friable

Short definition from Wiktionary

Adjective

1. 1
   Easily broken into small fragments, crumbled, or reduced to powder.
2. 2
   Of soil, loose and large-grained in consistency.
3. 3
   Of a poison, likely to crumble and become airborne, thus becoming a health risk.
4. 4
   Of a number: smooth, that factors completely into small prime numbers.