Llullaillaco has mostly erupted dacites with medium potassium content, with rocks becoming more felsic the younger they are. Rock samples taken from Llullaillaco are mostly porphyritic or vitrophyric, with a glassy or microcrystalline matrix. Phenocrysts are mostly plagioclase, with mafic phenocrysts being dominated by orthopyroxene and smaller amounts of biotite, clinopyroxene, and hornblende. Ilmenite, magnetite, and sulfide minerals are also present, magnetite especially in the more oxidized older lavas. Some phenocrysts show evidence of a complex history. Quartz is rare. Older lavas have developed a red colour and contain oxidized iron in form of hematite. Lavas from Llullaillaco I are grey. The content of SiO2 is 65-67%.