Siliceous sinter (Photo)
A kind of hot spring sinters composed of amorphous silicic acid or opal deposited from hot spring water. Silicic acid dissolved in hot spring water deposits as amorphous silicic acid, [SiOx(OH)4-2x]n, by water temperature lowering or pH increasing. Silicic acid can deposit stably in the wide pH range between 1.0-10.0, and pure white silicic acid (siliceous sinter) deposits from acidic hot spring water with a pH less than 1.0. Silicic acid deposits with amorphous iron oxides in the pH range between 3.0-7.0 from iron containing hot spring water, and colored silicic acid is formed. These silicic acid changes to jade or agate during diagenesis. Some locations produce transparent globular aggregates of siliceous sinter.