The type of soil with the smallest rock particles is clay.

Here's why:

* Clay has particles smaller than 0.002 millimeters. These tiny particles pack tightly together, making clay soils dense and often sticky when wet.

* Silt has particles between 0.002 and 0.05 millimeters, so it's larger than clay.

* Sand has particles between 0.05 and 2 millimeters, making it the largest of the three.

Let me know if you'd like to learn more about the different types of soil!