Sideromelane is a
vitreousbasalticvolcanic glass, usually occurring in
palagonite tuff,[1] for which it is characteristic. It is a less common form of
tachylite, with which it usually occurs together; however it lacks the
iron oxide crystals dispersed in the glass, and therefore appears transparent and pure, with yellow-brown color, instead of tachylite opaque black.[2] It forms at higher temperatures and with more rapid chilling. Presence of sideromelane indicates higher temperature of the lava, and solidifying of the flow closer to the vent, probably by rapid quenching in a wet environment.