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rock needs ID

Posted by craig johnson  
rock needs ID
March 07, 2013 07:41PM
sedimentary or metamorphic ?
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open | download - 000_0997.JPG (414.2 KB)
avatar Re: rock needs ID
March 08, 2013 04:54PM
Craig, from what I can see in the photo, I would go with sedimentary. The particles appear to be quite mixed in size and it looks like the rock broke around particles and not through them (although a real close-up would help determine that). There is a slight indication of what could be foliation at the top, but I think it is an artifact.
Re: rock needs ID
March 09, 2013 01:54AM
Thanks for your reply Donald, The rock does react with hydrochloric acid, There looks to be some grain break opposite of the planar cleavage surfaces and are shiny spots through the break of the rock. Would that indicate a marble or would it be limestone ?
avatar Re: rock needs ID
March 10, 2013 04:34PM
If it fizzes everywhere with acid It would be limestone. If only in isolated spots, it could be another type stone containing small amounts of calcite. I would put my money on the limestone.
Re: rock needs ID
March 10, 2013 07:38PM
Hi, The rock might be a calcite cemented clastic sedimentary rock, such as sandstone. I'd try scratching a piece of glass with it. It it scratches glass, it's probably calcite cemented sandstone. It might also be a calcarenite--essentially a calcite sandstone. In either case, it looks sedimentary to me.
Re: rock needs ID
February 23, 2015 11:19PM
I've busted another one of these rocks open and believe it shows phyllite texture and marble , it will polish very smooth, hydrochloric acid fizz once but not twice ..whats going on ?
The one just recently busted the middle inside is grey, outer is green or brown, with also a outer dark brown crust. one direction the micro-plates shine or glisten then approx 90 degrees of that there are areas of sparkling which like marble glistens as tilt around .. I cut one piece with diamond blade then sanded it with 320 , 400 , 600 grit wet/dry sandpaper its polished very smooth with grey meeting the brown colors.



Edited 1 time(s). Last edit at 02/23/2015 11:56PM by craig johnson.
Re: rock needs ID
February 24, 2015 02:08AM
A mudstone, so a slightly metamphosed rock? {is that the correct past-tense?} I hunt stuff on the beach and find that some veins and vugs that fizz like a carbonate when I use my eyedropper bottle of acid on them, when I take home and soak, they only partly dissolve. So it is a mix of silicate/carbonate, I don't get nice, clean veins and vugs usually, there is a lot of clay or other minerals, not pretty ones, cluttering up the 'landscape' under my microscope lens. I suspect you have the same mix, maybe even less carbonate...But I would not expect a phyllite to polish well. I would guess the 'crust' is just weathering. Iron rust stains everything.

Mike
Re: rock needs ID
February 24, 2015 03:23AM
From what I can see under the lens, there is crystalline areas with cleavage I'm assuming its calcite which looks recrystallized rather than such as clay particals or calcic fragments as would be in mudstone. That is what brings marble to mind. But also the phyllite looking texture and or schist like foliation. The rock being extra hard and taking quite a few hard hits with sledge hammer to bust it just dont seem sedimentary..but ?
Re: rock needs ID
February 25, 2015 04:22AM
Well, sedimentary rock hasn't yet changed, morphed. So if this has, and it is tough, I'd go with metamorphic. But limestones and shales are considered seds, and some limestones can be surprisingly tough, at least some I've hammered on. It seems everything grades into everything else in rocks, so the answer may not be so clear cut, without getting more technical than it is worth, at least to a hobbyist.
Just some thoughts..

Mike
Re: rock needs ID
February 25, 2015 06:20AM
    
Looks more like a schist rock.
Re: rock needs ID
February 25, 2015 10:15AM
For me, this looks like an arkoze, a sandstone with (slightly) larger fragments of feldspar and resistent minerals. As it reacts with HCl, it is probably cemented by calcite.
Re: rock needs ID
February 25, 2015 01:00PM
    
arkoze or greywack
Re: rock needs ID
February 25, 2015 04:34PM

avatar Re: rock needs ID
February 25, 2015 04:40PM
    
Looks like greywacke to me--a clastic sedimentary rock rich in iron-bearing silicates. That certainly explains the iron-oxide weathering rim. The graininess and deep weathering also suggest sedimentary. If metamorphosed the crystallinity would tend to retard weathering more than this shows. Argillite would be the metamorphic equivalent, and I've seen a lot of those and pounded on them, at times nearly breaking my hammer without harming the rock a bit.
avatar Re: rock needs ID
February 26, 2015 08:33AM
    
Norman,

Perhaps that's why it's called 'Greywacke' - you can whack it till you turn grey, and still it doesn't break...

Sorry !

Tim
Re: rock needs ID
February 26, 2015 04:36PM
The rock looks mostly micro mica that is silvery and all aligned through the entire rock, can see the mica from two opposite sides. I tumbled some from it ,polished with wet dry sandpaper. I'd about believe the rock is calcite and muscovite with silicates..too fine grain to see anything but the mica and calcite squished in there. If turn the rock 1/4 turn from the mica cleavage then sparkles nearly like marble which is what may be crystallized calcite forming like ice crystals would. I believe you all are thinking the specks are feldspar but that actually is mica reflecting, looks like the rock is mostly mica sericite ? Could this be a schist rock ?




Edited 1 time(s). Last edit at 02/27/2015 02:39PM by craig johnson.
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