A clastic sedimentary rock with particles that are 1.5 millimeters in diameter would be classified as a siltstone.

Here's why:

* Clastic sedimentary rocks are formed from fragments of other rocks.

* Particle size is a key factor in classifying clastic rocks.

* Siltstones are characterized by particles ranging from 0.0625 to 2 millimeters in diameter.

Here's a quick breakdown of common clastic rock classifications based on particle size:

* Claystone: < 0.0625 mm

* Siltstone: 0.0625 mm - 2 mm

* Sandstone: 2 mm - 64 mm

* Conglomerate/Breccia: > 64 mm