Table 1: The materials used in the fitting procedure. In the case of the amorphous silicates we use the names "olivine'' or "pyroxene'' to indicate the average silicate stoichiometry. For the coating of the coated GRF particles we use amorphous carbon as measured by Preibisch et al. (1993). The references in the last column refer to: [1] Dorschner et al. (1995), [2] Henning & Stognienko (1996), [3] Mutschke et al. (1998), [4] Spitzer & Kleinman (1960), [5] Servoin & Piriou (1973), [6] Jäger et al. (1998), and [7] Laor & Draine (1993).

Name	Composition	Lattice structure	Ref.
Olivine (x=0.5)	MgFeSiO4	Amorphous	[1]
Pyroxene (x=0.5)	MgFeSi2O6	Amorphous	[1]
Olivine (x=1)	Mg2SiO4	Amorphous	[2]
Pyroxene (x=1)	MgSiO3	Amorphous	[1]
Na/Al pyroxene	NaAlSi2O6	Amorphous	[3]
Silica	SiO2	Amorphous	[4]
Forsterite	Mg2SiO4	Crystalline	[5]
Enstatite	MgSiO3	Crystalline	[6]
Silicon carbide	SiC	Crystalline	[7]