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grey rock

Posted by craig johnson  
craig johnson February 04, 2013 06:01PM
Help ID
open | download - 000_0989.JPG (589.4 KB)
Bob Harman February 04, 2013 06:21PM
Good photo, but of a very generic looking rock; compatible with Indiana St. Genevieve or St Louis limestone or closely related. Also compatible with many worldwide limestones so a location wherefrom this rock was found would help!!! Looks identical to the matrix where dolomite vugs occur in Corydon Quarry; Harrison County Indiana. CHEERS..........BOB

Edited 1 time(s). Last edit at 02/04/2013 07:03PM by BOB HARMAN.
Norman King February 04, 2013 09:40PM
So, have you tried an acid test (on fresh rock as well as pulverized)?
Paul Brandes February 04, 2013 10:28PM
Looks to me like either limestone or a dirty quartzite.
A quick acid/vinegar check should tell which it is (if it fizzes, limestone).....
D Mike Reinke February 05, 2013 06:05AM
Why couldn't this be basalt? Not dark enough? The small white streaks?
craig johnson February 08, 2013 02:38AM
Norman and Paul .. yes the rock is reactive to acids (hydrochloric acid and vinigar) I placed a small chip in a cup with vinigar and alot of the carbonates formed a ring around the inside of the cup as the vinigar evaporated within a day or two and there is a layer of silica gel dried up on the bottom of the cup along with two or three small black magnetite grains/crystals. I had already noticed the rock was magnetic and seen the magnetite, even a piece of steel wire hanging next to the cup will pull to and cling to the cup. Some feldspars like oligoclase are reactive to acids from my understanding and even basalt will react to acid .. So I'm not sure that acids will actually determine that if there is reaction to it then it be limestone. furthermore with carbonatite type lava i'm sure it would react to acids. I'm not saying it is one thing or another because at this point I'll need better knowledge of why would the rock be magnetic with magnetite and knowing magnetite is known to be in basalt, the rock looks like basalt, and have not seen fossil or fragments of any remains of shells or organisms in the rock yet. Also there is yellow crusting material that might be palagonite ..and the magnetite seems more occuring near that as ?
Norman King February 08, 2013 02:22PM
If it dissolved at room temperature in a day or so it would be a carbonate. Where did it form? (What has been found in the same area might give us a clue.)
craig johnson February 08, 2013 08:06PM
There is green silty shale looking rock or sediment over this deposit that this rock seems to be grading into, Looks chlorite to me . located in se ohio. The vinegar dissolved a chip of the rock and revealed the magnetite remaining. I used a piece of wire from the twisty tie off from a loaf of bread to check if it suspending (next to the plastic cup) would attract to the remaining especialy the black grains and it pulled and cling to the cup. I could guess this being a chlorite schist but that would be metamorphic and possibly due to contacting igneous . ? This silty shale seems to be very reactive to hydrochloric acid.
cascaillou February 08, 2013 09:06PM
I'm not very in touch with petrography, but I can grab my book of rock identification and give it a try.

I need to know:

-it seems the rock is compact and homogeneous, and does not feature any visible individual elements (like grains, crystals, blades, fossils, gravels, lamellae, crystalline plates, or any kind contrasting elements) in it's structure. Is that right? I need to confirm that the rock does not feature any visible elements.

-the surface of a fresh fracture of the rock can be scratched by the blade of a penknife (I'm talking of a thin blade pocket knife such as the swiss army knife)? My guess is that it can't but I need confirmation.

-does it react with effervescence (bubbling) to concentrated hydrochloric acid? (dangerous acid, wear protective goggles, if you're a kid ask an adult for help). My guess is that there's no reaction but I need confirmation.

-if finely powdered, does it dissolve (after 24hours) in concentrated hydrochloric acid? My guess is that it doesn't dissolve but I need confirmation.

-does it scratch window glass? My guess is that it does, but I need confirmation.

