[Rockhounds] Geology in Progress

[R. Peter Richards]

I've seen similar concentrates on Great Lakes beaches, but I don't  
remember ever seeing that the garnet and magnetite were layered, one  
above the other.  In addition to possible grain size differences, is  
it possible that there are differences in affinity for water  
(hydrophilicity?) that might cause the magnetite to tend to float  
over the garnet, in spite of specific gravity differences?  Though I  
know little about it, differential flotation is a feature often used  
in recovering desired minerals from crushed rock.

Regards,
Pete Richards


On Jan 4, 2009, at 5:09 PM, Dennis Buffenmyer wrote:

> Nathan Martin wrote:
>> Larry,
>> Seems as if you made good use of one of the few mild days we have  
>> had.  Lets
>> hope that we get more of that "beach weather" soon!
>>
>> One of the things that I find interesting about your observations  
>> is that
>> the magnetite was on top of the garnet. Based on the relative  
>> densities of
>> magnetite (~5.15 gm/cm^3) and typical New England garnets like  
>> almandine
>> (~4.2 gm/cm^3) or grossular (~3.6 gm/cm^3) , I would have expected  
>> just the
>> opposite.  I wonder if it is possible that strong wave action  
>> could turn
>> over a density-stratified layer of sand, garnet and magnetite to  
>> produce the
>> layering that you saw?  I know that bulk granular materials can  
>> have some
>> pretty unusual properties but I don't really know if what you  
>> observed is
>> common.  It may be a really unusual occurrence.  Perhaps someone  
>> else on the
>> list can comment.  Any sand collectors out there care to venture  
>> an opinion?
>>
>>
> Not a sand collector, but could it be a safe venture to make a  
> statement that rocks float?? Most farmers know this and dont  
> realize it.
> I suspect that it is more likely that the garnet was able to be  
> worn and fragmented to a smaller size than the magnetite and so,  
> the smaller particles sink and the larger particles float. Try it  
> some time on your vibratory lap unit. fill a mason jar full of  
> various size grains/pebbles, chips, and invariably, after a long  
> period of time the big ones,,,,,, wind up on top
> Dennis Buffenmyer
>
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___________________________________
R. Peter Richards
rpr at heidelberg.edu
Morphological crystallographer





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