The 5- to 8- micron overtone-combination bands of silicates could be used to test the theory that grain growth occurs in the ISM of galaxies (Rowlands et al. 2014 and Zhukovska 2014) because overtone bands occur when near optically thick conditions exist for the 10-micron silicate absorption feature (Bowey & Hofmeister 2005). As a rule of thumb grains responsible for an optically-thin 10 micron feature have diameters ~0.1 to 0.3 micron. However, room-temperature laboratory silicate overtones are measured in films of compressed powder that are 5 to 15 micron-thick which is analogous to larger or clumped grains in space.

We shall use the Spitzer spectrum of the z = 0.89 absorber toward PKS 1830-211 published by Aller et al. (2012) as a test case because this spectrum exhibits uninterpreted bands near 5.9 micron and 6.9 micron similar in shape to those matched by the overtones of silicates in young stellar objects (Bowey & Hofmeister 2005). A high abundance of clumped grains could explain the absence of the more commonly observed smooth 10 micron feature in this line of sight, which led Aller et al. to conclude the structured 10-micron feature could be due to a blend of 95% crystalline silicates.