An amorphous solid lacks long-range order in the positioning of its constituent atoms; glass is an example. This contrasts with a crystalline solid, where such order is present, e.g., quartz. Both the ices and the silicates in interstellar dust grains are typically amorphous, although crystalline silicates are present in some circumstellar and cometary dust. The conversion of amorphous to crystalline water ice has often been invoked as an energy source in cometary outbursts at large heliocentric distances. The presence of crystalline silicates (presumably formed in the hot and dense inner solar system, possibly under the action of energetic particles from the young Sun) in comets, which are formed in the cold outer part of the solar system, suggests that mixing of material was important in the solar nebula.