Black Sand Gets Stranger

Classified a half bucket of sand and gravel and soil to 100 mesh. Got rid of the blond sand by panning. Black sand left. _Two_ levels of black sand. My musings in the video . . .

If anything is as mysterious as gold, it is black sand. Black sand may not indicate that placer gold is present, but a lack of black sand almost certainly rules it out. But little is settled.

There is a spot at the east end of Folsom Lake where black sand covers hundreds of yards of raggedy beach, all deposited by the north fork of the American River. All that sand is completely sterile, although, to be fair, I have never had a pound or two of it fire assayed. Let’s say visible gold isn’t present.

These heavy occurrences causes my White’s GMT to howl or grunt in protest. “Lift coil. Bad ground.” The best black sand happenings that I find are small streaks, not big ones.

My White’s has a black sand tracker feature which gives a numerical readout. I start sampling when it hits 45. Above 54 or so it will register bad ground. A reading of 5 to 20 or indicates well developed or poorly developed soil and not the sand and gravel I am looking for. Time to move on.

With this feature I can assess a large gravel bar or a desert wash fairly efficiently, instead of guessing at where to begin, or concentrating solely on bends.

Update: Tony Wirtanen from Facebook provides a good explanation, that it is simply particle size at work that makes the difference. “Magnetite I think is what it is called and the more ferrous is probably just a micron or two bigger giving it more mass for the magnet to pull on. There are many different types of black sands from granite to graphite and everything in between. But the ferrous material I am almost positive is all magnetite.”

Granite sand would result from the black color of biotite, a common rock building mineral that gives a granitic rock its sometimes salt and pepper look.

But my friend the mineral dealer, he with fifty years of collecting experience, disagrees with the mass solution.

“The magnetism in the sand is probably a mix of the magnetite and either hematite or goethite and that is my thought on why some is magnetic and some not. The particle size is not something I agree with, small would be even more attracted to the magnet. Then a second is that some magnetite is very magnetic, the extreme example is “lode stone” a magnet in itself, some is barely magnetic so that can vary also.

You did find a good crystal and that is often the key to which one you have. In our local black sand, the stuff we actually find right on our property, it is still quite sharp in the crystals. Not that far from the original source.

The photo you sent looks like the material was a bit farther from the original source so rounded by the rolling in the flooding that took it along. It is funny stuff and can have platinum in the black sand but finding that is something I have never attempted. I know by reading it is in there too but to differentiate it may be something I can’t figure out.

In your photo I can see that same tetragonal shape, even in the more rounded nodules too.”

Here’s a picture of some of the highly magnetic black sand under my microscope. Magnetite’s crystal form is an octahedron. Can you spot the lone well developed crystal?

This graphic is from Wikipedia, showing an octahedron’s form.

Crystallography is a dark art for beginners and I am keeping clear of it where I can. I just got a reference specimen of magnetite from the Marble Mountains in San Bernardino County, California. Could anyone possibly relate these well defined crystal faces to the form above? And I have seen a picture of magnetite which shows a cubic form. Good grief.


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