[Rockhounds] Geology in Progress

[Nathan Martin]

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?

best regards,
Nate Martin
Lexington, MA

On Sat, Jan 3, 2009 at 3:54 PM, Lawrence Rush <larryrush at worldnet.att.net>wrote:

> Nate mentioned some good collecting stories. Here is a minor one I
> experienced last week....
>
> On a warmer (60 degrees F.) day last week, we took a day off from the
> holiday madness and went up to walk the deserted Rhode Island beaches. The
> day was cloudy and foggy there, but it was wonderful to walk and explore at
> a time when no one else was anywhere to be seen. At one end of the
> Charlestown Beach, near the breachway, the winter storms had winnowed great
> strands of Magnetite sand into long surf driven "windrows", several feet
> wide, about six inches deep, and hundreds of yards long. Pure, black, and
> striking. When I dug into it to find the depth, I found a band of Garnet
> just below it, also just as long and wide. Evidently these were separated by
> density with the high, strong surf this Fall and Winter. Not really unusual,
> but surprising to see so much of the minerals, originally derived from the
> New England hard rock highlands, concentrated like that. I have sometimes
> collected Magnetite from beach sands with a magnet, but this time a shovel
> would have given me all I could ever hope for.
>
> Just to imagine the time involved to weather out these minerals from the
> granite, many miles away and hundreds of feet higher in altitude, then to
> have them transported by streams (and glaciers), down to the ocean, and
> ground up to be redeposited at my feet is to bring home Geology to be seen
> first hand!
> Of course, the quartz beach sand underwent the same process, but we are so
> used to seeing that, it doesn't make much of an impression.
>
> Not really a collecting trip, but interesting just the same, in Winter in
> New Engalnd!
>
> Micromounts, anyone??
>
> Larry
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