src: MSL Raw Images
This Sol 69 MAHLI image shows that the coating on the surface granules does not cover the entire granule. The interior of the granule looks like it might be a saltation particle. A possible explanation is that a layer of saltation particles collected at this location on top of a small sand drift and the fine powder coated them later. In a dust storm one would expect a light fine powder to be airborn the longest. Electrostatic or intermolecular forces might be what made them adhere together. On the surface light might play a role in breaking chemical bonds permitting new ones to form between adjacent particles. On Earth dampness helps things cling together.