-what is the specific gravity? My guess is that it is between 2.5 and 2.9 but I'd need some accurate measure.

if every single of my first guesses are right, then that would point either to quartzite, corneenne (aka hornstein, don't know what's the english term for this rock), phtanite, phonolite, andesite(I doubt it, doesn't look like it), or ultramylonite

then to make the difference between those, you need to answer additional questions:

-is it completely opaque (meaning that when strongly backlit, a very thin splinter of the rock is not at all transluscent, not even on its sharp edges)?

-what geological context is the rock from, I mean was it found nearby:
*nearby volcanic rocks?
*nearby sedimentary rocks?
*nearby endegeneous rocks
*or both sedimentary and endegeneous rocks?

Edited 7 time(s). Last edit at 02/08/2013 09:22PM by el cascaillou.
craig johnson February 08, 2013 11:06PM
Specific gravity seems to be 2.67 , There is cracks or fractures in the rock with observable crystals that appear to be chlorites and rather soft not scratching glass , the crystals may be layered or cleavage stacking as some look shifted off-set ///// like and also observed some crystal inside another crystal or void space of the crystal being previously present, some twinning possible. There is sedimentary in the area and there is possibly diabase, dolerite , diorite, and some arkose rocks in the area. This rock being massive outcrop and grades into layered more like slatey then shale-silty layers with silica sands overburden.

Edited 2 time(s). Last edit at 02/08/2013 11:49PM by craig johnson.
Norman King February 09, 2013 12:12AM

The information you provided is very helpful. I think this is a limestone concretion from a sedimentary rock sequence. The weathered, somewhat curved exterior surface is consistent with that. The uniform (i.e., non-bedded or foliated) structure is consistent with that. Presence of other minerals within the matrix such as iron minerals and clays and possibly other phyllosilicates is also consistent. If the concretion grew displacively, that might produce local concentrations of phyllosilicates having a foliated appearance. Thus, everything you show and describe fits the diagnosis of limestone concretion. Besides calcite, such concretions often contain dolomite, as well as sand, silt, and clay or other phyllosilicate grains.

Good job at coming up with the descriptive information!
cascaillou February 09, 2013 12:36AM
thanks for the answer Craig, however concerning the absence of visible elements, you misunderstood me I think:

here's a quartzite:

in this case, no individual elements (like grains, crystals, fossils, gravels, lamellae, crystalline plates, or any kind contrasting elements) are seen in the rock structure.

on the other hand, here's a granite:
in this case, the rock is shows different elements, which are crystals (quartz, feldspar and mica crystals).

and here's an oolitic limestone:
here again, the rock shows visible elements which are spheric grains.

here is a basalt which also show different elements (grey, white and black) in its structure:

Now here's a corneenne:
->no contrasting elements to be seen in the structure

So, from the picture you posted of your rock, it seems to me that just like quartzite or corneenne, it is homogeneous and no individual elements are seen, but I'm not sure I'm judging it correctly as this is not a close-up pic of the rock (and maybe I'm just not seeing elements because they are very small), which is why I ask you about it.
Just keep in mind that when identifying a rock, every observations and tests must be done from a fresh fracture (which is exposing the inside of the sample).

the presence of crystallisations inside fractures is not the point, the point is what is the general structure of the rock like: is it homogeneous (no contrasting elements) or is it heterogeneous (rock made of contrasted elements). This is an important distinction to be made so to identify a rock.

A close-up picture of the rock texture on a fresh fracture would be quite helful

Edited 6 time(s). Last edit at 02/09/2013 12:58AM by el cascaillou.
craig johnson February 09, 2013 02:03AM
el cascaillou ,
I agree with corneenne - hornfels this rock , your photo a match to my rock.
cascaillou February 09, 2013 02:37AM
there are actually quite a few rocks that visually match your rock, but you can't tell them apart without proceeding to the tests I suggested and answering the few questions I asked. Your rock might be a corneenne (does anyone know the english term for corneenne?), but it might also be something else.
Norman King February 09, 2013 03:32AM
Craig and El,

"Corneene" must mean "hornfels," because "corne" means "horn."

One cannot call a rock hornfels unless it has been established that it was produced by contact metamorphism. Thus, we cannot say this is hornfels, and therefore contact metamorphism occurred there. It is just the opposite: we can only say that after we have found a contact metamorphic aureole, we can say that this rock should be called hornfels. Craig has not found a contact aureole.

