latter implies >90% clay minerals). There will still be about 35-50% silt and sand. I dont like the name lime-clay concretion for this reason; lime-mud concretion would be more realistic, or just carbonate concretion.

Re the host rock concretion relationship: the concretion in petrologically a marl (with ~50% carbonates , plus mud); the matrix is mud and I think this is all fine?

10th Jan 2021 11:24 UTCLarry Maltby Expert

I vote for the name “carbonate concretion”. The shape is controlled by the accretion of a carbonate within a sediment. Without the carbonate there would be no shape. If the name is based on the sediment content within the shape, naming becomes problematic. As “carbonate concretions” are found from place to place, the naming would be dependent on analysis similar to what Ralph shows above.

10th Jan 2021 13:56 UTCHarold Moritz 🌟 Expert

The matrix is garden variety alumino-silicate clays and silts (mud) derived from weathering and physical crushing by continental ice sheet of common metamorphic rocks - overwhelmingly made of albite/quartz/muscovite/microcline/biotite/"hornblende". Your modal mineralogy is more or less correct, probably a mix of biotite and chlorite. The clay is glacially-derived "rock flour", the silt is described by Sheldon as "arkosic" in composition.

In the hierarchy it looks like "marl" applies to the unconsolidated sediment, as opposed to "marlstone"? In any case, "carbonate concretion" sounds applicable since the carbonate is what is holding it together and makes it different from the host matrix.

10th Jan 2021 21:31 UTCRalph S Bottrill 🌟 Manager

Thanks Larry and Harold, I agree with “carbonate concretion”; we could keep lime-mud concretion as a subtype?

Yes marlstone is an indurated or cemented marl, it does describe the concretions petrologically, but ignores their structure.

These muddy sediments could contain biotite, muscovite, feldspar, amphiboles etc; I did play around with these a bit, but these minerals will mostly be in the silt fraction also, and the clay content constrained by the LOI. So it’s a weakly calcareous, clayey silt, or just mud!

3rd Jan 2021 00:00 UTCKeith Compton 🌟 Manager

Just wondering if they form in a similar manner to ice discs on water, only on liquid clay. I could imagine the clay liquid flowing like water.

3rd Jan 2021 02:55 UTCHarold Moritz 🌟 Expert

No similarity to ice disks at all. The claystones form in-situ by lateral propagation of the cementation mostly along the varved clay/silt bedding planes, which has greater permeability than across the bedding. This results in the typically flat shapes. Then they erode out in gullies or found in clay pits where they were a nuisance.

Yes collector bias selects the more aesthetic ones, most are not particularly interesting, but they can be present in stream beds by the thousands as an erosional lag deposit.

9th Jan 2021 13:22 UTCLarry Maltby Expert

This series of photos shows the accretion of pyrite in the Antrim Shale in Michigan. It appears that there is a natural chemical attraction that causes pyrite to accumulate at a specific point in the shale sediment. As stated above, these clay concretions are the result of the accretion of calcite. There may be some mechanical reason (grain size?) that causes the soft rounded shapes as opposed to the sand calcite crystals from South Dakota.

11th Jan 2021 12:41 UTCRalph S Bottrill 🌟 Manager

Yes Larry the pyrite, calcite, baryte etc seem to find some odd grains to nucleate about and off they grow. Whether they form crystals, roses or balls seems to depend on growth rates and degrees of supersaturation in critical elements. Quite amazing what can happen in cold, damp mud really!

31st Oct 2021 20:32 UTCStephanie Parker

Clay concretion, self collected in Schoharie NY

31st Oct 2021 20:34 UTCStephanie Parker

Perfect shapes. I LOVE THEM.

1st Nov 2021 06:45 UTCHerwig Pelckmans

Hi Stephanie,

Welcome to Mindat!

Those are beautiful samples!

Thanks for sharing them with us!

Cheers, Herwig

2nd Feb 2022 21:32 UTCPaul F Hewitt

Middlesex, Vermont clay concretions

2nd Feb 2022 21:35 UTCPaul F Hewitt

Middlesex, Vermont clay concretion

2nd Feb 2022 21:36 UTCPaul F Hewitt

https://www.facebook.com/ClayConcretions

3rd Feb 2022 18:25 UTCGregg Little 🌟

In the field situation a rough and ready rule of thumb was used for simple wet chemical analysis of marlstones. The lithologic assumption was that marlstones, usually calcareous although dolomitic ones were occasionally observed, were roughly bracketed as 40 to 60% calcite to clay & silt insoluble residue (in 10% HCl). If the calcite dominated (>60%) then it was an argillaceous limestone. If clay & silt dominated (>60%) then we called it a calcareous mudstone or calcareous shale sediment.

The technique was only visual (binocular microscope) estimations. Noting the original size of the rock chip (usually less than 10 mm) in a watch glass, it was immersed in enough HCl to completely dissolve the calcite. If the insoluble residue disaggregated completely, leaving a cloudy layer in the bottom of the acid pool then its assumed to be argillaceous limestone. If the insoluble residue held together in a spongy porous mass soon after the effervescence stopped then it was labelled a marlstone. Finally if the rock chip residue was slow to give-up the interstitial calcite and continued effervescing slowly after an initial burst of activity then it was called an argillaceous limestone.

These marlstones ranged widely in sedimentary rock types and I encountered them in Devonian to Tertiary sediments. Since samples are from drill cuttings and core (HQ), larger structures usually could not be determined to be bedding or concretions.

and/or

Mindat.org is an outreach project of the Hudson Institute of Mineralogy, a 501(c)(3) not-for-profit organization.
Copyright © mindat.org and the Hudson Institute of Mineralogy 1993-2024, except where stated. Most political location boundaries are © OpenStreetMap contributors. Mindat.org relies on the contributions of thousands of members and supporters. Founded in 2000 by Jolyon Ralph.
Privacy Policy - Terms & Conditions - Contact Us / DMCA issues - Report a bug/vulnerability Current server date and time: May 11, 2024 19:12:02

Mudstone, Calcite : CaCO3Mack Brick Yard, Windsor, Hartford County, Connecticut, USA