Could some Identify a few rocks

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Offline Crnovich

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Could some Identify a few rocks
« on: 07/07/2012 00:49:14 »
My son is doing a 4h geology project and has a few rocks that he can't identify.  Could someone help us identify these rocks ASAP.  Thanks

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Offline RD

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Re: Could some Identify a few rocks
« Reply #1 on: 07/07/2012 02:02:19 »
#3 possibly hematite nodule encasing limestone ? ...

[attachment=16522][attachment=16524]

http://meteorites.wustl.edu/id/concretions.htm
« Last Edit: 07/07/2012 02:28:53 by RD »

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Offline Bass

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Re: Could some Identify a few rocks
« Reply #2 on: 07/07/2012 05:21:21 »
I agree with RD that the last photo is some sort of nodule, possibly quartzite or limestone.
It's really hard to identify rocks from photos only- some other properties, such as density, hardness, etc would be helpful.
That being said, Photo 1 is obviously igneous, with feldspar phenocrysts (large white crystals) in a very fine-grained (called aphanitic), dark groundmass.  The fine-grained groundmass makes it a volcanic rock- the dark color plus feldspar suggests andesite poryphyry (a porphyry is an igneous rock with abundant larger crystals).
Photo 2 could one of several white minerals.  Calcite is most likely, given the obvious cleavage.  The pearly sheen suggests talc, another possibility is barite.  These are all easily determined by hardness (talc can be scratched with a fingernail, barite and calcite with a knife) and density (barite is very heavy).
Keep in mind these are just my best guesses from the photos.
Old enough to have a grandson
Slow enough to study rocks
Thirsty enough to find a pub