(For example, see--
Matt Neuzil February 09, 2013 03:47AM
you guys probably hit it on the head first with limestone. It likely has some kind of silt, clay and or organic material in it also. End of story.

A buena hambre no hay pan duro
craig johnson February 09, 2013 04:14AM
close up photo
open | download - 000_0996.JPG (454.2 KB)
D Mike Reinke February 09, 2013 05:04AM
That is a lot greener than your other picture, so no way is it a basalt! It doesn't help being partly color'blind, the fairly common, particularly in guys, 'red-green deficient.' That is a bit frustrating in rock IDing, and staring at computr monitors too.

David Baldwin February 09, 2013 12:14PM
Certainly looks like a limestone to me. The beaches where I live are full of the stuff which comes from strata within clays, marls and shales. The iron staining on the exterior is consistent with this too. The green sediment could be what is known as 'greensand', a sort of stickly, clay-sand mixture containing glauconite mica, which forms in marine deposits.
cascaillou February 09, 2013 12:35PM
Yeah, is it gray or is it a bit greenish ???

I suggested two hydrochloric acid tests and two hardness tests. Maybe we'd better stop making shots in the dark and just wait for the results of these tests.

Edited 1 time(s). Last edit at 02/09/2013 12:36PM by el cascaillou.
craig johnson February 09, 2013 03:55PM
Its grey but i think when wet like first break it from the outcrop its greenish but its also wet where the outcrop is. That close up photo did appear too green it was night time I couldnt get the photo outside in daylight at that time.
Hardness it will scratch glass with pressure but not easy like quartz.
hydrochloric acid will dissolve it in couple days time, a small chip in vinegar dissolved most of the chip and softened the whole chip mushy 1 - 2 days.
cascaillou February 09, 2013 04:21PM
if hydrochloric acid can slowly but completely dissolve your rock then what you have is none of the rocks I suggested.

You said acid slowly dissolve it, but does hydrochloric acid provocate almost immediate bubbling (effervescence) too?

also, please watch closely at your rock (on a fresh fracture) and answer my previous question as to wether or not you can see different elements in its structure, or if it is homogeneous with no contrasting elements to be seen in the material?

Edited 2 time(s). Last edit at 02/09/2013 04:24PM by el cascaillou.
Donald Peck February 09, 2013 04:58PM
I, also, think it is a limestone. It certainly looks like one and the fact that it dissolves in acid indicates that it is one. The tip of a knife blade should scratch it.
craig johnson February 09, 2013 11:22PM
Hydrochloric acid reacts like mad on a fresh break immediatly and a chunk of the rock in a container of hydrochloric acid /water mix will react untill the acid lost strength and the chunk of rock mushy.
I can see small black dots that I think are magnetite because the rock is magnetitc .
Some breaks have a slight different texture like one breaks through the solid and one breaks through where two solids met , foliated in a way ? Even one break through the rock can show both of these types of textured breaks on one piece.
Matt Neuzil February 10, 2013 12:06AM
Is it really magnetic? I'd like to see a magnet hanging off of the piece. Living in ohio its likely limestone. It has the characteristics of a dirty limestone.

As said by someone else in the post it looks like limestone concretions. If anyone has been to the blue bridge locality on the huron river in monroeville they know how those concretions are. They are pretty hard to break although they're limestone and look similar to the pieces posted.

