Particles of rock in soil with a diameter between 0.004 and 0.006 mm are called

Silt

Rock particles found in soil are categorized into three types based on its size. They are sand, clay, and silt.

Sand particles are the largest with a  range in diameter from 0.0625 mm to 2 mm. The silt comes next with a width from 0.004 to 0.006 mm. while the clay is the smallest at <0.002 mm.