On a recent visit to Hawaii Volcanoes National park, my son and I discoverd some of Pele's Hair in a Aa flow near Muliwai a Pele. Since I have been lead to believe that Hawaiian magma is rich in both iron and sulfur, I guessed that Pele's hair is a crystalographic variant of "fools gold" or iron pyrite. Am I close? Is its appearance in cavaties just below the surface indicating a vapor phase reaction? Nerdish minds want to know.
Actually, Pele's hair is volcanic glass that has been stretched into thin strands by the physical pulling apart of molten material during eruptions. Most commonly it forms during fire fountain activity. I have seen it also on the surface of flows; commonly the wind blows it around and it ends up in little hollow places. Is it possible that the material you found could have got there by wind? If the pieces you found were covered by other rocks, then maybe we'll have to go back to the drawing board! But vapor phase deposition by volcanic gases is also very common. I'm not sure what the stability field of pyrite is, but it probably could form by the process you describe. The golden color of Pele's hair is, I believe, due to slight weathering effects.
Dr. Rodey Batiza, Professor
Department of Geology and Geophysics
University of Hawaii, Honolulu, HI 96822