A buena hambre no hay pan duro
Bill Cordua February 10, 2013 12:22AM
Hi. Pictures can be misleading and are incomplete. The facts are that the rock was found in southern Ohio, where the bedrock is all undeformed unmetamorphosed sedimentary rock, including abundant limestone and dolostone. The areas has few, if any, volcanic or intrusive igneous rocks. We also know the sample reacts to hydrochloric acid and vinegar. These facts tell us it is a limestone or dolostone. The green mineral could well be glauconite, or a green clay mineral of some sort, both common in that geologic setting. No need to invoke more complex explanation. That's why location information is so important - it greatly helps to reduce the "suspects" for an unknown.
craig johnson February 10, 2013 12:58AM
About it being magnetic I hung the rock from a string and held a magnet near it and the rock will pull toward the magnet or if the rock spins slowly I can stop and change its direction with the magnet , or the rock will cling to the magnet but will not hang onto the magnet without being suspended from the string. Then I tested the remains from the small chip that I dissolved with the vinegar in a plastic cup , I used a twist wire from a loaf of bread removed the paper off it of course and made a hook on the wire so I could hang the wire on the end of a pencil, edit ( I've now discovered that this wire is magnetic) I could see the black grains believed to be magnetite in the plastic cup and the carbonate or corrosive looking stuff on the sides of the cup from this rock dissolving and floating this to the inside sides of the cup crusting there ok the wire suspending near the cup it pulled and cling to the cup where this material obviously is magnetic. The rock does seem to be a type of limestone such as calcilutite or micritic and where this rock was outcropping there is massive rock like this that would take explosives or heavy equipment to break it out. The upper part of this outcrop is more layering and seems to grade into silty layers as would siltstone or shale and that is green .
I'd like to know how and why this rock is magnetic especialy if its limestone ..I tested another piece of the rock that does not have any irony crust on it , broke this piece out as to not have the iron crust , I hang the rock and sure enough its still magnetic pulling to the magnet. If limestone formed from this sediment of shallow sea or lake which Ohio was known to be before and being sedimentary here then where would it have got magnetite from and yet missing other minerals from sand ? ...I'm not even sure how to ask this question, Biological or Erosional and transported iron deposits, or environment durning the time this limestone placement. Is there some hydrated magnetic iron in this or Why if this is limestone then why magnetic like this ?

Edited 2 time(s). Last edit at 02/10/2013 05:47AM by craig johnson.
cascaillou February 10, 2013 03:25PM
if it bubbles in HCl and can be scratched with a knife blade then that is a limestone.
Mohammad Taslim February 17, 2017 05:33AM

someone can help me with these?

best regards

Reiner Mielke February 17, 2017 01:29PM
Hello Craig,

I am intrigued by this magnetic limestone. If you are willing to send me a small piece less than 2cm thick so it can go relatively cheaply as an oversized letter, or less than 5mm thick as a regular letter, I can do some tests on it. Private message me and I will give you my address.
Reiner Mielke February 17, 2017 01:35PM
Hello Mohammad,

The first looks like a tuff or a piece of concrete. The second like pieces of oolitic limestone like the picture posted by cascaillou in a previous post. Are any of them soluble in acid?
Georg Graf February 17, 2017 04:48PM
Hi Craig, on the Schwäbische Alb, near Urach, are limestone containing Magnetite, generated by volcanic heat from limestone containing a little bit Pyrite, Marcasite or/and limonite.

Just a guess. Georg
Gregg Little February 19, 2017 11:33PM

The fact that there may be magnetic detrital material in the rock is not that unusual considering the other insolubles with it (clay and possibly glauconite). Limestone can grade to other sedimentary rocks, as you have alluded to where you stated, "The upper part of this outcrop is more layering and seems to grade into silty layers as would siltstone or shale and that is green".

In a mixed depositional environment you can get silty and sandy limestone or marlstone (clay-rich limestone). It really depends on what the accessories are but to call it a limestone requires the single largest component to be calcite. For example you can have 40% calcite, 30% clay and 30% silt and this is still a limestone although quite unusual. One way of observing the distribution and percentage of insolubles is to acid etch the rock surface (10 minutes?) then flush with water to stop the reaction causing the insolubles to stand in relief and be observable. Observation would require a hand lense or binocular microscope as silt grains are in the 0.0039 to 0.0625 mm size range. If glauconite is also present it is often occurs as tiny pellets and these could also show up in the insoluble residue.

This type of limestone would react quickly at first then slow considerable as the acid has to work past all the insolubles coating the surface of the fragment. The hardness test could be misleading as a higher silt content (usually quartz grains) would give an apparent hardness between calcite and quartz. Like Reiner, I too am intrigued by the rock and would be interested in hearing what you have uncovered, or dissolve out, as the case may be.
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