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The broad 30 \mu m feature in carbon stars is commonly attributed to MgS dustparticles. However, reproducing the 30 \mu m feature with homogeneous MgSgrains would require much more sulfur relative to the solar abundance. Directgas-phase condensation of MgS occurs at a low efficiency. Precipitation of MgSon SiC precursor grains provides a more efficient formation mechanism, suchthat the assumption of homogeneous MgS grains may not be correct. Using a MonteCarlo-based radiative transfer code, we aim to model the 30 \mu m feature ofthe extreme carbon star LL Peg with MgS dust particles. We find that for LL Pegthis modeling is insensitive to the unknown MgS optical properties at \lambda <10 \mu m. When MgS is allowed to be in thermal contact with amorphous carbonand SiC, the amount of MgS required to reproduce the strength of 30 \mu mfeature agrees with the solar abundance of sulfur, thereby resolving thereported MgS mass problem. We conclude that MgS is a valid candidate to be thecarrier of the 30 \mu m feature when it is part of a composite grain populationthat has optical properties representative of an ensemble of particle shapes.

Observational evidence for composite grains in an AGB outflow: MgS in the extreme carbon star LL Peg