Any geologists in the house?

Shoki

Shoki

Joined
Apr 10, 2007
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I found a weird rock unlike any I've seen. It looks like a mass of tiny black crystals all fused together and appears to contain iron as there is what looks like rust in some spaces. I found it in the river amid the washed-out bank and it's starkly different than anything else in the area. I don't usually give rocks much attention but this one has me curious, for some reason.
These are the best shots I could get of it:
0602121639.jpg

0602121650.jpg
 
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Picture seems out of focus, but it may be a basalt or gabbro or diabase with large crystalline structure.
You may need to crack that puppy open to see a fresh surface.
 
I just broke it open and took these two pics, notice what appears to be rust:
DSCF0945.jpg

DSCF0946.jpg
 
It still looks like it has been weathered and altered inside. The rust probably is clay weathering or oxidation of individual grains in the rock. If you see black interlocking crystals in fresher areas, chances are it is igneous and mafic (high in iron and magnesium). See any biotite in it? These are shiny paper like flakes that you can tease or pick at with a pin or knife tip. Biotite (and black iron oxide minerals) can indicate a basalt or gabbro (they are the same except gabbro is coarse grained while basalt in finely crystalline to glassy). See any holes (vesicles) in it? That would indicate a volcanic rock and substantiate the basalt guess. Get a pin and try to scrath freshh looking grains to see if there is quartz or feldspar in it. Hardness of a pin is about 7 and it will not be able to scratch quartz. Mafic rocks do not have a lot of quartz in it (10% or less).


Sometime, despite the color and massive crystalline aspect, it could be a sedimentary rock or metamorphic. Put a drop of vinegar and see if it fizzles (efferveces). If it does this strongly and throughout the rock, it may be a dark colored limestone or similar carbonate sedimentary rock. Sometimes an igneous or metamorphic rock can have secondary (post formation) mineralization by carbonates, but so you have to be careful udentifying this (it could be just a few grains will fizzle - that would indicate it's not all throughout the rock).

Hornblendes are dark, prismatic and have cross sectional angles of cleavage of 120/60 (less than 90), iirc. The 90 degree ones are pyroxenes. You can tell by using a hand lens, a lot of times.

This is all from memory so ymmv. Nowadays, little children give me unsolicited lectures on quartz when I am in rock shops, even though I've forgotten more geology than they know. :)
 
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