[Rockhounds] Geology in Progress

Sun Jan 4 17:12:30 PST 2009

The sand along Lake Michigan in West Michigan contains magnetite. Wave 
action concentrates it at the high water reach on the beaches, leaving 
a black line. Each wave brings sand up with it, but the denser 
magnetite drops out and stays as the water flows back into the lake 
under the power of gravity.

Kreigh

On Sunday, Jan 4, 2009, at 19:33 America/Detroit, Lawrence Rush wrote:

> Nate: That's an interesting question. I have two thoughts as possible 
> answers....
>
> One....It was the holidays and everything is upside down at that 
> time....or...
>
> Maybe the layers were deposited at different times. The first storm 
> laid down
> Magnetite and Garnet in different places, and a subsequent one washed 
> away the Magnetite to be redeposited on top of the Garnet?
>
> I don't think there was any overturning where I saw the layers. They 
> were even and discrete, no interruptions. They were at the higher 
> portion of the beach, where the wave action had to be stronger than 
> normal, but nothing really of super strength.
>
> Are there such layers to be found in the old sedimentary quartzite 
> deposits?
> Is layered granular Magnetite ever an economic deposit in quartzite? I 
> don't know that I ever saw that in the literature or in the field.
>
> One other interesting fact about that same area of that beach. Some 
> years ago, a strong storm washed away a great deal of sand at the 
> upper levels and exposed the wooden ribs and keel of a large old 
> sailing vessel. We happened to be there on vacation just afterwards. 
> The State had posted a warning that the wreck should not be disturbed. 
> It was found to be not a pirate ship laden with gold and treasures 
> (Darn!), but a rum-runner which plied between New England and the 
> Caribbean. It subsequently has been covered up again with sand.
>
> Not really relevant, but a little more stimulating for one's fantasies 
> than Magnetite sand deposits!
>
> Larry
>
>
>
>
>
>
> ----- Original Message ----- From: "Nathan Martin" <rocknate at gmail.com>
> To: "Rockhounds at drizzle.com: A mailing list for rock and gem 
> collectors" <rockhounds at lists.drizzle.com>
> Sent: Sunday, January 04, 2009 3:12 PM
> Subject: Re: [Rockhounds] Geology in Progress
>
>
>> 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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>>
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