From the picture, it looks like all silt.
If you want to go beyond the shake-in-a-jar test, you are going to have to put it through some sieves to get a better idea of the particle size. If it is 15% "coarse fragments" you
should be able to get a fine screen that will hold back that fraction. Some bacon spatter screens that you can buy at the dollar store have a mesh that size.
And on the other end, separating the silt from the clay, you can probably use a cloth filter to see if there is any clay that is small
enough to wash through. There is probably a relation between the
thread count of sheets and the size of particles they will pass, but it's getting too late for me to look that up today. If it's really important to you, let me know and I can work up a test method for you.