"Dirt" vs. "Sand" vs. "Clay" vs. "Grit"

   [Those of you grubbing around your garden or walking on the beach might be interested in this old Why Things Are item, circa 1995. Note the Leatherman quote: When I did Why, Leatherman was one of those people I could call up on horrid deadline on a Friday afternoon. In America there is an exerpt on everything, and Leatherman is an expert on BEACHES, and specifically beach sand. Why does that sound like a cool job. Best of all, he spoke in simple, jargon-free sentences that anyone could understand.]

     Why is dirt dirty? And why does beach sand seem so clean?

     It's the clay. Dirt is dirty because it contains clay, which sticks to
everything, getting into the pores of your skin and the fabric of your
clothes. It is so sticky because of what the experts call the
"double-gooey layer."
     Clay is by definition a substance with no grit whatsoever. You can
rub it in your hand and feel nothing but goo. At the microscopic level, clay is a compound of thousands of particles, adhering to one another
not just with water but with electrically charged particles. The charged
particles form a layer above and below every little wad of clay: thus clay is "double-gooey."
     "Clay is what makes things really dirty. If you just had silt it
wouldn't be as much mess," says Stephen Leatherman, director of the
Laboratory for Coastal Research at the University of Maryland.

     To wash a pair of dirty blue jeans, you need something that can
break into those charged particles, wedge them apart, and dissipate the
little wads of clay. (We recommend detergent.)
     As for beach sand: It's already washed. (We are itching to make a
Tide joke but never mind.) All the silt and clay and tiny wads of gunk
and bacteria and flecks of skin and whatnot that you might find in
regular soil can be easily suspended in water and washed out to sea. All
that remains are the large grains of sand, by definition between 64
microns and 2 millimeters in diameter.
     Those grains are crystals, mostly of quartz. They don't adhere to
anything, not even to each other. Now you may argue that they clearly
stick together, as when making a sand castle. That, however, is just the
surface tension of the water at work.
     Wander onto a sand dune, and you'll see that the sand is so dry it
will easily brush right off your clothes. Of course days later you will
still find some of it wedged here and there.