(a) Extinction efficiency for composite spherical particles of silicate grown on the surface of alumina grains, calculated for core-mantle grains with fixed total radius a = 0.1 μm and different core-radii from 0 to 1 of the total radius in steps of 0.1 (black lines). The green and red lines correspond to pure silicate and alumina grains, respectively. The blue line is for a grain with a core of 0.5 of the total radius. (b) Superposition of -values for pure silicate and pure alumina grains weighted according to the volume portions in the core-mantle grains.