QUESTION: Where I live there are large amounts of metallic black dust (a bit smaller than the size of salt) that a magnet can easily pick up. They are all over the forest floor and lake shores. The size is uniform with very little variance. What is it and how did it get there?
ANSWER: What you are seeing is heavy mineral fraction of sand.
Sand weathers from rock, usually igneous rock such as granite. If you look closely at granite, you will see the majority of the mineral crystals are quartz and the second largest component is feldspar, a pink or whitish to off white mineral with cubic cleavage. Feldspar weathers to clay, and quartz weathers to the all too ubiquitous quartz sand. Hidden in the granite is another 10% or so of accessory minerals. These are ferromagnesian minerals such as mica (the flat shiny bits that stick to your skin) and others. The "ferro" is what is important here as it means iron containing. The list of these minerals is pretty long, but Ilmenite is an important one along with Magnetite(yes its magnetic) hematite, siderite and others.
Because of their relative weight, they tend to be concentrated by the action of moderate surf. I went to school in Norfolk,Va and we would see the black layers a lot in some areas. In Oklahoma, I would see them along the shores of lakes I used to sail, where the moderage wave action of the lake would concentrate the heavy mineral fraction out of the sediment being deposited in the reservoir.
In the geologic past and even at present sometimes these concentrations are exploited as a kind of ore if they are thick and wide spread enough. Ilmenite and Sphene both present in the fraction are a source of Titanium.
In school we went through a very laborious and hazardous process of separating out the fraction from sand by a gravity separation. Using a heavy and toxic liquid called bromoform, we would float off the quartz, its specific gravity being lighter than the liquid, but less than the heavy mineral fraction. The stuff was nasty and had to be used in a vent hood. Think of very thick and odiferous finger nail polish remover. Once we floated off the quartz we would dry and clean the remaining heavy fraction and then run it through a magnetic separator, basically a vibrating inclined V channel along which an electro magnet was positioned. The heavy fraction would move down the V and the part that was magnetic would stick to the side and the rest would vibrate on down into a vessel and be caught.
So why did we go to all this trouble? Well separating out the fractions allows us to determine a thing called provenance, or where the minerals came from. Where the parent rock was located. Being in Virginia it allowed us to determine that a lot of the sand came from up in Canada, and had moved down about a million years ago during the glaciation.
This can be determined by the makeup of the accessory minerals. Each granite or parent rock has a unique makeup of accessory minerals due to the magma it came from. So comparing the types of heavy minerals we found we can make an educated guess at to the parent rock by comparing what we have to the makeup of the kinds of granites in the watershed. From the drainage shed of the Appalachians rivers run east to the coast and dump sand. So we would look at the types of granites only along the east coast that would have fed those rivers.
Its not as hard as it sounds. In essence the black sand gives us a fingerprint of the rocks from which they came. Not all grantites contain all of them, so of course there will be a sort of hash, but we would expect them to be a blend, so we can say that granite x and granite z "sourced" the sands along this section of a given coast line.
Now as to the varience in size or lack there of. That is due to the process of weathering and transport.
If you go to an area where there is granite and look at the soil, you will see saprolite, or rotten granite. It will be a coarse angular and clayey soil, exactly what you would expect to find due to the weathering of the minerals in the rock. Over time all this gets transported to a stream and then to a river and along the way it gets worked.
A whole school of study has sprung up over the years based on this, called sedimentology.
It looks at the power of water flow and categorizes the power of a stream to do work. It is abstracted into what they call the Reynolds Number. Clear water flowing at a certain velocity can do work. The work it can do is erosion or picking up a load. Which it does, always. The load is usually clay and silt which forms mud when it drops out of the water when the water ceases moving. If it is moving over solid rock it may not pick up mud, but be clear, in which case it will still do work by moving and bouncing larger rocks or sand along the bottom in a process called saltation. This is where the large particles get broken up and ground down. High flow means bigger rocks get moved. This is why when you walk along a creek you see large cobbles buried in sand or silt in the bars or along the sides and in the channel itself. They got there during high flow stages and are just waiting till the next flood that has a higher Reynolds number in order to continue their trip down stream. In the meantime the reduced flow moves smaller particles and gravel along. This constant grinding and bouncing reduces large rocks into sand eventually and that is what you see along the beach.
Now in the North East you see cobble beaches, or rocky beaches this is due to the high surf conditions moving and winnowing the sand out, plus the source rock is right there on the beach too, not far inland. So the transport part is non existent. Farther south a huge wedge of sand silt and mud separates the mountains from the rocks in the mountains from the coast.
Now once the sand reaches the sea, additional winnowing takes place and much like a gold miners panning, the surf tends to separate out the size fractions so that the sand on the beach is fairly uniform. Just like the way the wind separates out the size sand particles it can blow. The smaller fraction ends up in the sand dunes behind the beach, while the coarser fraction stays on the beach for additional "working" till it too is reduced to smaller size by the constant "grind" of being in the surf.
Geologist, what we call sedimentary petrologists, look at the sand under microscopes and we can tell what environment the sand was worked in, and how old it is relative to other sands.
This can be done by looking at its angularity, the rounder it is the more working it has undergone. Sand due sand looks like frosted bathroom glass since the wind action works it differently and gives it a frosted rounded appearance. An angular sand grain is judged to be younger than a well rounded similar grain.
So if we pick up a piece of sandstone in the rocky mountains, and we see that it is composed of well rounded cemented grains with a frosted character, we know that that sandstone rock started out as a sand dune that was buried and cemented into the rock we now see and part of which we are holding in our hand.
Here is more information of the process of studying the heavy minerals:
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QUESTION: Thanks for your thorough answer. Here a few more characteristics: it is actually much finer than salt, it does not rust, it is not attracted to metal, but is very attracted by magnets. I do not live near any factory. I live in Maryland. Knowing this has your answer changed? Do you think I could sell it?
No change. What you have is magnetite, its a form of magnetic iron. It does not rust, it is not native iron, which will, but is part of the Spinel group of minerals.
It is found in most igneous rocks as an accessory mineral. It is the same thing as Lodestone.
You could sell it as a curiosity. If you fill small vials and put them on necklaces or something like that. People will buy anything.
Lodestone is how magnetism was discovered.