Rock ID for one of my students

[jr.nation88]

One of my students brought in an interesting stone today hoping I could help her identify it. We are currently learning about geology and sometimes they believe I know it all...I told her I was not for sure but would know where to turn for help. Can you give me any leads? I've also posted this in the Native American forums because I've come across some posts about gaming balls and the such. At first I was thinking it was in the Geode family and was doing a little research and the closest thing I could find was possibly a geode nodule but not sure. Thanks in advance for the help, Nick.



This is only one half, she has the other half and is going to bring it in next week to class. The outside is extremely smooth except for some small bits. Seems a bit too smooth to be purely by erosion. This one half is also pretty heavy feeling for its side. The inside has no hollow spots but does contain some mineral deposits. Quite a few crystals of quartz and what appears to be hematite/iron because it has turned rusty brown.

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[Eu_citzen]

The last picture would have been the most helpful if it was in focus. Is the stone wetted?
Skip that next time.

Try using the macro feature ("flower symbol") of the camera and take the shot in natural day light. (if possible)

If I'm not mistaking I think I can see quartz and feldspar in there. Can you see any mica? (Biotite or muscovite?)

A freshly exposed surface would help, i.e. chip of a piece to see what really is going on there.
It allows you to more easily see certain features for example the cleavage of feldspar.

P.S. Some magnification might help - i.e. a loupe. 10x should be plenty.

[jr.nation88]

i'll try and get some better pics, we just tried to use what we had at the time. I'll also see if she minds if we chip a small piece out. There is definitly quartz but I'll have to take a closer look to see if its feldspar. I was thinking a bit of hematite because of the 'rust' it shows, especially with the line feature

[Eu_citzen]

I would be more thinking in the lines of Pyrite which has a tendency to form Limonite (iron hydroxides) in moist environments. A quick streak test with a piece of unglazed porcelain will separate Pyrite and Hematite.
Hematite gives a brownish-red line, pyrite a greyish-black colour.

There's also another possibility - I know some mafic (quartz poor) rocks tend to be rich in iron and iron silicates.
It is not rare to see limonite on those. Could be due to magnetite altering to limonite. I'm not 100% sure.

The 'line' could just be an older crack filled by something else. It is quite common in volcanic rock.
I really can't see much which would make me assume it is sedimentary, but metamorphic could also fit the bill.

So far I'm leaning towards either granite or quartzite.

[jr.nation88]

Thanks for the input. I'm gonna try and get better pics tomorrow but either way, these comments will be exciting for my students to read as they are currently studying the exact things yall speak of.

[StoneWhisper]

Common River rock which can contain mica and in many cases, small garnets. Rounded and worn to to the flowing waters of a stream or river.. Sometiems these so called river rocks because of their round shape are used in mass mixers for rounding or knocking of sharp corners within a larger tumbler.. (commercially)

[jr.nation88]

interesting input about the garnets...i was thinking they were looking more like quartz, how could I determine for sure? I have more pics that we took which are alot clearer and from different angles but am going to have to transfer them from my school computer which wont let me get on this site...standby

[Eu_citzen]

I would worry more about confusing cordierite with quartz.
There is nothing in it which I can see that would suggest garnet.

Garnets occur mostly in metamorphic rock, although it can occur in volcanically related rocks and in quartz veins to.
It just isn't as common.

[StoneWhisper]

Firstl;y, I never suggested that the above stoen had garrnets in it.. I'm mearly saying that typically these types of river rocks can have garrnets within them..

[Eu_citzen]

That wasn't directed to you Jeep, hope you didn't get me wrong. I was just trying to help Jr.nation.

[StoneWhisper]

EU.. I feel at times yoru attempting to discredit anything and everything I try to tell thsoe who are interested in learning.. and I would agree that you are the best at helping others.. I just don't agree with the tackets used in these cases.. There arent any right or wrong answers.. and I persoanl have given my best guess many times not knowing from the image what soemthing truly is.. Often I get a better since of what something is when holding the items.. Therefore we all work differntly..

[Eu_citzen]

Sorry budd, that is not my intent. I to only give off my best guesses.
I suppose I might come over sounding a bit harsh. Just has to do with the way I write.

That is why I wanted to empathize that garnet occurs mostly in metamorphic rock.
But it can occur in eclogites, pegmatites and even quartz veins.

[jr.nation88]

Sorry its taken me a bit, but hopefully these pictures are a bit clearer and can help give a better idea. I have appreciated everyone's input. Wish it could be as easy as all agreeing and saying "yep, thats a.... but my class loves reading all the ideas and then going from there on their own search" so thank you for that

[jr.nation88]

more pics

[Eu_citzen]

A lot of quartz, that's for sure. And better pics, thanks.
It looks like a lot of quartz "grains" has been fused together.

I hope the students got a chance to notice the difference in looks, among other lustre.
Does the lustre look "greasy"? I.e. did it break "through" the quartz grains or around them?

I'm leaning towards either sandstone or quartzite, looks like plenty of quartz around.

But rocks specifically are difficult to ID from a picture since you can't for sure say what minerals make it up.
Usually You'd be inspecting the rock with a 10x loupe.

[jr.nation88]

they broke around the quartz instead of through them. We have had alot of fun with this stone at school and have enjoyed reading this site as well. I told them, like you mentioned, that with just pics sometimes it is hard to get an exact answer.

[yakker]

One thing I didn't catch was *where* this rock was found/ where you're located. That would make a difference, wouldn't it?
Thnks. and Cheers!
Yakker

[jr.nation88]

I am located in SE Missouri and the rock was found in this area around a creek (Bollinger County)

[rock]

Id say quartzite by the grainy look.