The only international standards I am aware of are the world reference base and the FAO - UNESCO Soil Map of the World. The FAO - UNESCO Soil Map of the World Revised Legend, 1990 defines the particle-size classes as follows: Sand 50 to 2000 microns; Silt 2 to 50 microns; and clay as <2 microns. This agrees with 0.05 mm as the sand-silt border.