[Rockhounds] Geology in Progress

[Kreigh Tomaszewski]

The layers were obviously deposited in sequence, not at the same time. 
IIRC, the original report talked about six inch layers.

As an example, suppose the wind were from the north-east and was 
bringing in garnet sand, and then switched to the south-east to bring 
in magnetite sands.

All that would be necessary is for the collection point to be between 
two rivers that come from different eroded rock to produce garnet 
concentrates at one side, and magnetite concentrates at the other.

Geology in action indeed.

I've got a specimen of sand/silt stone clearly showing ripple marks on 
the surface. It is a fossil of an ancient beach or lake bottom near a 
shoreline. I collected it along Lake Superior. What makes it 
interesting is that it has an inch and a half pebble sticking up out of 
the face. The pebble is a fossil of newer erosion than that which 
produced the sand it was imbedded in.

It is from study of such sediments that geologists can deduce ancient 
mountain ranges and deposits that no longer exist. Seeing geology in 
action today makes it easier to understand the clues we find in the 
rock formations we visit.

Kreigh



On Sunday, Jan 4, 2009, at 21:04 America/Detroit, R. Peter Richards 
wrote:

> 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
>>
>> -- 
>> _______________________________________________
>> Rockhounds at drizzle Mailing List
>> Subscription Services:
>> http://lists.drizzle.com/mailman/listinfo/rockhounds
>> List Home Page, with a link to the List Usage Policy:
>> http://www.eclecticlapidary.com/Rockhounds/index.html
>
> ___________________________________
> R. Peter Richards
> rpr at heidelberg.edu
> Morphological crystallographer
>
>
>
>
>
> --- StripMime Report -- processed MIME parts ---
> multipart/alternative
>  text/plain (text body -- kept)
>  text/html
> ---
> -- 
> _______________________________________________
> Rockhounds at drizzle Mailing List
> Subscription Services:
> http://lists.drizzle.com/mailman/listinfo/rockhounds
> List Home Page, with a link to the List Usage Policy:
> http://www.eclecticlapidary.com/Rockhounds/index.html
>