[Rockhounds] Geology in Progress

[Jay Bates]

Pete,

At various times gold has been panned from the black sands on that 
beach. Also diamonds have also been found there. It is now part of the  
Golden Gate National Recreational Area and off limits to collecting.

Magnetite is quite common in the Coast Range. The soil from the 
Benitoite Gem Mine is so full of magnetite that if you drop a magnet on 
the ground it can be lost by being buried by dirt attracted to the magnet.

pmodreski at aol.com wrote:
> Back to the beach sand puzzle...
>
> After I read this, I wanted to share my own curious observation of beach sand in California.? I was in the San Francisco area a few weeks ago, and was surprised to see a very thick layer of nearly pure magnetite sand on the upper part of a beach at Ocean Beach (the name of that Pacific Ocean beach section), on the west side of San Fran.? I've often seen thin layers of magnetite-rich sand at beaches but here, the entire upper part of this section of beach was solid magnetite, not just on the surface but an inch or more thick--several inches, I think--I should have, but didn't, tried to scoop out a little trench to see how thick it actually was--I thought of that now.? If you go to this link, you'll see two pictures of that beach and the black sand layer:
>
>
> http://picasaweb.google.com/Pete3555/MagnetiteSandOnOceanBeachSanFranciscoCA?feat=directlink
>
> When I saw this black beach from a distance, I thought maybe it was from an old oil spill or something.? The sand closer to the surf was more "normal" quartz sand with just scattered grains of magnetite, and sandy bluffs you'll see above the beach, though somewhat dark-colored, did not seem unusually magnetite-rich (I didn't bring back a sample of them).? I did bring back a little baggie of the magnetite sand, and looked at it under a microscope; and it is indeed magnetite, somewhat rounded but often subhedral showing crystal shapes, and very uniform in size, around 1/8 mm.? The black sand contains a scattering of other heavy minerals, including some orange garnet and amber to pale colored what I believe is zircon; many of the zircons and garnets are remarkably euhedral--very pretty crystals, doubly terminated clear tetragonal prisms of zircon and some trapezohedral garnet.???? :? )
>
> There was such an abundance of magnetite there, I wondered if it was all a natural accumulation, or whether humans had played some role--was there an industrial dump of magnetite, or a shipwreck of a ship carrying iron ore?? But it does all look like natural magnetite-rich sand.? Next time anyone on this List is near that beach--go check it out!? (I think I was?near the southern end of Ocean Beach--just north of the San Francisco Zoo.)
>
> I'll take a stab at getting some closeup macro/micro pictures of the crystals in the sand, and try to post those online too.
>
> Cheers, Pete
>
>
>
> -----Original Message-----
> From: Lawrence Rush <larryrush at worldnet.att.net>
> To: Rockhounds at drizzle.com: A mailing list for rock and gem collectors <rockhounds at lists.drizzle.com>
> Sent: Mon, 5 Jan 2009 8:05 am
> Subject: Re: [Rockhounds] Geology in Progress
>
>
> Thank you Earl! Great answer! Logical and good science! Am I am happy to think that water pollution is NOT responsible! These are the best beaches in New England, with uncommonly clean surf and sand, a pleasure to visit and swim (and collect Magnetite) in!?
> ?
> Thanks to all!?
> ?
> Larry?
>
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>
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