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 ®rst deposited, sinters consist largely of noncrystalline spheres (<1±8 lm diameter) of opal-A exhibiting varying degrees of closepacking. Particle densities range from 1.5 to 2.1 g cm, total water 4±10 wt%, and porosities 35±60%. Changes over 10,000 years following deposition are slight although the spheres may be invested by an additional ®lm of secondary silica. For the next 10,000 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 and coalesce in microbotryoidal aggregates ( 10±30 lm diameter). Amygdaloidal ®brous clusters occur with lepispheres. As silica lattice ordering becomes enhanced, total water content drops to <7 wt%, particle density increases to 2.3 g cm, and porosity reduces to<30%. The change from opal-A to opal-C takes place over a briefer periods ( 50 years) in silica sinters that contain other materials (e.g. calcite, sulfur, alunite, plant remains). Sinters older than 50,000 years have recrystallised to microcrystalline quartz. With the onset of quartz crystallisation at 20,000 years, total water is <0.2 wt%, particle density approximates quartz (2.65 g cm), and porosity is <4%. The progressive changes in silica species and texture yield ageing pro®les for sinters that may serve as guides to the paleohydrology of geothermal systems and/or epithermal ore deposits in areas where surface thermal activity has declined or ceased.