encrusted siliceous rocks, primarily opaline, occurring as deposits of primarily hot mineral springs. Siliceous sinter is typical of deposits of geysers and other springs in volcanic areas (geyserites). The color (brown, greenish, red, yellow) depends on different mineral admixtures (primarily hydrous oxides of iron) and on the presence of organic matter. Deposits of siliceous sinter are usually porous, sometimes in the form of stalactites and stalagmites, and occupy small areas near the source of water from which the siliceous matter is precipitated.