The main rocks of the volcano are andesite and dacite, which contain biotite and pyroxene as well as blebs containing iron oxide. The iron-containing rocks are a less important component. The whole rock falls into the calc-alkaline class of volcanic rocks, similar to these erupted by the neighbouring volcanoes Lascar and Llullaillaco. The andesites contain plagioclase clinopyroxene, orthopyroxene, and phenocrysts of magnetite.:681-682 Magnetite, and in lesser measure hematite, are the most abundant iron minerals;:685 anhydrite, diopside, goethite, limonite, maghemite, pyrite,:685 and diadochite are also found. Erupted magma was probably gas-rich, as the magnetite lavas would otherwise have melting points of over . The lavas lost most of their sulfur and phosphorus after their eruption. High oxygen-18 amounts in the Laco magmas indicate either crustal contamination or isotopic effects during fractional crystallization. Some atmospheric water influence has been inferred from isotope data as well.