Geyserite is the name given to siliceous or calcareous mineral deposits at & in the vicinity of hot springs and geysers. Such mineral deposits are typically composed of opaline silica (SiO2·nH2O), but sometimes they are calcareous, resulting in a surficial travertine deposit (e.g., Mammoth Hot Springs, Wyoming). Siliceous geyserite is also known as siliceous sinter. It tends to be porous, lightweight, and somewhat friable. The hydrous silica forms by precipitation from the hot waters emerging from hot spring or geyser vents.
Geyserite from the Yellowstone Hotspot, northwestern Wyoming, USA (public display, Field Museum of Natural History, Chicago, Illinois, USA).