mineral deposit with a porous or vesicular texture (having small cavities). At least two kinds are recognized: siliceous and calcareous. Siliceous sinter (geyserite; fiorite) is a deposit of opaline or amorphous silica that occurs as an incrustation around hot springs and geysers and sometimes forms conical mounds (geyser cones) or terraces. The deposition of siliceous sinter is largely due to the action of algae and other forms of vegetation in the thermal waters; near fumaroles and in the deeper channels of hot springs, it may be formed by alteration of wall rocks. Locations include Iceland; New Zealand; Yellowstone National Park, Wyo., U.S.; and Steamboat Springs, Colo., U.S.

Calcareous sinter, sometimes called tufa, calcareous tufa, or calc-tufa, is a deposit of calcium carbonate, exemplified by travertine. So-called petrifying springs, not uncommon in limestone districts, yield calcareous waters that deposit a sintery incrustation on objects exposed to their action. The cavities in calcareous sinter are partly due to the decay of mosses and other vegetable structures that have assisted in its precipitation.