I can't think of any way to buy silt.
I look for the coarsest sand possible, but have never found a bag that was more than 30% COARSE sand. So instead I buy crushed stone for coarse sand.
Actually, for grit-sized particles, I use pine bark chopped and screened. Lighter, cheaper, holds more water and gradually adds organic matter.
Grit can be bought as crushed granite grit, as #2 chicken grit. But look for the 50# bag "back in the warehouse". Don't let them sell you a half-gallon bag of grit for almost as much money as a 50# bag!
Clay, silt and sand are mostly about particle size, although it seems that particles small enough to be called "clay" are usually one of a few kinds of minerals also called "clay minerals" that break up into very thin, flat platelets that have highly charged surfaces. These minerals grab soluble plant nutrients like NPK and micro-nutrients when available in excess, because they have charged surfaces. Then they release the nutrients when their concentration in ground water falls lower. They buffer the nutrient concentrations and slow down their leaching away. Clay isn't all bad, in moderation! ("Moderation" is around 10-15% clay!!)
Once source "clarified" that silts are fine-grained soils that do not include clay minerals. I think it was Wikipedia that said that the word "silt" may be used differently by geologists, soil scientists, sedimentologists , colloid chemists and geotechnical engineers.
(I'm not positive, but I think that high-quality "rock dust" might be even smaller: colloidal dust, smaller than 1 micron. Colloidal particles tend not to settle out of water.)
Clay particles are smaller than 1-2 microns, but apparently sedimentologists just have to be difficult, and use 4-5 microns as the breakpoint. A micron is 1/1,00th of a millimeter. Say, "smaller than a bacterium". Clay properly so called is also one of several specific minerals: "Clays are formed from thin plate-shaped particles held together by electrostatic forces".
Clay feels slippery or slimy rubbed between fingers, and it settles very slowly in water.
Silt particles have sizes from 1-2 microns (or 4-5 microns) up to 63 microns.
Another system says "1/256 mm to 1/16th mm" .
USDA decided that sand starts when it won't pass through a #200 sieve (75 microns).
Silt feels "like flour" when rubbed between fingers and settles rather slowly in water.
Sand particles range in diameter from 0.0625mm (1⁄16 mm) to 1 or 2 millimeters.
Sand feels "gritty" when rubbed between fingers and "sinks like a rock" when it settles out of water.
Some people call it "grit" above 1 mm, others call 1-2 mm "very coarse sand".
Above grit, I only know gravel, pebble, cobble, stone and boulder.
