Mineralogical and textural changes accompanying ageing of silica sinter

Twenty nine samples of silica sinter, ranging in age from modern to Miocene , record temporal changes in both mineralogy and texture. When first deposi ted, sinters consist largely of noncrystalline spheres (<1-8 mu m diameter! of opal-A exhibiting varying degrees of close-packing. Particle densities range from 1.5 to 2.1 g cm(-3), total water 4-10 wt%, and porosities 35-60% . Changes over similar to 10,000 years following deposition are slight alth ough the spheres ma!: be invested by an additional film of secondary silica . For the next 10,000 to similar to 50,000 years, the silica incrementally crystallises to become poorly crystalline opal-CT and/or opal-C; spherical particles of thin-bladed crystals (lepispheres) replace opal-A particles an d coalesce in microbotryoidal aggregates (similar to 10-30 mu m diameter). Amygdaloidal fibrous clusters occur with lepispheres. As silica lattice ord ering becomes enhanced, total water content drops to <7 wt%, particle densi ty increases to similar to 2.3 g cm(-3), and porosity reduces to <30%. The change from opal-A to opal-C takes place over a briefer periods (similar to 50 years) in silica sinters that contain other materials (e.g. calcite, su lfur, alunite, plant remains). Sinters older than similar to 50,000 years h ave recrystallised to microcrystalline quartz. With the onset of quartz cry stallisation at similar to 20,000 years, total water is <0.2 wt%, particle density approximates quartz 12.65 g cm(-3)) and porosity is <4%. The progre ssive changes in silica species and texture yield ageing profiles for sinte rs that may serve as guides to the paleohydrology of geothermal systems and /or epithermal ore deposits in areas where surface thermal activity has dec lined or ceased.