Naked Science Forum

Non Life Sciences => Geology, Palaeontology & Archaeology => Topic started by: Crnovich on 07/07/2012 00:49:14

Title: Could some Identify a few rocks
Post by: Crnovich 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

Title: Re: Could some Identify a few rocks
Post by: RD on 07/07/2012 02:02:19

#3 possibly hematite nodule encasing limestone ? ...

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Title: Re: Could some Identify a few rocks
Post by: Bass 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.